Faculty of Science
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Date: Thursday 15 December 2022
Speaker: Lucia Reining 
Methods, models and materials: joining forces to deal with the many-body problem

In this series of seminars the many-body problem is approached using a large variety of methods, and exploring systems ranging from models to complex materials. The aim of my talk is to highlight the impact of combinations. On one side, I will present recent results concerning the use of vertex corrections beyond GW derived from time-dependent density functional theory, both for spectra and for total energies. These results include benchmarks on analytically solvable problems, and an exploration of the low-density homogeneous electron gas with its exotic excitations. An alternative strategy to benefit from a combination of methods is the use of models in the framework of ab intio calculations. I will explain our approach termed connector theory [1], which can be used to speed up calculations and to design new functionals.

[1]  M. Vanzini, et al., npj Computational Materials 8, 98 (2022)

Date: Thursday 1 December 2022
Speaker: Christoph Friedrich
Electron–plasmon and electron–magnon scattering in ferromagnets from first principles by combining GW and GT self-energies

This work combines two powerful self-energy techniques: the well-known GW method and a self-energy recently developed by us [1,2] that describes renormalization effects caused by the scattering of electrons with magnons and Stoner excitations. This GT self-energy, which is fully k-dependent and contains infinitely many spin-flip ladder diagrams, was shown to have a profound impact on the electronic band structure of Fe, Co, and Ni. In the present work [3], we refine the method by combining GT with the GW self-energy. The resulting GWT spectral functions exhibit strong lifetime effects and emergent dispersion anomalies. They are in an overall better agreement with experimental spectra than those obtained with GW or GT alone, even showing partial improvements over local-spin-density approximation dynamical mean-field theory. The performed analysis provides a basis for applying the GWT technique to a wider class of magnetic materials.
[1] M.C.T.D. Müller, S. Blügel, and C. Friedrich, Phys. Rev. B 100, 045130 (2019)
[2] C. Friedrich, Phys. Rev. B 100, 075142 (2019)
[3] D. Nabok, S. Blügel, and C. Friedrich, Npj Comput. Mater. 7, 178 (2021)

Date: Thursday 17 November 2022
Speaker: Shiwei Zhang 
More is different: multi-method studies of many-electron systems

Capturing the effects of interacting electrons is essential for understanding and predicting material properties. Reliable treatment of the many-electron Schrodinger equation is thus a central theme in modern physics and chemistry, and an outstanding challenge. Recent collaborative studies combining multiple cutting-edge computational methods have led to significant progress on several fronts. I will discuss an in-depth study of the quantum-mechanical ground state of what is perhaps the simplest realistic model for a bulk material: an infinite chain of equally spaced hydrogen atoms. Our study reveals a rich phase landscape, including dimerization, metal-insulator transition, and magnetic phases. The hydrogen chain will serve as a key benchmark for further methodological developments and an important model for correlated electron systems beyond the Hubbard model. The results will motivate experimental realizations and stimulate further efforts to characterize phase diagrams of low-dimensional materials.

Date: Thursday  20 October 2022
Speaker: Alex Zunger
Could it be that d- electron oxide perovskites are not necessarily strongly correlated, just complicated?

Open shell isolated gas-phase atoms and ions have been known for ages to manifest spectroscopically numerous narrow lines (multiplets) indicative of the dominance of many-body strong correlation effects. Yet, such multiplets often disappear when the atoms are assembled into solids (such as diamond from carbon), leading to the question when exactly are many - body effects need, and where has the strong correlation gone. The 3d oxides perovskites have formed a playing ground of theoretical approaches claiming the exclusive need of strong correlation as a minimal required theoretical framework. Interestingly, such conclusions were often based on models focusing on what the (interacting) electrons do, disallowing the atomic positions, local moments and other lattice modes to respond by breaking symmetries. Recent study [1] point to the possibility that mean-field like Density Functional Theory (DFT) even without recourse to strong correlation --yet permitted to break symmetry as it lowers the total energy-- can provide without recourse to strong correlation excellent intuition into such questions. This includes gapping of Mott insulators as well as appearance of Mott metals, bond dimerization, Orbital Order, Mass enhancement, both for magnetic as well as nonmagnetic ordered as well as Para phases of oxides. Local experimental probes are reporting observations of such local symmetry breaking motifs.

[1]A. Zunger, Comment to Nature Computational Science | VOL 2 | September 2022 | 529–532

Date: Thursday 6 October 2022
Speaker: Emanuel Gull 
Let’s get real – Adapting the toolkit of many-body theory to realistic materials simulation

Quantum many-body theories, including diagrammatic perturbation theories and non-perturbative embedding theories, describe the physics of many interacting particles in solids. These theories are typically applied to effective low-energy lattice models, which are designed to capture the essential degrees of freedom of a solid.

This talk will summarize recent progress on solving the many-body problem ab-initio, i.e. without adjustable parameters and without the construction of effective low-energy models, with quantum many-body theories. We will show how algorithmic and computational advances have enabled the adaptation of tools that were previously only available on lattice models to real-materials simulations, and how these simulations avoid several common uncontrolled approximations. A path towards controlled and adaptive many-body simulations is outlined.

Date: Wednesday 13 July 2022
Speaker: Theo Nieuwenhuizen 
Analysis of a quantum measurement: from dynamics to interpretation

The contact between quantum mechanics and the reality in a laboratory lies in measurement, so its dynamics should guide the interpretation. In a solvable model a spin-1/2, is measured by an apparatus, an Ising magnet with $N \gg 1$ spins coupled to a bath. The Born rule follows since $s_z$ is conserved, while the decay of $\langle s_{x,y}\rangle$ Implies the disappearance of Schr\”odinger cat states. The outcome is registered when the magnet goes from its initial paramagnetic state to its stable up or down ferromagnetic state, dumping free energy in the bath. In this approach, the density matrix is an abstract object that codes our best information about the ensemble of systems on which an ensemble of measurements is performed. Finally, postulates are needed, weaker than the Copenhagen ones. In this epistemic description, there is no role for various ontological interpretations of quantum mechanics.

Date: Friday 8 July 2022
Speaker: Igor Gornyi
Measurement-induced steering of quantum systems

Two standard approaches for preparing quantum states are suggested by the laws of quantum and statistical mechanics. One of these is to perform projective measurements of a set of observables represented by commuting operators that fully specify the target state. Alternatively, if the target state is the ground state of the Hamiltonian for the system, it can be reached by putting the system in thermal contact with a heat bath that is at a sufficiently low temperature.
We set out an alternative general framework for steering toward a chosen quantum state from an arbitrary initial state by coupling the system to auxiliary quantum degrees of freedom (“detectors”). The steering harnesses back-action of the projectively measured detectors on the system, which arises from entanglement generated during the coupled evolution. We establish principles for the design of system-detector couplings and illustrate our general ideas using both few-body examples and a many-body example (a spin-1 chain steered to the Affleck-Kennedy-Lieb-Tasaki state). We further address the stability of measurement-induced steering with respect to various types of errors in the protocol. We extend our analysis by including an active-decision choice of the system-detector interactions during the steering, which can be viewed as "navigation in many-body Hilbert space" based on the information extracted through measurements. We also consider the topological classification of steered states in the context of open systems. Finally, we discuss the relation of our approach to the phenomenon of measurement-induced entanglement transition, in particular, in connection to Anderson (and many-body) localization.

Date: Friday 17 June 2022
Speaker: Jan Tomczak
Title: Transport properties of correlated narrow-gap semiconductors

Combining numerical simulations with analytical insight, we establish a comprehensive and thermodynamically consistent methodology for transport properties in semiconductors[1]. First, we consider heavy-fermion insulators: Their resistivity typically saturates below a characteristic temperature T*. In our scenario[2], finite lifetimes of intrinsic carriers drive residual conduction, impose the existence of a crossover T*, and control---on par with the charge gap---the quantum regime emerging below. We showcase this mechanism for the Kondo insulator Ce3Bi4Pt3 and elucidate how its saturation regime evolves under external pressure and varying disorder. Deriving a phenomenological formula for the quantum regime, we also unriddle the ill-understood bulk conductivity of SmB6---demonstrating a wide applicability of our mechanism in correlated narrow-gap semiconductors[3]. Finally, we extend the discussion to signatures of finite electronic lifetimes in the coefficients of Hall, Seebeck and Nernst, with a focus on FeSb2. The described methodology will soon be available in the open-source package "LinReTraCe" [4] that can be easily interfaced any electronic structure or tight-binding code.

[1] M. Pickem, E. Maggio, J.M. Tomczak, Phys. Rev. B 105, 085139 (2022)
[2] M. Pickem, E. Maggio, J.M. Tomczak, Commun. Phys 4, 226 (2021)
[3] J.M. Tomczak, J. Phys.: Condens. Matter 30, 183001 (2018)
[4] https://github.com/LinReTraCe

Date: Friday 3 June 2022
Speaker: Riccardo Rossi
Title: Phase transitions and crossovers in the Hubbard model with Diagrammatic Monte Carlo

In order to determine phase transitions and crossover behaviors numerically, it is often necessary to simulate large system sizes, a task which is particularly challenging for strongly-correlated fermionic systems. Advancements in the Diagrammatic Monte Carlo technique allows us to obtain numerically-exact results for previously inaccessible system sizes and temperatures. As we show, this is important when studying the interplay of spin and charge correlations of the doped two-dimensional Hubbard model at finite temperature, and the superfluid transition in the three-dimensional attractive polarized system.

Date: Friday 20 May 2022
Speaker: David Nelson
Statistical Mechanics of Mutilated Sheets and Shells

Understanding deformations of macroscopic thin plates and shells has a long and rich history, culminating with the Foeppl-von Karman equations in 1904, a precursor of general relativity characterized by a dimensionless coupling constant (the "Foeppl-von Karman number") that can easily reach  vK = 10^7 in an ordinary sheet of writing paper.  However, thermal fluctuations in thin elastic membranes fundamentally alter the long wavelength physics, as exemplified by experiments that twist and bend individual atomically-thin free-standing graphene sheets (with vK = 10^13!)   A crumpling transition out of the flat phase for thermalized elastic membranes has been predicted when kT is large compared to the microscopic bending stiffness, which could have interesting consequences for Dirac cones of electrons embedded in graphene.   It may be possible to lower the crumpling temperature for graphene to more readily accessible range by inserting a regular lattice of laser-cut perforations, an expectation an confirmed by extensive molecular dynamics simulations.    We then move on to analyze the physics of sheets mutilated with puckers and stitches.   Puckers and stitches lead to Ising-like phase transitions riding on a background of flexural phonons, as well as an anomalous coefficient of thermal expansion.  Finally, we argue that thin membranes with a background curvature lead to thermalized spherical shells that must collapse beyond a critical size at room temperature, even in the absence of an external pressure.

Date: Friday 13 May 2022
Frank Koppens
Title: Near-field optical and photocurrent microscopy of twisted 2D materials

Twisted two-dimensional materials are formed by stacking two atomically thin layers with a small twist angle. This causes an interference pattern in the atomic lattice called a moiré pattern, which affects the electronic and optical properties dramatically. The first studies on twisted graphene near the “magic angle” of 1.1° revealed strongly correlating states and topological features, making it a host of tunable exotic phases that may shed light on the origins of unconventional superconductivity. These phenomena motivate us to study the optical properties of twisted materials on the nanoscale. To this end, we use cryogenic (10K) near-field optical microscopy and near-field photocurrent mapping. This technique allows for 20-nm resolution spatial mapping of the local optical properties, collective excitations and Seebeck coefficient. Our observations allow us to relate the spatial variations of correlated electronic states to spatial variations of the twist angle. Moreover, we observe spatial anisotropies, which shed light on the microscopic mechanisms of symmetry breaking.

Date: Friday 29 April 2022 at 15:30 hours
Speaker: Francisco Guinea
Title: Electron-electron interactions and superconductivity in twisted bilayer graphene and related materials
Location: Transitorium 00.012 (Toernooiveld 5) & via ZOOM (request Malte Rösner)

The effect of electron-electron interactions in twisted bilayer graphene is enhanced by the narrowness of the central bands, and by the complexity of the electronic wavefunctions. The long range electrostatic interaction is the strongest coupling in twisted bilayer graphene. The way in which this interaction determines the phase diagram of twisted bilayer graphene will be discussed. Long wavelength charge fluctuations, modified by the Coulomb interaction, and also by longitudinal acoustic phonons, can lead to a net attraction, even at long distances, and contribute to the existence of superconductivity. Comparison between twisted bilayer graphene and other graphitic systems with narrow bands will also be presented.

Date: Friday 8 April 2022
Speaker: Alexey Kimel
Ultrafast magnetism of antiferromagnets

It is believed that antiferromagnets have a huge potential to accelerate the speed of magnetic recording and information processing in spintronics and magnonics devices. However, experimental studies of antiferromagnets and the development of antiferromagnetic spintronics are significantly hindered by the absence of magnetization in antiferromagnets in the ground state. This makes conventional magnetometry impossible and requires an extremely strong magnetic field to control the spins. The purpose of this work is to demonstrate that ultrashort THz pulses of an electromagnetic field open up new possibilities in spin control in antiferromagnets and practically become the game-changer in the field. At the beginning of the talk, I will start with the simplest example showing that a relatively weak, but rapidly changing magnetic field is an effective tool to control spins in antiferromagnets [1-3]. Afterwards I will demonstrate that ultrafast and intense excitation of magnonic resonances pushes spin dynamics into anharmonic regime promoting energy transfer between otherwise noninteracting magnonic and phononic modes via fully coherent magnon-phonon interactions [4].

[1] A. K. Zvezdin, JETP 29, 553 (1979).
[2] A. F. Andreev, V. I. Marchenko, Sov. Phys. Uspekhi 23 21 (1980).  
[3] V.G. Baryakhtar, B. A. Ivanov, M. V. Chetkin, Sov. Phys. Usp. 28 563 (1985).
[4] E. A. Mashkovich et al, Science 374, 1608-1611 (2021).

Date: Friday 1 April 2022
Speaker: Michele Kotiuga
Title: Designer materials for neuromorphics and more!

In this talk I will discuss our work done in our joint theory-experiment collaboration with the Ramanathan group, in which nickel-based materials were manipulated to create neuromorphic devices, sensors, ionic conductors and more. I will present a series of first-principles calculations that elucidate the on-site Mott transition that occurs in electron-doped rare-earth nickelates. The charge-localization in this system is facilitated by the strong nickel-oxygen hybridization present in the pristine materials. Specifically, I will show how the structure, nickel oxidation state and band gap evolve with high-density electron doping. These systems, and the charge localization that occurs with doping make them highly tunable and relevant for many types of functional applications from neuromorphic devices to energy storage.  I will also discuss recent extensions of this work in nickel oxide which present a novel platform to investigate neuromorphic learning.
Relevant references: 
Zuo, Panda, Kotiuga et al., Nat Comm 8, 240 (2017); 
Zhang, Schwanz, Narayanan, Kotiuga et al., Nature, 553, 68 (2018); 
Sun, Kotiuga et al., PNAS, 115, 9672 (2018); 
Kotiuga et al., PNAS 116, 21992 (2019); 
Kotiuga & Rabe, PRM, 3, 115002 (2019); 
Kotiuga & Rabe, arXiv:2007.01744
Zhang et al., PNAS  118, e2017239118 (2021)

Date: Friday 25 March 2022
Speaker: Carl M. Bender
Title: PT symmetry

By using complex-variable methods one can extend conventional Hermitian quantum theories into the complex domain. The result is a huge and exciting new class of non-Hermitian parity-time-symmetric (PT-symmetric) theories that still obey the fundamental laws of quantum mechanics. These new theories have remarkable physical properties, which are currently under intense study by theorists and experimentalists. Many theoretical predictions have been verified in recent beautiful laboratory experiments.

Date: Friday 11 March 2022
Speaker: Swagata Acharya
A theory for colors of strongly correlated insulators

Color of a material is complementary to the wavelengths it absorbs. Strongly correlated electronic systems appear in different colors. Many such systems have large gaps and no quasi-particle excitations in the visible range, suggesting that the color in them can only emerge through selective absorption of visible part of the spectrum which are purely many-body in nature. We theoretically reproduce the colors for several such strongly correlated Mott systems by employing an advanced first principles approach that combines a sophisticated many body perturbative approach with a locally exact method.

Date: Friday 4 March 2022
Speaker: Guy Cohen
Inchworm Monte Carlo in the Steady State

Numerically exact real time quantum Monte Carlo methods simulate steady state dynamics by propagating a tractable initial condition to long times. Though inchworm algorithms make this tractable, the computational cost for accurately accessing nonequilibrium steady states often remains prohibitive. We overcome this issue by reformulating the inchworm equations directly in the steady state. We validate the resulting method against analytic results and other numerically exact techniques, and discuss applications to extended correlated systems by way of the dynamical mean field theory.

Date: Friday 25 February 2022
Speaker: Margaret Murnane 
Title: Uncovering New Light-Induced Phases in Magnetic and 2D Materials

The ability to probe the full dynamic response of quantum materials on the length- and time-scales (Å to attoseconds on up) fundamental to charge, spin and phonon interactions is leading to a host of new discoveries. Under thermal equilibrium conditions, materials can be tuned by varying the temperature, pressure, chemical doping or dimensionality. Ultrafast light sources have undergone remarkable advances in recent years, achieving what was merely a dream three decades ago, i.e., full coherent control of light fields spanning the THz to the X-ray regions. These new capabilities are providing powerful new tools for coherently manipulating and probing quantum materials using light.

Date: Friday 4 February 2022
Speaker: Uwe Bovensiepen
Microscopic insights into photo-excited reactions at complex interfaces: Polar vs. non-polar solvents

Screening of ions at interfaces as a consequence of solvation is of central importance for electrochemical, photo- and electrochemical properties and essential in technology for energy conversion. In comparison to solvation processes in bulk liquids the presence of the interface increases the level of complexity. Microscopically, on the atomic scale, interactions of the ion with the solvent and the underlying substrate (or electrode) compete and potentially alter the bulk properties fundamentally.
In this talk recent findings of a combined experimental and theoretical investigation at model interfaces of Cs+/Cu(111), onto which D2O and Xe were coadsorbed as examples for polar and non-polar solvents, will be reported.
We find that attraction between Cs+ and Xe counterbalances the screened Coulomb repulsion between Cs+ ions on Cu(111) and observe that the Cs 6s electron is repelled from Cu(111) due to xenon’s electron density [1]. Together, this yields a dual, i.e., attractive or repulsive, response of Xe depending on the positive or negative charge of the respective counterparticle, which emphasizes the importance of the Coulomb interaction in these systems. In case of water coadsorption the solvent-solute arrangement is fundamentally different from the bulk. Ring like structures, in which the Cs+ ions are located at the perimeter, form due to the competition between the Cs+-water and the hydrogen bonds in combination with the interaction with the substrate [2]. Furthermore, relaxation dynamics of the Cs 6s electron which is photo-excited by electron transfer from Cu(111), shows indications of a collective solute-solvent response. This work outlines that the relationship between coverage, geometric and electronic structure will have important consequences on the chemical reactivity of such interfaces in general.
This work is supported by the Deutsche Forschungsgemeinschaft through the Cluster of Excellence RESOLV and the Collaborative Research Center 1242 Non-equilibrium dynamics of condensed matter in the time domain.
[1] J. Thomas, C. Bertram, J. Daru, J. Patwari, I. Langguth, P. Zhou, D. Marx, K. Morgenstern, U. Bovensiepen, Phys. Rev. Lett. 127, 266802 (2021).
[2] C. Penschke, J. Thomas, C. Bertram, A. Michaelides, K. Morgenstern, P. Saalfrank, U. Bovensiepen, see arXiv.org.

Date: Friday 28 January 2022
Speaker: Atac Imamoglu 
Strongly correlated electrons in atomically thin semiconductors

In this talk, I will describe recent experiments in atomically-thin transition metal dichalcogenides (TMDs) where Coulomb interactions between electrons dominate over their kinetic energy. Our measurements provide a direct evidence that the electrons at densities < 3 · 1011 cm-2 in a pristine MoSe2 monolayer form a Wigner crystal even at B = 0 [1]. This is revealed by our low-temperature (T = 80 mK) magneto-optical spectroscopy experiments that utilize a newly developed technique allowing to unequivocally detect charge order [2]. This method relies on the modification of excitonic band structure arising due to the periodic potential experienced by the excitons interacting with an electronic lattice. Under such conditions, optically-inactive exciton states with finite momentum matching the reciprocal Wigner lattice vector k = kW get Bragg scattered back to the light cone, where they hybridize with the zero-momentum bright exciton states. This leads to emergence of a new, umklapp peak in the optical spectrum heralding the presence of periodically-ordered electronic charge distribution.
Twisted bilayers of TMDs in turn offer a wealth of new phenomena, ranging from dipolar excitons to correlated insulator states. Another striking example of qualitatively new phenomena in this system is our recent observation of an electrically tunable two-dimensional Feshbach resonance in exciton-hole scattering [3], which allows us to control the strength of interactions between excitons and holes located in different layers. Our findings enable hitherto unexplored possibilities for optical investigation of many-body physics, as well as realization of degenerate Bose-Fermi mixtures with tunable interactions.
[1] T. Smoleński, P. E. Dolgirev, C. Kuhlenkamp, A. Popert,1 Y. Shimazaki, P. Back, X. Lu, M. Kroner, K. Watanabe, T. Taniguchi, I. Esterlis, E. Demler, and A. Imamoglu, arXiv:2010.03078 (2020).
[2] Y. Shimazaki, C. Kuhlenkamp, I. Schwartz, T. Smolenski, K. Watanabe, T. Taniguchi, M. Kroner, R. Schmidt, M. Knap, A. Imamoglu, arXiv:2008.04156 (2020).
[3] I. Schwartz, Y. Shimazaki, C. Kuhlenkamp. K. Watanabe, T. Taniguchi, M. Kroner, A. Imamoglu, arXiv:2105.03997 (2021).

Date: Friday 21 January 2022
Speaker: Jonas Fransson 
Title: Electron correlations in the setting of many-body operators
- no abstract available -

Date: Friday 17 December 2021
Speaker: Victor Galitski 
Title: Strongly-Interacting Electron-Photon Systems and Superconductivity

Following the recent success of realizing exciton-polariton condensates in cavities, I will discuss the hybridization of cavity photons with collective modes in interacting two-dimensional materials. First, I will examine the closest analog of excitons within a superconductor, states called Bardasis-Schrieffer modes. Though Bardasis-Schrieffer modes do not typically couple directly to light, one can engineer a coupling with an externally imposed supercurrent, leading to the formation of hybridized Bardasis-Schrieffer-polariton states. These new excitations have a non-trivial overlap with both the original photon states and d-wave superconducting fluctuations, implying that their condensation could produce  an exotic s±id superconducting state. Next, I will discuss how the Higgs mode in various symmetry-broken states of matter  can be stabilized and excited by resonantly coupling it to cavity photons. In conclusion, I will discuss how cavities can be used to enhance  superconductivity and other coherent quantum states in strongly correlated materials.

Date: Friday 10 December 2021
Speaker: Philipp Werner
Title: Nonequilibrium dynamical mean field theory

Recent experiments on laser-driven solids have revealed interesting nonequilibrium effects such as light-induced superconducting states [1,2] or switching into long-lived metastable states with novel structures and electronic properties [3]. To investigate and understand such phenomena, new theoretical and computational tools need to be developed. I will present the dynamical mean field approach, which over the past 15 years has been extended into a powerful framework for the simulation of real-time dynamics in correlated lattice systems [4]. After introducing this nonequilibrium Green’s function based technique, I will discuss benchmarks against cold-atom simulators [5], and present recent applications to laser-driven lattice models. These investigations demonstrate the possibility of effectively cooling correlated electron systems [6], and inducing magnetic, superconducting or excitonic order in long-lived nonequilibrium states [7,8]. I will comment on the implications of these findings for the experiments on light-induced superconductivity.

[1] S.Kaiser, C. R. Hunt, D. Nicoletti, W. Hu, I. Gierz, H. Y. Liu, M. Le Tacon, T. Loew, D. Haug, B. Keimer, and A. Cavalleri, Phys. Rev. B 89, 184516 (2014).
[2] M. Mitrano, A. Cantaluppi, D. Nicoletti, S. Kaiser, A. Perucchi, S. Lupi, P. Di Pietro, D. Pontiroli, M. Ricco, S. R. Clark, D. Jaksch, and A. Cavalleri, Nature 530, 461 (2016).
[3] L. Stojchevska, I. Vaskivskyi, T. Mertelj, P. Kusar, D. Svetin, S. Brazovskii, and D. Mihailovic, Science 344, 177 (2014).
[4] H. Aoki, N. Tsuji, M. Eckstein, M. Kollar, T. Oka, and P. Werner, Rev. Mod. Phys. 86, 779 (2014).
[5] K. Sandholzer, Y. Murakami, F. Goerg, J. Minguzzi, M. Messer, R. Desbuquois, M. Eckstein, P. Werner, and T. Esslinger, Phys. Rev. Lett. 123, 193602 (2019).
[6] P. Werner, M. Eckstein, M. Mueller, and G. Refael, Nature Comm. 10, 5556 (2019).
[7] P. Werner, J. Li, D. Golez, and M. Eckstein, Phys. Rev. B 100, 155130 (2019).
[8] P. Werner and Y. Murakami, Phys. Rev. B 102, 241103(R) (2020).

Date: Friday 26 November 2021
Speaker: Alexander Wietek
Mott insulating states with competing orders in the triangular lattice Hubbard model

The physics of the triangular lattice Hubbard model exhibits a rich phenomenology, ranging from a metal-insulator transition, intriguing thermodynamic behavior, and a putative spin liquid phase at intermediate coupling, ultimately becoming a magnetic insulator at strong coupling. We combine a finite-temperature tensor network method, minimally entangled thermal typical states (METTS), with two Green function-based methods, connected-determinant diagrammatic Monte Carlo (DiagMC) and cellular dynamical mean-field theory (CDMFT), to establish several aspects of this model. We elucidate the evolution from the metallic to the insulating regime from the complementary perspectives brought by these different methods. We compute the full thermodynamics of the model on a width-4 cylinder using METTS in the intermediate to strong coupling regime. We find that the insulating state hosts a large entropy at intermediate temperatures, which increases with the strength of the coupling. Correspondingly, and consistently with a thermodynamic Maxwell relation, the double occupancy has a minimum as a function of temperature which is the manifestation of the Pomeranchuk effect of increased localisation upon heating. The intermediate coupling regime is found to exhibit both pronounced chiral as well as stripy antiferromagnetic spin correlations. We propose a scenario in which time-reversal symmetry broken states compete with nematic, lattice rotational symmetry breaking orders at lowest temperatures.

[1] A. Wietek, R. Rossi, F. Šimkovic IV, M, Klett, P. Hansmann, M. Ferrero, E. M. Stoudenmire, T. Schäfer, A. Georges, Phys. Rev. X 11, 041013 (2021)
[2] A. Wietek, Y.-Y. He, S. R. White, A. Georges, and E. M. Stoudenmire, Phys. Rev. X 11, 031007 (2021)

Date: Friday 12 November 2021
Speaker: Lincoln Carr
Entangled quantum cellular automata, physical complexity, and Goldilocks rules

Cellular automata are interacting classical bits that display diverse emergent behaviors, from fractals to random-number generators to Turing-complete computation. We discover that quantum cellular automata (QCA) can exhibit complexity in the sense of the complexity science that describes biology, sociology, and economics. QCA exhibit complexity when evolving under 'Goldilocks rules' that we define by balancing activity and stasis. Our Goldilocks rules generate robust dynamical features (entangled breathers), network structure and dynamics consistent with complexity, and persistent entropy fluctuations. Present-day experimental platforms—Rydberg arrays, trapped ions, and superconducting qubits—can implement our Goldilocks protocols, making testable the link between complexity science and quantum computation exposed by our QCA.

The inability of classical computers to simulate large quantum systems is a hindrance to understanding the physics of QCA, but quantum computers offer an ideal simulation platform. If time allows, I will discuss our recent experimental realization of QCA on a digital quantum processor, simulating a one-dimensional Goldilocks rule on chains of up to 23 superconducting qubits. Employing low-overhead calibration and error mitigation techniques, we calculate population dynamics and complex network measures indicating the formation of small-world mutual information networks. Unlike random states, these networks decohere at fixed circuit depth independent of system size, the largest of which corresponds to 1,056 two-qubit gates. Such computations may open the door to the employment of QCA in applications like the simulation of strongly-correlated matter or beyond-classical computational demonstrations.


  1. LE Hillberry, MT Jones, DL Vargas, P Rall, N Yunger Halpern, N Bao, S Notarnicola, S Montangero, LD Carr, “Entangled quantum cellular automata, physical complexity, and Goldilocks rules,” Quantum Science and Technology, v. 6, p. 045017 (2021)
  2. EB Jones, LE Hillberry, MT Jones, M Fasihi, P Roushan, Z Jiang, A Ho, C Neill, E Ostby, P Graf, E Kapit, and LD Carr, “Small-world complex network generation on a digital quantum processor,” https://arxiv.org/abs/2111.00167 (2021)
  3. LE Hillberry, M Fasihi, L Piroli, N Yunger Halpern, T Prosen, and LD Carr, “Thermodynamics, scrambling, chaos, and integrability in quantum cellular automata,” in preparation (2021)

Date: Friday 5 November 2021
Speaker: Friedrich Krien
Pseudotwins | Pseudogaps - The remarkable transformation of paramagnon scattering from weak to strong coupling

A momentum-selective suppression of spectral weight, the pseudogap, is observed in models of strongly correlated electrons and in experiments, for example, on high-temperature superconducting cuprates. I review and compare pseudogaps induced by antiferromagnetic fluctuations (paramagnons) in the single-band Hubbard model from weak [1,2,3] to strong [4] coupling. At weak coupling paramagnon scattering is subject to the nesting condition and hence opens a gap near so-called `hot spots' [1]. On the other hand, strong coupling and particle-hole asymmetry give rise to a second scattering mechanism [5]. I explain in detail the remarkable features of paramagnon scattering at strong coupling, such as antidamping of quasiparticles, gaps opening on extended parts even of a frustrated Fermi surface, and emergence of a classical regime at short correlation length [6]."

[1] Vilk PRB 55, 3870 (1997)
[2] Schäfer et al. PRX 11, 011058 (2021)
[3] Krien et al. PRB 102, 235133 (2020)
[4] Wu et al. PRB 96, 041105(R) (2017)
[5] Krien et al. arXiv:2107.06529
[6] Vilk & Tremblay J. Phys. I France 7, 1309 (1997)

Date: Friday 29 October 2021
Speaker: Cedric Weber
Title: Optimisation of the superconducting temperature by many body computational approaches

Since its discovery in the mid 1980's, the quest for unconventional superconductors with higher Tc, based on oxides of copper, have challenged and pushed the frontiers of our theoretical and experimental abilities. Finding a unified description or single tuning parameter that drives a superconductor towards higher critical temperatures continues to be a key but elusive question in this field.

We have recently suggested that lattice vibrations might provide an aim to increase the critical temperature on short timescales. In this work, we investigate this early hypothesis and carry out a three tier calculation of the estimated critical temperature with a combined approach of quasi-particle GW, dynamical mean-field theory and the Bethe Salpeter approach. We deploy this approach
for a frozen phonon approach, matching the lattice excitations observed by ultra-fast pump probe spectroscopy.

Our work provides guidance for optimising and controlling superconducting correlated materials with guided ab initio design. If time allows, we will also review a recent extension of our formalism to high temperature superconductivity in the lanthanide hydrides family, with the ability of our formalism to tackle the calculations of forces within many-body perturbation theory.

PNAS 117,  6409 (2020)
PRX 8, 021038 (2018)

Date: Friday 22 October 2021
Speaker: Vladimir Mazurenko
Patterns of quantum state bitstrings

The rapid development of quantum computing technologies already made
it possible to manipulate a collective state of several dozen of
qubits. This success poses a strong demand on efficient and reliable
methods for characterization and verification of large-scale quantum
states. Traditional methods, such as quantum tomography, which require
storing and operating wave functions on classical computers, become
problematic to use in the regime of large number of degrees of
freedom. In this talk, I will discuss a numerically cheap procedure
[1] to describe and distinguish quantum states which is based on a
limited number of simple projective measurements in at least two
different bases and computing inter-scale dissimilarities of the
resulting bit-string patterns via coarse-graining. By concrete
examples, it will be shown that our approach can be used to
characterize quantum states with different structure of entanglement,
including the chaotic quantum states. The last part of the talk will
be devoted to detecting phase transitions in quantum magnetic systems
with the dissimilarity approach.

[1] O. M. Sotnikov, I. A. Iakovlev, A. A. Iliasov, M. I. Katsnelson,
A. A. Bagrov, V. V. Mazurenko, arXiv:2107.09894

Date: Friday 15 October 2021
Speaker: Kieron Burke
DFT and strong correlation:  Successes, Failures, and Machine Learning

I will discuss density functional theory, the starting point of most realistic calculations of electronic structure of materials.  I will use the Hubbard dimer to illustrate truths and falsehoods about DFT and many-body theory.

I will show results of DMRG calculations of strongly correlated systems with Steve White, and what standard functionals get wrong.  I will end with recent machine-learned functionals that work for strongly correlated systems.

Date: Friday 8 October 2021
Speaker: Tianyu Zhu
Condensed Phase Chemical Physics from Full Cell Quantum Embedding

Solid-state materials with strong electronic interactions exhibit exotic quantum phenomena including high-temperature superconductivity and metal-insulator transition, and form a crucial part in light harvesting and catalysis applications. However, quantitative first-principles description of spectral properties in strongly correlated materials remains a fundamental challenge in computational physics and chemistry. In this talk, I will describe a quantum embedding framework to compute electronic charged excitations and spectra in correlated solids towards quantitative accuracy. First, I will introduce the basics of quantum embedding, particularly dynamical mean-field theory (DMFT), and discuss the strengths and challenges in previous embedding formulations. Second, I will detail a “full cell” quantum embedding formulation that provides a different and new avenue for studying condensed phase chemical physics. I will describe how molecular many-body quantum chemistry methods, such as coupled-cluster theory, can be utilized for accurate description of photoemission spectra in a range of semiconducting, insulating, and metallic periodic materials as well as Kondo systems. Finally, I will conclude with a brief outlook on this approach in studying strongly correlated materials and heterogeneous chemical systems.

Date: Friday 24 September 2021
Speaker: Guillaume Salomon 
Title: Exploring strongly correlated fermions at the single particle level

The manipulation and detection of quantum many-body systems down to the level of single particle and spin offer a totally new paradigm to study strongly correlated phases. In particular, spin-resolved quantum gas microscopy allows to directly measure arbitrary N-point correlations involving both spin and density which opens fascinating perspectives for experiments.
I will discuss here recent equilibrium and out of equilibrium experimental studies of the Fermi-Hubbard model realised by trapping ultracold fermions in optical lattices, focusing on the interplay between doping and magnetism. In particular, fundamental differences between doped one- and two-dimensional Mott insulators will be discussed based on the observation of spin-charge separation signatures in 1d, magnetic polarons and Fermi-liquid in 2d.

Date: Friday 17 September 2021
Speaker: Hermann Dürr
Non-equilibrium nanoscale control of charge, spin & lattice motion in magnetic materials

The idea to probe, change and control functional materials properties with the help of light has long intrigued researchers in materials science. I will show for several examples the unique potential of using femtosecond pulses from x-ray free electron lasers to probe in real time the ultrafast dynamics in nanoscale systems. Although engineering the evolving electron, spin and lattice motion on the time- and lengthscales associated with the relevant magnetic interactions promises new ways for information technologies their understanding remains a grand challenge for basic science.

Date: Thursday 24 June 2021
Speaker: Erik van Loon
Title: Two-particle correlations in Dynamical Mean-Field Theory

Strong repulsive interactions between electrons can lead to a Mott metal-insulator transition. A theoretical description of this transition is provided by dynamical mean-field theory (DMFT). This is usually in done terms of single-particle concepts, such as the spectral function and the quasiparticle weight. Here, I will reconsider the hysteresis region and the critical end point of the metal-insulator transition on the level of two-particle correlations. I will show that the eigenvector structure of the nonlocal Bethe-Salpeter equation provides a unified picture of the hysteresis region and of the critical end point of the Mott transition. In particular, it simultaneously explains the thermodynamics of the hysteresis region and the iterative stability of the DMFT equations. I will argue that this type of two-particle analysis can provide a deeper understanding of phase transitions in correlated materials.

Date: Thursday 17 June 2021
Speaker: Elton Santos
Exploring the Limits of Magnetism in Two-Dimensional Materials

The family of 2D compounds has grown almost exponentially since the discovery of graphene and so too the rapid exploration of their vast range of electronic properties. Some family members include superconductors, Mott insulators with charge-density waves, semimetals with topological properties, and transition metal dichalcogenides with spin-valley coupling. Among several compounds, the realization of long-range ferromagnetic order in van der Waals (vdW) layered materials has been elusive till very recently. Long searched but only now discovered 2D magnets are one of the select group of materials that retain or impart strongly spin correlated properties at the limit of atomic layer thickness. In this presentation I will discuss how different layered compounds (e.g. CrX3 (X=F, Cl, Br, I), VI3, MnPS3, Fe3GeTe2, FePS3, CrGeTe3) can provide new playgrounds for exploration of spin correlations involving quantum-effects, topological spin-excitations and higher-order exchange interactions. I will show that this new generation of vdW magnets can help to revolutionize several technological applications from sensing to data storage, which can lead to new magnetic, magnetoelectric and magneto-optic applications in industry. Moreover, I will discuss some challenges at the forefront of 2D vdW magnets and new opportunities to understand fundamental problems.

Date: Thursday 3 June 2021
Speaker: Sergey Simak
Title: Iron in Earth’s core

The Earth solid inner core is mostly iron, however its crystal structure is still uncertain. Currently, two major candidates are considered – hexagonal close-packed (hcp) and body centered cubic (bcc) structures. Neither of these structures received unanimous support. Experimental evidence is controversial.

The earlier ab initio based computational studies, whether supporting stabilization of the body-centered cubic phase or rejecting it, suffered from the small size of simulation cells. I will demonstrate that the size is important and show that if the size of the computed structure is increased the bcc phase becomes more stable than the hcp phase. The bcc phase is stabilized by the unique self-diffusion related to its low temperature dynamical instability.

This suggests that the iron in the Earth inner core could be stable in the bcc phase. The results of the latest experiments, if interpreted correctly, support this conclusion.

Date: Thursday 27 May 2021
Speaker: Edoardo Baldini
Exciton-driven quantum phases in correlated insulators

Collective excitations of bound electron-hole pairs—known as excitons—are ubiquitous in condensed matter and offer a unique platform to discover new many-body physics. Through a plethora of microscopic interactions, bound excitons can drive exotic correlated and topological phases in and out of equilibrium, most of which have remained so far undetected. In this talk, I will describe how advanced ultrafast spectroscopy methods that probe the low (i.e. meV) energy scale of materials are key to uncovering these emergent quantum phases. In particular, I will focus on the tailored driving of spin–orbit-entangled excitons that arise from Zhang-Rice states in correlated insulators. I will show how this excitation protocol enables the realization of an emergent antiferromagnetic metallic phase and the simultaneous coherent manipulation of the underlying magnetic moments. Finally, I will discuss the opportunities offered by the development of novel driving schemes in the terahertz range and their first-time integration into advanced electronic structure probes.

Date: Thursday 20 May 2021
Speaker: Markus Wallerberger
Unleashing the Matsubara technique: Storing and manipulating many-body response functions with exponential convergence.

Many-body response functions such as the spectral function and the electronic
susceptibility are play a crucial role in understanding correlated fermions. 
For realistic systems, they usually have to be computed in imaginary time
rather than real time.  Still, the cost of storing and manipulating them
usually rises quickly in the bandwidth of the system, the inverse temperature,
and desired accuracy; linearly for one-particle, cubically for two-particle
responses.  This puts many interesting systems out of reach.

In this talk, I show that imaginary time comes with an intrinsic compression
which allows us to reliably reconstruct one- and two-particle responses from a
sparse grid. It also allows us to confine any diagrammatic computations to such a grid.  Both memory and runtime cost then only rises logarithmically in bandwidth, inverse temperature, and accuracy; moreover, the accuracy of the computation can be established a priori.

I showcase the method on two challenging examples: (1) GW calculations on 
molecular systems, where we reduce the storage and computation of millions of
imaginary-time points to a sparse grid of 200, and (2) the Bethe--Salpeter equation, where we show how to achieve hitherto inaccessible accuracies for a set of benchmark systems.

Date: Thursday 6 May 2021
Speaker: Lucas Wagner
Title: Excited states and effective models from first principles quantum Monte Carlo calculations

The workhorse of materials simulation is generally density functional theory (DFT), which avoids the complicated many-body wave function at the cost of an unknown functional. While DFT can often be sufficient, current functionals often fail when high accuracy is needed or strong electron correlations must be described. Quantum Monte Carlo (QMC) techniques use statistical methods to treat the many-electron wave function. These methods are quite old, dating back to at least the 1960's, but have had sustained development over the years and obtain high accuracy at a relatively low computational cost.[1] Until recently, QMC methods have been mainly limited to either ground state or thermal distributions, and have not been applied very much to excited states.

Recently, several new algorithms have been proposed that allow access to excited states within QMC, and may enable scalable treatment of correlated electronic states. I will review one that we recently developed in my group,[2] which allows for stable optimization of very flexible wave functions for excited states. I will present applications of this method to some model systems and demonstrate the attainment of high accuracy. However, this algorithm, like most, proceeds in a state-by-state manner, limiting its application to extended systems.

To fill the gap from model systems to realistic materials, we have developed a way to develop downfolded effective models by analyzing a finite sample of excited states using modern data science techniques.[3]  This method is similar in spirit to Wannier interpolation of band structures from a sparse sampling of k-space; however, it treats one- and two-body terms on the same footing, and is based on correlated solutions of the Schroedinger equation. I will compare effective models derived this way to those derived using RPA techniques.

  1. Simons Collaboration on the Many-Electron Problem et al. Direct Comparison of Many-Body Methods for Realistic Electronic Hamiltonians. Phys. Rev. X 10, 011041 (2020).
  2. Pathak, Busemeyer, Rodrigues, Wagner. Excited states in variational Monte Carlo using a penalty method. J. Chem. Phys. 154, 034101 (2021).
  3. Zheng, Changlani, Williams, Busemeyer, Wagner, From Real Materials to Model Hamiltonians With Density Matrix Downfolding. Front. Phys. 6, (2018).

Date: Thursday 29 April 2021
Speaker: Jonas Fransson
Shaky moment — molecular vibrations causing magnetism

In condensed matter, vibrations are often regarded as a nuisance and merely a source for decoherence. While new results, both theoretical and experimental, emerge showing that this may not be a complete, or even good, picture to construct a viable theory for ordered structures, e.g., magnetic order, it is challenging to overcome the concept of thermal destruction of order due to vibrational excitations. Here, I will discuss new theoretical results that point towards the possibility of stabilizing magnetic configurations (order?) when electrons, or spins, are coupled to lattice vibrations (phonons). These results can be connected to recent experimental results which can be explained in terms of those theoretical ideas.

Reference material:

  • J. Fransson, D. Thonig, P. V. Bessarb, S. Bhattacharjee, J. Hellsvik, and L. Nordström Microscopic Theory for Coupled Atomistic Magnetization and Lattice Dynamics, Phys. Rev. Mater. 1, 074404 (2017).
  • A. K. Mondal, N. Brown, S. Mishra, P. Makam, D. Wing, S. Gilead, Y. Wiesenfeld, G. Leitus, L. J. W. Shimon, R. Carmieli, D. Ehre, G. Kamieniarz, J. Fransson, O. Hod, L. Kronik, E. Gazit, and R. Naaman, Long-Range Spin-Selective Transport in Chiral Metal-Organic Crystals with Temperature-Activated Magnetization, ACS Nano, 14, 16624 (2020).
  • J. Fransson, Vibrational origin of exchange splitting and chiral-induced spin selectivity, Phys. Rev. B, 102, 235416 (2020).
  • J. Fransson, Charge Redistribution and Spin Polarization Driven by Correlation Induced Exchange in Chiral Molecules, Nano Lett. (2021); acs.nanolett.1c00183.
  • Y. S.Sang, Mishra, F. Tassinari, K. S. Kumara, R. Carmieli, R. D. Teo, A. Migliore,D. N. Beratan, H. B. Gray,  I. Pecht, J. Fransson, D. H. Waldeck, R. Naaman, Temperature dependence of charge and spin transfer in azurin, submitted for publication – unpublished, 2021.


Date: Thursday 22 April 2021
Speaker: Eugene Demler
A pointillist approach to quantum matter

What can we learn about a many-body system when we measure every constituent particle? Current experiments with ultracold atoms provide snapshots of many-body states with single particle resolution. I will discuss new insights into strongly correlated states that came out of analyzing snapshots of the Fermi Hubbard model. For a broad range of fermion densities, experiments indicate an existence of magnetic polarons that can be understood as spinon-chargon pairs bound by geometric strings, in close analogy to quark-antiquark bound pairs forming mesons in QCD. I will review evidence for the crossover between the polaronic metal and the Fermi liquid state. The application of machine learning techniques to the analysis of snapshots of many-body states will also be discussed.

Date: Thursday 8 April 2021
Speaker: Alessandro Toschi 
Title: Nonperturbative fingerprints of the many-electron physics and their surprising implications

In this talk I start by illustrating how to read fundamental physical features of electronic correlations from the two-particle description of the Anderson impurity model. In particular, I will discuss the precise way in which the formation of a local magnetic moment and its Kondo screening are encoded in the generalized charge susceptibility. The sharpness of this identification even pinpoints a novel criterion to determine the Kondo temperature of strongly correlated systems in the charge sector [1].

The identified structures on the two-particle level directly reflect the appearance of divergences of the irreducible vertex functions. These singularities were long believed to be a mere mathematical formality, associated to the breakdown of many-body perturbation theory and to the multivaluedness of the Luttinger-Ward functional. Instead, novel DMFT calculations of the Hubbard model demonstrate that they also have a precise physical meaning [2]: They are responsible for flipping the sign (from repulsive to attractive!) of the effective electronic interaction in specific scattering channels. As a result, entering the non-perturbative regime triggers an enhancement of the uniform charge response. This mechanism is ultimately responsible for the phase-separation instabilities emerging close to the Mott metal-insulator transitions as soon as the condition of perfect particle-hole symmetry is released [3].

[1]: P. Chalupa, T. Schäfer, M. Reitner, S. Andergassen, and A. Toschi, arXiv: 2003.07829, PRL 126 056403 (2021).
[2]: M. Reitner, P. Chalupa, L. Del Re, D. Springer, S. Ciuchi, G. Sangiovanni, and A. Toschi, PRL 125 196403 (2020).
[3]: D. Springer, P. Chalupa, S. Ciuchi, G. Sangiovanni, and A. Toschi, PRB 101, 155148 (2020).


Date: Thursday 1 April 2021
Speaker: Chin Shen Ong
Excitons in Quasi-2D Semiconductors

Using the GW and GW-BSE theoretical approaches, linear and nonlinear optics, we investigated selected phenomena for excitons in quasi-2D semiconductor. First, we will discuss screening and antiscreening effects when a quasi-2D semiconductor is supported by insulator and metal respectively [1]. Next, in collaboration with experimentalists, we demonstrated the splitting of the 2p exciton states due to Berry phase effect and found that these excitons have very large transition dipole moments [2]. Finally, we will report the observation of bright excitons with negative-mass electrons, its implication and their strong coupling to phonons [3].

[1] M. I. B. Utama, H. Kleemann, W. Zhao, C.-S. Ong, F. H. da Jornada, D. Y. Qiu, …, A. Zettl, S. G. Louie, F. Wang. Nature Electronics 2, 60–65 (2019). (Also featured on Nat. Electron News & Views)
[2] C.-K. Yong, M. I. B. Utama, C.-S. Ong, T. Cao, …, A. Zettl, S. G. Louie, F. Wang. Nature Materials, 1 (2019)
[3] K.-Q. Lin, C.-S. Ong, ..., J. Fabian, A. Chernikov, D. Y. Qiu, S. G. Louie, J. M. Lupton (2020) arXiv:2006.14705


Date: Thursday 25 March 2021
Speaker: Nikolaj Bittner
Title: Photoinduced coherent dynamics in correlated systems: string states and Higgs modes

The recent advances in nonequilibrium pump-probe time-domain spectroscopy opened new perspectives in studying different properties of strongly correlated materials, which were previously inaccessible using equilibrium probes. For instance, time-resolved optical conductivity has been used to disentangle the pairing glue in several families of cuprates and to demonstrate that the short-time dynamics provide useful microscopical information about the electron-spin coupling in these materials.

Using various numerical methods, we present a theoretical study of the non-equilibrium dynamics in strongly correlated materials. Firstly, employing the non-equilibrium dynamical mean-field theory to doped Mott insulators we study the time-resolved relaxation process in these systems and focus on the interplay between the charge and spin dynamics. Particular emphasis is on the appearance of string states in nonequilibrium probes, which are a direct consequence of strong spin-charge coupling in these systems. [1] The string states we analyze in a view of a nontrivial scaling of the relaxation time with the exchange coupling and in a coherent reshuffling of kinetic and spin interaction energy in the relaxation dynamics. [2] Secondly, within the framework of the density matrix formalism, we study Higgs oscillations in single-band superconductors, which allow to detect directly properties of the superconducting condensate as a function of time [3]. In particular, the results of the time-dependent optical conductivity calculations are discussed.

1] K. Gillmeister, D. Golež, C.-T. Chiang, N. Bittner, Y. Pavlyukh, J. Berakdar, P.Werner, W. Widdra, Nat. Commun. 11, 4095 (2020) [2] N. Bittner, D. Golež, H. U. R. Strand, M. Eckstein, and P. Werner, PR B 97, 235125 (2018) [3] L. Schwarz, B. Fauseweh, N. Tsuji, N. Cheng, N. Bittner, H. Krull, M. Berciu, G.S. Uhrig, A.P. Schnyder, S. Kaiser, and D. Manske, Nat. Commun. 11, 287 (2020)


Date: Thursday 11 March 2021
Speaker: Mark van Schilfgaarde
Title: High-fidelity Green’s functions in correlated materials

To manage electron correlations, Green’s function methods have proven to be powerful tools in the theorist’s toolbox. Ab initio Green’s function methods have traditionally divided into two tracks: low-order many-body perturbation theory (MBPT), applicable to systems with weak or moderate correlations, and nonperturbative Dynamical Mean Field Theory (DMFT) when the independent particle picture is no longer adequate. Traditionally, both are added as a correction to density functional theory, taken as a standard reference one-body hamiltonian H0 . However, uncontrolled approximations in H0 propagate to the higher level theory, which obscures and also limits the range of validity, in either track. By using MBPT itself to construct an optimal H0 , we show that the approximations become much better controlled. The fidelity of the theory significantly improves, and discrepancies with experiments become more systematic, making it possible to both clarify the limits of either theory. The optimization scheme, called Quasiparticle Self-Consistent GW (QSGW ) approximation, can be systematically improved by adding higher order diagrams (QSGW ++). On the MBPT side, I show some illustrations for various kinds of weakly and moderately systems. For strongly correlated systems, I show recent progress in joining QSGW with DMFT (an alternative approach to QSGW ++) to characterize one- and two- particle spectral functions with much higher fidelity than either separately. With an added step to make the particle-particle vertex, the instabilities to unconventional superconductivity can be obtained. We illustrate this by comparing bulk and monolayer forms of FeSe.


Date: Thursday 25 February 2021
Speaker: Dmitri Basov
Live from New York: Programmable Quantum Materials

Experimentally realizing quantum phases of matter and controlling their properties is a central goal of the physical sciences. Novel quantum phases with controllable properties are essential for new electronic, photonic, and energy management technologies[i]. Quantum materials offer particularly appealing opportunities for the implementation of on-demand quantum phases.  This class of materials host interacting many-body electronic systems featuring an intricate interplay of topology, reduced dimensionality, and strong correlations that leads to the emergence of “quantum matter’’ exhibiting macroscopically observable quantum effects over a vast range of length and energy scales. Central to the nano-optical exploration of quantum materials is the notion of polaritons: hybrid light-matter modes that are omnipresent in polarizable media[ii]. Infrared nano-optics allows one to directly image polaritonic waves yielding rich insights into the electronic phenomena of the host material supporting polaritons. We utilized this novel general approach to investigate the physics of on-demand hyperbolic exciton-polaritons in a prototypical atomically layered van der Waals semiconductor WSe2 in which polaritons are prompted by femto-second photo-excitation[iii].

Dmitri N. Basov (PhD 1991) is a Higgins professor and Chair of the Department of Physics at Columbia University [http://infrared.cni.columbia.edu], the Director of the DOE Energy Frontiers Research Center on Programmable Quantum Materials and co-director of Max Planck Society – New York Center for Nonequilibrium Quantum Phenomena. He has served as a professor (1997-2016) and Chair (2010-2015) of Physics, University of California San Diego. Research interests include: physics of quantum materials, superconductivity, two-dimensional materials, infrared nano-optics. Prizes and recognitions: Sloan Fellowship (1999), Genzel Prize (2014), Humboldt research award (2009), Frank Isakson Prize, American Physical Society (2012), Moore Investigator (2014, 2020), K.J. Button Prize (2019), Vannevar Bush Faculty Fellowship (U.S. Department of Defense, 2019), National Academy of Sciences (2020).

[i] D.N. Basov, R.D. Averitt and D. Hsieh, “Towards properties on demand in quantum materials” Nature Materials 16, 1077 (2017).
[ii] D. N. Basov, Ana Asenjo-Garcia, P. J. Schuck, X. Zhu & Angel Rubio, “Polariton panorama” Nanophotonics 10, 549 (2021) https://infrared.cni.columbia.edu/research/polariton-panorama-2-2/
[iii] A. J. Sternbach, S. Chae, S. Latini, A. A. Rikhter, Y. Shao, B. Li, D. Rhodes, B. Kim, P. J. Schuck, X. Xu, X.-Y. Zhu, R. D. Averitt, J. Hone, M. M. Fogler, A. Rubio, and D. N. Basov, “Programmable hyperbolic polaritons in van der Waals semiconductors” Science 371, 617 (2021).

D.N. Basov, Columbia University, https://infrared.cni.columbia.edu

Date: Thursday 18 February 2021
Speaker: Samir Lounis
Title: A new view on the origin of zero-bias anomalies of Co atoms atop noble metal surfaces

Many-body phenomena are paramount in physics. In condensed matter, their hallmark is considerable on a wide range of material characteristics spanning electronic, magnetic, thermodynamic and transport properties. They potentially imprint non-trivial signatures in spectroscopic measurements, such as those assigned to Kondo, excitonic and polaronic features, whose emergence depends on the involved degrees of freedom. With the help of scanning tunneling microscopy (STM), zero-bias anomalies assigned to be Kondo features were identified early on in Co adatoms on Au(111) surface [1]. This gave birth to a very active and exciting research field devoted to Kondo physics on surfaces that led to tremendous theoretical and experimental developments and discoveries.

In this talk, I will present our recent work [2] based on time-dependent density functional and many-body perturbation theories accounting for spin-orbit coupling. After a systematic first-principles investigation of Co adatoms on Cu, Ag and Au substrates, we find zero-bias anomalies quasi-identical to those measured by STM. The obtained features originate, however, from gaped spin-excitations induced by a finite magnetic anisotropy energy (MAE) in contrast to the usual widespread interpretation. Furthermore, we unveil a new many-body feature, the spinaron, resulting from the interaction of electrons and spin-excitations. I will show ways of reducing the MAE, for example by attaching Co atoms to Cu-wires, which enable easier magnetic-field measurements of the transport anomalies. Finally, I will address unconventional spin-excitations [3] before giving an overview of potential future experimental and theoretical investigations.

[1] Madhavan, Chen, Jamneala, Crommie, Wingreen, Science 280, 567 (1998)
[2] Bouaziz, Guimarães, Lounis, Nature Communications 11, 6112 (2020)
[3] Küster, Montero, Guimarães, Brinker, Lounis, Parkin, Sessi, Nature Communications – in press (2021)

Date: Thursday 11 February 2021
Speaker: Johannes Flick
First-principle approaches to strong light-matter coupling in molecular and extended systems

In recent years, research at the interface of material science, chemistry, and quantum optics has surged and now offers new possibilities to study light-matter interactions. The combination of theoretical concepts from these fields presents an opportunity to create a predictive theoretical and computational approach from first principles that describes the correlated dynamics of electrons, nuclei, and the electromagnetic field on the same quantized footing.

In this talk, we discuss how density-functional theory can be generalized to quantum-electrodynamical density-functional theory

(QEDFT) and show how new exchange-correlation potentials arise. We discuss the linear-response theory for QEDFT to access excited state properties of such systems, the emerging ab initio lifetimes and incorporation of losses. By considering electrons, nuclei and photons on the same quantized footing, we find polaritonically induced vibrational mode mixing, cavity-modulated molecular motion of molecules in optical cavities, as well as new light-matter correlated observables for a new type of spectroscopy.

Further, we use this novel framework to study how chemical reactivity is altered in this regime, by studying the modification of potential-energy surfaces under strong-light matter coupling. Beyond molecular systems, we will discuss how strong light-matter coupling can be used to make nonlinear phonon processes more efficient and discuss first principle methods to characterize novel single-photon emitters.

Our work opens an important new avenue in introducing ab initio methods to the nascent field of strong light-matter interactions and demonstrates the novel abilities to alter material properties and chemical reaction pathways.


Date: Thursday 4 February 2021
Speaker: Fedor Šimkovic
Title: New Diagrammatic Routes for Strongly Correlated Systems

In this talk I will give an introduction to the Diagrammatic Monte Carlo (DiagMC) approach for strongly correlated systems and showcase recent results obtained for the two- and three-dimensional fermionic Hubbard model in the paramagnetic as well as symmetry broken regimes. I will discuss the range of applicability of DiagMC, and in particular the Connected Determinant Diagrammatic Monte Carlo algorithm, as compared to other numerical techniques and show how one can use perturbation theory to obtain results in non-perturbative regimes. Finally, I will state the current limitations of DiagMC in terms of the number of accessible expansion coefficients and the convergence properties of resulting perturbative series and will present some of the most recent attempts at overcoming these limitations.


Date: Thursday 10 December 2020
Speaker: Liviu Chioncel 
Towards an ab-initio theory of Anderson localization with correlated electrons

Great progress has been made in recent years towards understanding the properties of disordered electronic systems. This is made possible by recent advances in quantum effective medium methods which include Dynamical Mean-Field Theory and the Coherent Potential Approximation, and their cluster extension, the Dynamical Cluster Approximation. The recently developed typical medium dynamical cluster approximation captures disorder-induced localization and provides an order parameter for the Anderson localized states. We present an overview of various recent applications of the typical medium single-site and dynamical cluster approximation to the Hubbard model, and its combination to realistic systems in the framework of Density Functional Theory.

  1. Ab initio typical medium theory of substitutional disorder
    A. Östlin, Y. Zhang, H. Terletska, F. Beiuşeanu, V. Popescu, K. Byczuk, L. Vitos, M. Jarrell, D. Vollhardt, and L. Chioncel Phys. Rev. B 101, 014210 (2020)
  2. Locally self-consistent embedding approach for disordered electronic systems, Y. Zhang, H. Terletska, Ka-Ming Tam, Y. Wang, M. Eisenbach, L. Chioncel, M. Jarrell, Phys. Rev. B 100, 054205 (2019)
    3. Systematic Quantum Cluster Typical Medium Method for the Study of Localization in Strongly Disordered Electronic Systems, H. Terletska, Y. Zhang, Ka-Ming Tam, T. Berlijn, L. Chioncel, N. S. Vidhyadhiraja, M. Jarrell, Review: Appl. Sciences 8 (12), 2401 (2018)
    4. Typical-medium multiple-scattering theory for disordered systems with Anderson localization, H. Terletska, Y. Zhang, L. Chioncel, D. Vollhardt, M. Jarrell, Phys. Rev. B 95, 134204 (2017)


Date: Thursday 3 December 2020
Speaker: Alexander Steinhoff
Title: Many-body correlations and excitonic effects in two-dimensional
materials: equation-of-motion approach

Monolayers of transition metal dichalcogenide (TMD) semiconductors
exhibit strong long-range Coulomb interaction of their charge carriers
giving rise to bound electron-hole states, known as excitons, with
remarkable oscillator strength in optical spectra. Along with the
potential of engineering material properties by assembling van der Waals
heterostructures from different two-dimensional materials, this
recommends TMD semiconductors as active materials in optoelectronic
devices such as light-emitting diodes, solar cells, and lasers. At the
same time, these strong interaction effects render TMD semiconductors a
fascinating playground to study many-body correlation effects from an
experimental and theoretical perspective.

In my talk, I will first discuss the semiconductor Bloch equations
(SBE), which represent an equation-of-motion approach to optical spectra
in the time domain. In the linear regime, the SBE are equivalent to a
Bethe-Salpeter equation. The equations allow to include many-body
effects due to photoexcited electron-hole pairs such as band-structure
renormalizations, phase-space filling and screening. Moreover,
nonequilibrium carrier kinetics after strong pulsed photoexcitation can
be described by systematically extending the SBE, including exciton
formation and exciton-exciton interaction.

In the second part, the SBE are applied to address the question how a
finite density of excited carriers modifies the optical properties of
TMD monolayers. Absorption spectra for excited carrier densities up to
10^13 cm^-2 reveal a redshift and mild bleaching of the excitonic ground
state absorption, whereas higher excitonic lines are found to disappear
successively due to Coulomb-induced band-gap shrinkage of more than 500
meV and binding-energy reduction. For investigations of the carrier
kinetics in MoS2, I will present results for the carrier-carrier Coulomb
scattering after pulsed optical excitation of the monolayer.


Date: Thursday 26 November 2020
Speaker: Sangeeta Sharma
Title: Ultrafast spin dynamics: ab-initio description

I will talk about all-optical switching of long-range magnetic order. The type of coupling between the constituent atoms of a magnetic solid, usually ferromagnetic (FM) or anti-ferromagnetic (AFM), is a fundamental property of any magnetic material. This coupling is governed by the exchange interaction, for which the time scale of a typical magnetic material is of the order of a few 100s of femtoseconds. In our work, using time-dependent density functional theory (TDDFT), we demonstrate that a rich control over magnetization at  sub-exchange time scales (of the order of few tens of femtoseconds) is possible[1,2,3,4,5,6,7]. This even includes changing the magnetic order from AFM to FM[8]. By investigating a wide range of multi-sublattice magnetic materials we are able to formulate three simple rules that predict the qualitative dynamics of magnetization for ferromagnetic, anti-ferromagnetic, and ferri-magnetic materials on sub-exchange time scales.

[1] Dewhurst et al. Nano Lett. 18, 1842, (2018)
[2] Elliott et al. Scientific Reports 6, 38911 (2016)
[3] Shokeen et al. Phys. Rev. Lett. 119, 107203 (2017)
[4] Chen et al. Phys. Rev. Lett. 122, 067202 (2019)
[5] Willems et al. Nat. Comm. 11, 1 (2020)
[6] Dewhurst et al. Phys. Rev. Lett. 124, 077203 (2020)
[7] Hofherr et al. Sci. Advs. 6, eaay8717 (2020)
[8] Siegrist et al. Nature 571, 240 (2019)


Date: Thursday 19 November 2020
Speaker: Oscar Grånäs
Title: Time-dependent density-functional theory as a first-principles framework for matter out of equilibrium

Electro-magnetic fields strong enough to rival atomic interactions can disturb the balance between kinematic effects due to electrons hopping between lattice sites and the Coulomb repulsion between electrons that limits the band formation. This opens the possibility to create and control physical properties in systems driven out of equilibrium. The computational complexity of the fully interacting many-body problem, in particular in out of equilibrium situations, demands that relevant physical processes can be described in therms of effective electronic parameters. A key is hence to find these effective parameters for realistic materials settings, and to understand how these can be modulated by external electromagnetic fields.

In this talk, I will review the basic concepts of time-dependent density functional theory in the real-time domain, and how it can be used to give insights to materials out of equilibrium. The main approximations in the current state of the art is outlined, as well as a route to address (some) shortcomings in the description of correlated materials.

Additionally, I will discuss a joint computational and experimental study of how ultrashort optical fields of ~0.2 V/Å leads to a transient inter-site charge transfer in the prototypical correlated electron insulator NiO. We analyse the results in terms of the polarizability of electronic states, which leads to an effective modulation of inter-site hopping.


Date: Thursday 12 November 2020
Speaker: Evgeny Stepanov
Title: Consistent partial bosonization of collective fluctuations in correlated electronic systems

A simplified description of various many-body effects in correlated electronic systems can be done in the framework of mean-field theory performing a partial bosonization of collective fluctuations in leading (charge, spin, and etc.) channels of instability. A simultaneous account for different bosonic channels gives rise to a famous Fierz ambiguity in decomposition of the local Coulomb interaction into considered channels, which drastically affects the final result of the calculation. This issue remains yet unsolved for all mean-field-based theories, although many of them, such as GW and GW+EDMFT, are intensively used for solution of realistic problems in and out-of equilibrium. Recently, we have found a receipt for a consistent partial bosonization of the fermionic model that finally solves the Fierz ambiguity problem. We have applied our method to extended Hubbard model and derived an effective theory that is formulated in terms of fermionic degrees of freedom, new bosonic fields, and the renormalized fermion-boson interaction. We have shown that the fermion-fermion interaction can be safely excluded from the model action, which results in a very simple dual triply irreducible local expansion (D-TRILEX) approach [PRB 100, 205115 (2019)] that significantly improves all existing partially bosonized theories. In addition, our method allows to include spin fluctuations in a GW scheme in a consistent way, which for a long time was a very important issue for realistic calculations of magnetic materials. In my talk I will present a brief derivation of the theory and discuss first promising results of the application of the D-TRILEX method to realistic description of spatial magnetic fluctuations in a highly-anisotropic three-orbital model for transition metal oxide compounds [arXiv:2010.03433 (2020)].


Date: Thursday 29 October 2020
Speaker: Attila Szilva
Title: Symmetry analysed results in non-collinear exchange formalism

Abstract: The presentation will be an overview of the interatomic exchange formalism for general, non-collinear, classical spin systems. The formalism that is derived from the two-site perturbation will be in the main focus together with non-trivial symmetry-resolved results for 3d metals, particularly for bcc Fe. The connection to the one-site derivation (presented already by Lars Nordström) will be also discussed, too, with presenting an explicit non-collinear Weiss-field formula based on the one-site variation. Orbital resolved symmetry analysis for a special non-collinear spin system in which one spin is rotated with a finite angle will be also presented with a detailed discussion of the bulk, surface, relativistic and non-relativistic cases. A few slides will be on the case when the spin-orbit coupling is present as a perturbation. This could be the way to recover the collinear-relativistic (Udvardi) limit.


Date: Thursday 22 October 2020
Speaker: Saikat Banerjee
Title: Phonon induced emergent charge order and its hallmarks in underdoped cuprates

Charge-density wave order is now understood to be a widespread feature of underdoped cuprate high-temperature superconductors, although its origins remain unclear. While experiments suggest that the charge-ordering wavevector is determined by Fermi-surface nesting, the relevant sections of the Fermi surface are featureless and provide no clue as to the underlying mechanism. Here, focusing on underdoped YBa2Cu3O6+x, we propose that charge-density waves form from the incipient softening of a bond-buckling phonon (B1g phonon). The momentum dependence of its coupling to itinerant electrons favourably selects the wavevector found in experiments. But, it requires quasiparticle renormalization by strong electronic correlations to enable a unique enhancement of the charge susceptibility near the B1g-phonon selected wavevector. The B1g phonon frequency softens by a few percent, and finite-range charge-density wave correlations will form locally, if nucleated by defects or dopant disorder. These results suggest that underdoped cuprates cannot be understood in the context of strong electronic correlations alone.

  1. Banerjee et al. Communications Physics 3, 161 (2020)
  2. Banerjee et al, arXiv:200801401 (provisionally accepted in Phys. Rev. B)


Date: Thursday 15 October 2020
Speaker: Misha Galperin
Title: Green’s function methods for single molecule junctions

We discuss theoretical Green's function methods applicable to open quantum systems out of equilibrium, in general, and single molecule junctions, in particular.

Two characteristic energy scales governing the physics are many-body interactions within the junctions and molecule–contact coupling. We, therefore, identify weak interactions and weak coupling as two limits that can be conveniently treated within, respectively, the standard nonequilibrium Green’s function (NEGF) method and its many-body flavors: pseudoparticle and Hubbard NEGF. In particular, we show that the Hubbard NEGF is convenient in studies of nanoscale optoelectronics and current induced molecular dynamics in junctions.

Finally, the intermediate regime, where the two energy scales are comparable, can in many cases be efficiently treated within the nonequilibrium dual approaches. We discuss recently developed auxiliary quantum master equation - dual fermion (aux-DF) and dual-boson (aux-DB) approaches. We combine ideas of exact mapping of non-Markov dynamics onto Lindblad type evolution in an auxiliary system with dual superperturbation expansions. This combination capitalizes on strong sides of both techniques which leads to formulation of relatively numerically inexpensive universal impurity solvers of high accuracy. Viability of the aux-DF and aux-DB approaches is illustrated within generic junction models, where the schemes are benchmarked against numerically exact results.

[1] F. Chen , M. I. Katsnelson and M. Galperin, Phys. Rev. B 101, 235439 (2020)
[2] G. Cohen and M. Galperin, Chem. Phys. 152, 090901 (2020)
[3] F. Chen, G. Cohen and M. Galperin, Phys. Rev. Lett. 122, 186803 (2019)
[4] F. Chen, E. Arrigoni and M. Galperin, New J. Phys. 21, 123035 (2019)
[5] M. Kuniyuki, I. Hiroshi, I.-I. Miyabi, K. Kimura, Galperin and Y. Kim, Nano Lett. 19, 2803 (2019)
[6] K. Miwa, A. M. Najarian, R. L. McCreery and Galperin, J. Phys. Chem. Lett. 10, 1550 (2019)
[7] F. Chen, K. Miwa and M. Galperin, Phys. Chem. A 123, 693 (2019)
[8] F. Chen, M. A. Ochoa and M. Galperin, J. Chem. Phys. 146, 092301 (2017)


Date: Thursday 8 October 2020
Speaker: Giuseppe Carleo
Title:  Neural-network quantum states for fermions: ab-initio quantum chemistry and two-dimensional lattice models

In this talk I will review recent efforts in extending neural-network based ansatz wave functions [Carleo and Troyer, Science 355, 602 (2017)] to fermionic matter.
I will start by discussing applications to small scale ab-initio electronic structure [Choo et al., Nature Comm. 11, 2368 (2020)].
I will then focus on the ongoing applications to a two-dimensional lattice model of spinless fermions [with Javier Robledo Moreno and James Stokes].
Challenges and possible extensions of these approaches will be addressed.


Date: Thursday 1 October 2020
Speaker: Jernej Mravlje 
Charge and spin-transport at high temperatures

At high temperatures the resistivity of correlated metals exceeds the Mott-Ioffe-Regel value where the scattering length is equal to the lattice spacing and grows further as the temperature is increased. The phenomenology of associated transport is described well in terms of the Nernst-Einstein relation in terms of diminishing charge susceptibility and saturated diffusion constant. I discuss this behavior based on the numerical results for the Hubbard model within dynamical mean-field theory, the high-temperature expansion and finite temperature Lanczsos methods. I will compare the results to the cold-atom experiments. In cold-atom experiments also spin-transport is measured. I will discuss how the phenomenology of charge-transport carries over to that of the spin-transport, discuss the behavior of diffusion constant, and of the corresponding Mott-Ioffe-Regel value.


Date: Thursday 17 September 2020
Speaker: Giacomo Mazza 
 Symmetry aspects of the excitonic insulators transition

The excitonic insulator has been proposed several years ago as a state of matter stemming from condensation of excitons. Despite several examples have been proposed, the notion of an excitonic insulator state remains elusive from both the theoretical and experimental points of view. In this talk I will address the question of what an excitonic insulator is focussing on the symmetry aspects of this phase transition.I will first consider the case of the material Ta2NiSe5 that has been proposed to host this kind of transition and I will discuss the breaking of the lattice symmetry issuing from a spontaneous hybridization between valence and conduction bands.Then I will extend these ideas to a generalized system and discuss the implication of the symmetry constraints on the properties of the symmetry broken phase. I will discuss how this kind of transition should be considered as a general mechanism for the breaking the symmetry of the solid. The properties of the resulting low temperature phase are set by the low symmetry phase which is eventually established due to coupling with lattice degrees of freedom.

Date: Thursday 10 September 2020
Speaker: LarsNordström 
Title:  Exchange coupling in non-collinear magnets; the appearance of non-relativistic Dzyaloshinskii-Moriya like interaction

The Liechtenstein-Katsnelson-Antropov-Gubanov (LKAG) approach are well established to calculate exchange parameters for collinearly ordered magnetic structures, which works both for metals and insulators. There have been attempts to generalise the method to general non-collinear ordering, but there is yet no-one established. We will here present a generalisation that has several advantages, but also a few surprising side effects. This approach has many things in common with the first order derived form of the LKAG formula, which probably is lesser known than the second order derived form. In fact it is identical in the non-relativistic and non-collinear limit. However, away from this limit there are some novel observations;
(i) the bi-linear exchange parameters depend strongly on the reference state for which they are calculated,
(ii) there are Heisenberg, Dzyaloshinskii-Moriya and anisotropic symmetric interactions also for non-relativistic non-collinear reference systems,
(iii) the interactions that determine the energetics are not identical to the ones that describe the excitations (spin waves etc).

These results are illustrated with calculated results for the metallic, non-collinear, co-planar antiferromagnet Mn3Sn, which has favourable transport properties and potential applications in spintronics. This material also possesses an enigmatic weak ferromagnetic moment. We will take some time to discuss how it in a natural way can be understood from our novel approach.

Finally we observe that the reference dependence is hard to avoid in a LKAG-approach but is nevertheless cumbersome when mapping to spin models. We will therefore, if time allows, discuss how such bi-linear interactions can, under certain conditions, be transformed into multi-linear interactions, i.e. involving an even number of moments. These interactions can still be calculated in first order in deviations from a reference state. This then results in reference independent multi-spin models.


Date: Thursday 16 July 2020
Speaker: Denis Golež
Title: Energy conversion in photo-excited charge-transfer  insulators  

Charge excitations across electronic band gaps are a key ingredient for transport in optoelectronics and light-harvesting applications. In contrast to conventional semiconductors, studies of above-band-gap photoexcitations in strongly correlated materials are still in their infancy. I will start with a comparison of the photo-doped state in the Mott and charge-transfer insulator. The later is described within the three-band Emery model as relevant for copper oxides. We will employ a non-equilibrium extension of dynamical mean-field theory taking into account changes in the screening environment (GW+EDMFT) [1]. In contrast to Mott insulators, a strong renormalization of the charge-transfer gap and a substantial broadening of bands is present in charge-transfer insulators [2,3]. The inclusion of dynamical screening leads to an ultra-fast conversion of excess kinetic energy into plasmonic excitations. The comparison with different experimental pump-probe techniques, like time-resolved ARPES and optical conductivity, shows qualitative agreement and exemplifies that the dynamical screening and correlations are essential for a proper description of the photo-doped state. In the second part, I will extend the theoretical description to nickel oxides and compare the dynamics after the photo-doping with the time-resolved photo-emission spectroscopy [4]. The short time dynamics reveals the importance of Hund physics, photo-induced in-gap states, and antiferromagnetic physics. The conversion of energy between Hund and magnetic degrees of freedom leads to long-lived coherent THz oscillations whose frequency corresponds to the superexchange coupling.

[1] DG, L. Boehnke, H. U. R. Strand, M. Eckstein, P. Werner, Phys. Rev. Lett. 118, 246402 (2018) [2] DG, L. Boehnke, M. Eckstein, P. Werner, Phys. Rev. B 100, 041111 (2019) [3] DG, M. Eckstein, P. Werner, Phys. Rev. B 100, 235117 (2019) [4] K. Gillmeister, DG, C. Chiang, N. Bittner, P. Werner, Y. Pavlyukh, J. Berakdar, and W. Widdra, arXiv:1909.00828 (2019) [accepted to Nat. Comm.].


Date: Thursday 2 July 2020
Speaker: Sergei V. Kalinin
Title: Machine Learning Beyond Correlative Models: Bayesianity, Parsimony, Causality, and Automated Experiment

Machine learning has emerged as a powerful tool for the analysis of mesoscopic and atomically resolved images and spectroscopy in electron and scanning probe microscopy. The applications ranging from feature extraction to information compression and elucidation of relevant order parameters to inversion of imaging data to reconstruct structural models have been demonstrated. However, the fundamental limitation of the vast majority of machine learning methods is their correlative nature, leading to extreme susceptibility to confounding factors and observational biases. In this presentation, I will discuss several examples of extending machine learning methods towards the analysis of causative physical mechanisms. One such approach is based on the Bayesian methods that allow to take into consideration the prior knowledge the system and evaluate the changes in understanding of the behaviors given new experimental data. The second pathway explores the parsimony of physical laws and aims to extract these from the set of real-world observations. Finally, the Bayesian networks can be used to explore the causative relationships in the multimodal data sets. These concepts will be illustrated using several examples of causal machine learning, including analysis of phase transitions on a single atom level in 2D materials and interplay between physical and chemical effects in the ferroelectric perovskites. Ultimately, we seek to answer the questions such as whether electronic instability due to the average Fermi level guides the development of the local atomic structure, or frozen atomic disorder drives the emergence of the local structural distortions, whether the nucleation spot of phase transition can be predicted based on observations before the transition, and what is the driving forces controlling the emergence of unique functionalities of morphotropic materials and ferroelectric relaxors. The unique aspect of Bayesian methods is their potential to quantify uncertainty, and harnessing this for automated experimentation is discussed on example of ferroelectric domain patterning and atomic fabrication via electron beams.

This research is supported by the by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division and the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, BES DOE.


Date: Thursday 25 June 2020
Speaker: Vladimir Nazaikinskii
Partial Spectral Flow and the Aharonov–Bohm Effect in Graphene

The following simple model where the Aharonov--Bohm effect manifests itself is studied. Consider a graphene tube in the shape of a right circular open-ended cylinder whose height and radius are both much greater than the distance between neighboring carbon atoms.

A magnetic field everywhere vanishing on the tube surface (or at least tangent to it) is adiabatically switched on, causing the eigenvalues of the tight-binding Hamiltonian describing the electron  states in graphene in the nearest neighbor approximation to move in the process. If the magnetic flux through the tube in the final configuration has an integer number of flux quanta, then the electron energy spectrum eventually returns to the original position. Moreover, the spectral flow of the Hamiltonian, defined as the net number of eigenvalues (counted with multiplicities) which pass through  in the positive direction, proves to be zero. A more detailed analysis reveals that the eigenfunctions corresponding to these eigenvalues can be chosen to be localized near the Dirac points  and  in the momentum space, and if one separately counts the spectral flow for the eigenfunctions localized near  and near , then one obtains two “partial” spectral flows, which have opposite signs and the same modulus (equal to the number of magnetic flux quanta through the tube). The physical interpretation is that switching on the magnetic field creates electron--hole pairs (or, more precisely, pairs of “electron” and “hole” energy levels) in graphene, the number of pairs being determined by the magnetic flux. If an electron level is created near , then the corresponding hole level is created near , and vice versa. Further, the number of electron/hole levels created near  equals the spectral flow of the family of Dirac operators approximating the tight-binding Hamiltonian near  (and the same is true with  replaced by ).

We assign a precise meaning to the notion of partial spectral flow in such a way that all the preceding assertions make rigorous sense (and are true).

Date: Thursday 18 June 2020
Speaker: Matija Culo
Planckian dissipation in overdoped cuprates: key for hightemperature superconductivity?

The family of mixed metallic copper oxides - cuprates - is one of the most studied materials in condensed matter physics primarily due to its record high superconducting (SC) transition temperatures (at ambient pressure) [1]. However, despite intense research over more than 30 years, high-temperature SC in cuprates still remains one of the biggest mysteries in the field. All cuprates have a layered perovskite structure with conducting CuO2 layers sandwiched between other metal oxide layers giving rise to quasi-2D electronic properties. In their undoped state, cuprates are Mott insulators with an antiferromagnetic ground state caused by strong on-site Hubbard repulsion. Doping either by electrons or holes progressively destroys the Mott insulating state (more effectively on the hole-doped side) leading to a metallic state that at low temperatures transits to the SC state. As in many other superconductors, the SC transition temperature Tc traces out a dome (higher and wider on the hole-doped side) with a maximum Tc at optimal doping. Interestingly, the metallic state out of which SC emerges is no less mysterious than the SC state itself. On the underdoped side (Tc < Tcmax) the metallic state is characterized by a pseudogap (momentum-dependent depletion of density of states at the Fermi level) with Fermi arcs, instead of a large metallic Fermi surface (FS), which is either a cause or a consequence of many additional intriguing phenomena such as charge ordering, nematicity and spin glass. On the overdoped side (Tc > Tcmax) the full FS is recovered but there exists a strange metal phase above the SC transition where marked deviations from the standard metallic Fermi liquid (FL) behavior have been found in many physical properties. It is only on the far overdoped side, when superconductivity is lost, that the metallic state shows the behavior expected from the standard FL theory. In this presentation we focus on hole-doped cuprates, particularly on the overdoped side of the phase diagram within the strange metallic regime. The basic characteristic of a strange metal is the linear temperature dependence of its resistivity (instead of the standard FL quadratic temperature dependence) that is believed to be related to the maximum dissipation allowed by quantum mechanics – Planckian dissipation. Planckian dissipation is often ascribed to the existence of a quantum critical point (QCP) located inside the SC dome in vicinity of which quasiparticles lose their coherence due to a strong coupling to
quantum critical fluctuations of the associated order parameter. In hole-doped cuprates, such a QCP has been postulated to exist at a doping p* that coincides with the vanishing of the normal state pseudogap. Here we present our recent high-field magnetoresistance measurements [2] on two overdoped cuprates, Tl2Ba2CuO6+δ and Bi2Sr2CuO6+δ, which at all dopings p > p* where SC appears exhibit quadrature scaling in field and temperature, scaling that provides, through simple dimensional analysis, a direct link to Planckian dissipation. These observations, combined with analysis of the Hall resistivity, establish that the cuprate strange metal harbors two charge sectors, one with coherent quasiparticles and the other with incoherent Planckian dissipators. Given the presence of these two sectors, it is pertinent to pose the question: which sector is responsible for (high-temperature) superconductivity? According to BCS theory, only the coherent sector is relevant. Our recent considerations [3], however, show, with a minimal set of assumptions, that with decreasing doping, the growth of the superfluid density at zero Kelvin in Tl2Ba2CuO6+δ and La2-xSrxCuO4 matches quantitatively the decrease in the density of coherent carriers deduced from the Hall effect. This correspondence leads us to postulate that superconductivity within the strange metal phase of overdoped cuprates is not, as expected, an instability of the FL, but rather an instability of the incoherent sector.
[1] A. Damascelli, Z. Hussain, Z.-X. Shen, Rev. Mod. Phys. 75, 473 (2003) [2] J. Ayres, M. Berben, M. Čulo, Y.-T. Hsu, E. van Heumen, Y. Huang, J. Zaanen, T. Kondo, T. Takeuchi, J. R. Cooper, C. Putzke, S. Friedemann, A. Carrington, N. E. Hussey, submitted (2020) [3] M. Čulo, C. Duffy, J. Ayres, M. Berben, Y.-T. Hsu, R. D. H. Hinlopen, B.Bernáth, N. E. Hussey, submitted (2020)

Date: Thursday 11 June 2020
Speaker: Oleksandr (Sasha) Zheliuk
Title: Josephson coupled Ising pairing in superconducting TMD bilayers

On the basis of the monolayer superconductivity configured by spin-orbit coupling, more exotic pairing schemes can be prepared by coupling two identical layers, for which two types of systems have been proposed theoretically. One type requires the coupling between two superconducting layers with Rashba-type SOC. Whereas the other type is based on Zeeman-type SOC involving two Ising pairings with opposite spin configurations coupled through Josephson interaction. Bilayers of transition metal dichalcogenides can serve as a host platform for the latter scenario [1]. The coupled state, having a finite centre-of-mass momentum q, is predicted as a Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) state [2, 3]. The realization of such a coupled system is not only of theoretical interest. Technically, the ability to control the Ising state at a specific location can build superconducting junctions formed by adjacent regions having a similar Tc0 but drastically different Bc2.

[1] O. Zheliuk, J. Lu, Q. Chen, A. El Yumin, S. Golightly, and J. Ye, Nat. Nanotech. 14, 1123 (2019). [2] Liu, C.-X. Unconventional superconductivity in bilayer transition metal dichalcogenides. Phys. Rev. Lett. 118, 087001 (2017). [3] Nakamura, Y. & Yanase, Y. Odd-parity superconductivity in bilayer transition metal dichalcogenides. Phys. Rev. B. 96, 054501 (2017).


Date: Thursday 4 June 2020
Speaker: Yunhua Wang (Wuhan university, China)
Title: Piezoelectricity in 2D hexagonal crystals, RSOC-induced topological phases in Xenes, and bilayer Xenes straintronics

Recent experimental works show that some 2D hexagonal crystals have considerable in-plane piezoelectricity while the piezoelectric effect is absent in their 3D bulks. An another interesting question is whether some of these 2D materials possess the out-of-plane piezoelectricity. In the former part of this presentation, I will show the built tight-binding piezoelectric theory for transition metal dichalcogenide monolayers, the obtained large out-of-plane piezoelectricity of oxygen functionalized MXenes and their piezoelectric device simulations. Recently, it is also shown that, the intrinsic 2D topological insulating states exist in post-carbon 2D elemental materials, monolayer and bilayer Xenes (silicene, germanene and stanene), owing to the sizeable spin-orbit coupling (SOC). The vertical buckling geometry in these materials offers engineering platforms for electronic and optical properties by external electric field, substrate interaction and mechanical strain. In the latter part, I will first present the external Rashba SOC-induced topological phases and their optical signatures in Xenes, and then show the bilayer Xenes’ electronic response to the uniaxial strain and bending.


Date: Thursday 28 May 2020
Jans Henke (UVA - University of Amsterdam)
Title and abstract
: Charge order in real materials

The prototype model for charge order is the Peierls instability in a one-dimensional, monatomic, monovalent, and uniformly spaced chain of atoms. The charge order in that highly artificial setting is unavoidable and easy to understand, leading it to be commonly used as a model for charge order in real materials, despite the fact that it has been repeatedly shown to be severely oversimplified.

In this talk we show that by applying some techniques borrowed from quantum field theory, it is possible to quantitatively model charge order in real materials. This approach brings to the fore the all-important role of the structure of electron-phonon coupling in determining many of the properties of the charge ordered state. We show that it can be used to understand a range of results in the literature surrounding charge order, and in particular in the transition-metal dichalcogenide VSe2.


Date: Thursday 14 May 2020
Yuriy Mokrousov (Head of Topological Nanoelectronics Group, Institute for Advanced Simulation, Forschungszentrum Juelich)
Title: Skyrmions, Non-commutative Geometry and Hall Effect

Magnetic skyrmions are fascinating particle-like objects, whose key properties are governed by their non-trivial real-space topology. Microscopically, this topology manifests in the presence of the so-called emergent gauge field, which directly couples to electronic degrees of freedom thus giving rise to such fundamental effects as for example the topological Hall effect. In strongly spin-orbit coupled systems our perception of skyrmions as gauge-field generating particles has to be conceptually altered, however, and we show that this can be naturally done by referring to the paradigm of non-commutative geometry [1]. We show that in terms of this powerful language, also utilized in the realm of quantum Hall effect, nuclear physics and string theory, skyrmions re-emerge as entangled objects living in a complex non-commutative phase space. Inspired by our previous work [2], we will demonstrate the emergence of a Hall effect in chiral magnetic textures which is neither proportional to the net magnetization nor to the topological emergent magnetic field. We show that this “chiral” Hall effect receives a natural interpretation in the language of non-commutative geometry, thus conceptually relating magnetic skyrmions to quantum Hall systems [3]. Moreover, we argue that the chiral Hall effect could provide a distinct magneto-transport signature of non-commutative geometry of complex spin textures which is distinctly different from that driven by the topological Hall effect [4].

We acknowledge funding by the Deutsche Forschungsgemeinschaft (DFG) through Priority Programme SPP 2137 “Skyrmionics” and by “Topology and Dynamics” initiative of the University of Mainz. We also gratefully acknowledge the Jülich Supercomputing Centre and RWTH Aachen University for providing computational resources.

[1] A. Connes, Non-commutative geometry, San Diego (1994)
[2] F. Lux, F. Freimuth, S. Blügel, and Y. Mokrousov, Comm. Phys. 1, 60 (2018) 
[3] J. Bellissard, A. van Elts, H. Schulz-Baldes, J. Math. Phys. 35, 5373 (1994)
[4] F. Lux, F. Freimuth, S. Blügel, and Y. Mokrousov, arXiv:1910.06147 (2019)

Date: Thursday 30 April 2020
Speaker: Achille Mauri (Radboud university)
Title: Anomalous scaling laws in fluctuating two-dimensional crystals: an epsilon-expansion method

​Free-standing two-dimensional crystals under a vanishingly small external tension exhibit large thermal fluctuations, which tend to deform the lattice and fold it in three-dimensional space. In contrast with a simple expectation based on the Mermin-Wagner theorem, two-dimensional crystalline membranes exhibit a thermodynamically stable flat phase, characterized by long-range order in the local normal to the surface and a macroscopically planar average configuration. It has long been recognized that fluctuations in the flat phase exhibit anomalous scaling laws, characterized by noninteger critical exponents.
​​After reviewing theories of scaling behavior in crystalline membranes, this seminar will present a field- theoretic epsilon-expansion approach which is based on a nonstandard dimensional continuation of the model to internal dimensions different from two.

Date: Thursday 23 April 2020
Speaker: Robert Sokolewicz (Radboud university)
Title: Uncovering anisotropic Gilbert damping in rashba honeycomb antiferromagnets

I study an antiferromagnetic honeycomb lattice with conducting electrons, Rashba spin-orbit interaction and a local exchange interaction between conducting and local spins. This model has many interesting properties that can be used to model, understand and predict physical properties relevant for spintronics. From this model we derive spin-orbit torques and Gilbert dampings in our recent publication [1].  The spin-orbit torques can be used to manipulate magnetic domains with electric currents, while Gilbert dampings describe the relaxation and dissipation of angular moments and are important for the manipulation of domains as well. We find that one of the dampings transitions from purely isotropic to highly anisotropic when spin-orbit is switched on. This anisotropy leads to the existence of a magnon mode that is essentially undamped.

In this brief seminar I will introduce the main result from our paper and some new findings I am working on right now.

[1] Giant anisotropy of Gilbert damping in a Rashba honeycomb antiferromagnet
Baglai, R. J. Sokolewicz, A. Pervishko, M. I. Katsnelson, O. Eriksson, D. Yudin, and M. Titov
Phys. Rev. B 101, 104403 (March 2020)


Date: Wednesday 15 April 2020 
Speaker: Swagata Acharya (King’s College London) 
Title: Prediction and control of superconducting Tc in unconventional superconductors from first principles

To realize superconductors at a practicable temperature has been one major focus of research and innovation for nearly last four decades. It has been a challenge for scientific acumen to explore the conditions under which Cooper pairs can form at high temperatures.  Theoretically, lack of ability to build and solve material specific Hamiltonians has hindered a systematic understanding of the desired conditions for Cooper pairing. A possible resolution to the problem involves understanding both the candidates reliably: the nature of the quasi-particles and the nature and strength of the glue. We have built an ab-initio theoretical scheme[1] for excited states that provides quantitative insights into quasi-particle excitations and spin, charge and orbital fluctuation glue at high temperatures and how such normal phases become unstable towards  superconductivity on lowering temperature. Large scale application of the technique in range of cuprates[2,3], iron based superconductors[4,5,6] and ruthenates[7] led to systematic understanding of origin of Cooper pairing and control parameters for Tc therein. In the process we understand the approximations of the present theory that should be bettered to better estimate Tc’s.

1. Questaal: a package of electronic structure methods based on the linear muffin-tin orbital technique by D. Pashov et al.,  Computer Physics Communications, Volume 249, 107065 (2020).
2. Metal-Insulator Transition in Copper Oxides Induced by Apex Displacements by S. Acharya et al., Phys. Rev. X 8, 021038 (2018).
3. Electron-Phonon-Driven Three-Dimensional Metallicity in an Insulating Cuprate by E. Baldini et al., PNAS, vol. 117, no. 12, 6409-6416 (2020).
4. Hund's coupling mediated colossal increment in Tc in multi-band FeSe manifold, S. Acharya et al.,  arXiv 1908.08136 (2019).
5. Controlling Tc through band structure and correlation engineering in collapsed and uncollapsed phases of iron arsenides, S. Acharya et al.  (under review).
6. Interplay between band structure and Hund's correlation to increase Tc in FeSe, S. Acharya et al. (under review)
7. Evening out the spin and charge parity to increase Tc in unconventional superconductors by S. Acharya et al.,  Communications Physics volume 2, Article number: 163 (2019)


Date: Tuesday 18 February 2020
Speaker: Danis Badrtdinov (Ural Federal university, Ekaterinburg, Russia)
Title: Control of magnetic interactions between surface adatoms via orbital repopulation

Within this talk, I will present a reversible mechanism for switching isotropic exchange interactions between transition metal adatoms from ferromagnetic to antiferromagnetic by the example of cobalt dimer deposited on the black phosphorous monolayer. This mechanism is based on electrically-controlled orbital repopulation of cobalt adatoms, as recently demonstrated by scanning probe techniques [Nat. Commun. 9, 3904 (2018)]. Using first-principles calculations, it will be shown that field-induced repopulation not only affects the spin state, but also causes considerable modification of exchange interaction between adatoms, including its sign. The additional model analysis demonstrates that variable adatom-substrate hybridization is a key factor responsible for this modification. Simulating inelastic tunneling characteristics, it will be discussed the possible ways to verify the proposed mechanism experimentally.

2019 Previous seminars

Date: Thursday 19 December 2019
Speaker: Luc Bouten (Q1t BV )
Title: Simulating light-matter interactions on a quantum computer

We study light-matter interactions in a Markovian limit. In this limit the unitary interaction is given by a quantum stochastic differential equation. For these equations there exists discretization theory where the electromagnetic field is represented by a spin chain. Using this theory, we discretize several problems of interest and show how this leads to quantum algorithms for simulating these systems on a quantum computer. We will show the simulation results obtained on the quantum simulator q1tsim and also some results from simulations on the publicly available IBM-Q quantum computers.

Date: Thursday 12 December 2019
Erik van Loon (universiteit Bremen, former TCM colleague)
The constrained Random Phase Approximation: Why and When it works

The constrained Random Phase Approximation (cRPA) is an important part of the modern theory of correlated electron materials. In the downfolding from the full band structure to an effective low-energy (Hubbard) model, the cRPA provides a way to calculate the interaction parameters for the low-energy model, taking into account screening by the other electronic states. Although it is a popular tool, a formal justification for the cRPA has been lacking. Here, in analogy to Migdal’s
theorem on the absence of vertex corrections for electron-phonon coupling, we show when the cRPA is a good approximation.


Date: Thursday 28 November 2019
Speaker: Tom Westerhout (Raboud university)
Title: Neural Quantum States of frustrated magnets: generalization and sign structure

Neural quantum states (NQS) attract a lot of attention due to their potential to serve as a very expressive variational ansatz for quantum many-body systems. We study the main factors governing the applicability of NQS to frustrated magnets by training neural networks to approximate ground states of several moderately-sized Hamiltonians using the corresponding wavefunction structure on a small subset of the Hilbert space basis as training dataset. We notice that generalization quality, i.e. the ability to learn from a limited number of samples and correctly approximate the target state on the rest of the space, drops abruptly when frustration is increased. We also show that learning the sign structure is considerably more difficult than learning amplitudes. Finally, we conclude that the main issue to be addressed at this stage, in order to use the method of NQS for simulating realistic models, is that of generalization rather than expressibility.

Date: Thursday 21 November 2019
Speaker: Jose Lado (Department of Applied Physics, Aalto University, Espoo, Finland) 
Title: Engineering and detecting unconventional superconductivity with disguised Dirac points

Materials hosting Dirac points at the Fermi energy have attracted a lot of attention due to their anomalous transport properties. However, in many instances, potential Dirac points do not appear at the Fermi energy in the electronic spectra, either due to additional gap openings or due

to their finite energy shift. Here, we show that materials hosting Dirac points remote from the chemical potential provide a powerful playground to both create [1] and detect [2] unconventional superconducting states. On the one hand, we show [1] that antiferromagnetic insulators that host

Dirac crossings in their parent paramagnetic state provide a robust platform to engineer topological superconductivity, giving rise to propagating Majorana modes. This topological superconducting state arises at the interface between the antiferromagnetic and a conventional s-wave superconductor, stems from the emergence of interfacial solitonic excitations between the two compounds, and provides a venue to engineer topological superconductivity in antiferromagnetic oxides. On the other hand, [2] we show that gap openings in "buried" Dirac crossings are a signature of non-unitary multi-orbital superconducting order. This allows us to identify specific features invisible in the low energy phase, such as superconducting domains of different symmetry breaking or non-unitary order in the orbital flavor, by analyzing high-energy spectral properties of a superconductor. As a result, we show that angle-resolved photo-emission spectroscopy measurements can be used to detect non-unitary multi-orbital superconductivity. Our results [1,2] highlight the versatility of hidden Dirac crossings in the electronic structure, turning possibly overlooked Dirac materials into an attractive platform to engineer and probe unconventional superconducting states.

[1] J. L. Lado and M. Sigrist, Phys. Rev. Lett. 121, 037002 (2018)
[2] J. L. Lado and M. Sigrist, arXiv:1908.04820 (2019)

Date: Thursday 7 November 2019
Speaker: Marcos Guimarães (Zernike Institute for Advanced Materials, University of Groningen)
Title: Spin-Orbit Torques Using 2D Materials

The manipulation of magnetization using materials with large spin-orbit coupling is very promising for applications in magnetic memory devices. In these devices, a charge current flowing through the high spin-orbit coupling material generates a spin current which is used to apply torques on the magnetization of a ferromagnet. In addition to promising applications, the study of these spin-orbit torques can unveil many of the material’s spintronic properties. The large family of layered two-dimensional materials has shown to be an excellent candidate for the generation of spin-orbit torques. They provide large atomically-flat single crystals with various properties and very pristine interfaces which leads to an efficient transfer of spins from the layered material to the ferromagnet.

In this talk I will show how the crystal structure and electronic properties of 2D materials can dictate the magnitude, direction, and symmetries of spin-orbit torques. In particular, I will discuss results for devices based on WTe2 or NbSe2 showing torques which are forbidden by symmetry in conventional heavy metal/ferromagnet devices. I will also discuss recent results on the layered insulating antiferromagnet NiPS3, where we observe large interfacial torques, with torque efficiencies comparable to best devices based on heavy metals.


Date: Thursday 17 October 2019
Speaker: Robert Sokolewicz (TCM, RU)
Title: Spin-orbit torque in a Rashba honeycomb anti-ferromagnet

This will be a practice talk for next week's sIMMposium. I studied spin-orbit torques that can appear in a conducting anti-ferromagnet in the presence of an electric current. The torques can be obtained by considering a tight-binding hamiltonian (that describes conducting electrons) that includes a local exchange interaction with an antiferromagnet and Rashba spin-orbit interaction. These two interactions allow for the exchange of angular momentum between the conducting electrons and the (localized) magnetic and anti-ferromagnetic order of the antiferromagnet. I will explain how the system is solved numerically to obtain the spin-orbit torques. When sublattice symmetry is preserved I demonstrate that both so-called Neel spin-orbit torque and anti-damping torques are completely absent, whereas when sublattice symmetry is broken these torques are finite.


Date: Thursday 10 Octobter 2019
Speaker: Vladimir Mazurenko (Ural Federal University, Russia)
Neural Network for material science and quantum computing

Amazing progress in development of machine learning techniques changing our everyday life can also facilitate the solution of challenging problems in material science and related fields in physics. 
In my talk, I will discuss neural network approaches we developed for recognition and classification of the complex non-collinear magnetic structures  formed in two- and three-dimensional materials at finite temperatures and magnetic fields [1,2]. 
In contrast to standard methods of machine learning such approaches allow one to analyse transitional areas between different phases.  A special focus will be on recurrent neural network classifier of ultrafast magnetization processes in systems with Dzyaloshinskii-Moriya interaction. 
The last part of my talk will be devoted to a new neural network eigensolver for quantum computers [3].

  1. I.A. Iakovlev, O.M. Sotnikov, V.V. Mazurenko, PRB 98, 174411 (2018)
  2. I.A. Iakovlev, O.M. Sotnikov, V.V. Mazurenko, PRB 99, 024430 (2019)
  3. O.M. Sotnikov, V.V. Mazurenko, arXiv:1904.02467

Date: Monday 19 August
Zhihao Jiang (PhD student from the University of Southern California in Los Angeles)
Coulomb Engineered Plasmonics in 1D and 2D systems

Monday 5 August
Speaker: Eric DeGiuli (École Normale Supérieure)
: What is the landscape of natural language? Insights from a random language model

Many complex systems have a generative, or linguistic, aspect: life is written in the language of DNA; protein structure is written in a language of amino acids, and human endeavour is often written in text.

Are there universal aspects of the relationship between sequence and structure? I am trying to answer this question using models of random languages. Recently I proposed a model of random context-free languages [1] and showed using simulations that the model has a transition from an unintelligent phase to an ordered phase. In the former, produced sequences look like noise, while in the latter they have a nontrivial Shannon entropy; thus the transition corresponds to the emergence of information-processing in the language.

In this talk I will explain the basics of natural language syntax, without assuming any prior knowledge of linguistics. I will present the results from the model above, and explain how the model is related to complex matrix models with disorder [2].

[1] https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.122.128301
[2] https://arxiv.org/abs/1902.07516

Date: Thursday 4 July
Speaker: Clement Dutreix (ENS de Lyon, France) 
: Band-structure topology from wavefront dislocations in Friedel oscillations

The knowledge of electronic band structures is central to describe the optical and electrical properties of solids. If the energy-band spectrum is routinely measured through various techniques, it is more challenging to access the topological properties of wave functions, such as quantised Berry phases. Such topological Berry phases are traditionally measured through the electron dynamics in response to external electromagnetic forces [1]. For instance, the topological Berry phases in semimetals are often probed through magneto-oscillations in transport measurements [2].During this seminar, we shall investigate a complementary approach and study such topological properties without external electromagnetic forces, through a static interference phenomenon in the local density of states. Indeed, we have recently imaged wavefront dislocations in the Friedel oscillations induced by hydrogen adatoms on graphene via scanning tunnelling microscopy experiments [3]. These dislocations are nothing but a quantum mechanical realisation of the wave topological defects introduced by Nye and Berry [4], nowadays at the heart of singular optics [5]. We will see that the dislocation charge we observe is actually a real-space measurement of the topological Berry phase that characterises the massless relativistic electrons in graphene. Next, we shall generalise the discussion of such topological defects to the cases of 1D and 2D semiconductors. This should highlight the electronic density as a powerful observable to investigate the band-structure topology of relativistic and gapped phases in solids.[1] D. Xiao, et al., Rev. Mod. Phys.82, 1959 (2010) [2] K. S. Novoselov, et al., Nature 438, 197 (2005); Y. Zhang, et al., Nature 438, 201 (2005); S. Pezzini, et al., Nature Physics 14, 178 (2017)[3] C. Dutreix, H. Gonzalez-Herrero, I. Brihuega, M. I.

Katsnelson, C. Chapelier, and V. T. Renard, unpublished[4] J. F. Nye & M. V. Berry, Proc. R. Soc. Lond. A336, 165 (1974) [5] M. R. Dennis, et al., Chapter 5 Singular Optics: Optical Vortices and Polarization Singularities. vol. 53 of Progress in Optics, 293 (Elsevier, 2009)


Date: Thursday 6 June
Speaker: Anton Akhmerov (TU Delft)
: On graphene, mirrors, and smoke

The mirror surface reflects incoming light specularly despite not being atomically flat. This happens the waves are not sensitive to the disorder that exists on scales shorter than the wave length, by effectively averaging it out. Much to our surprise, when analyzing the reflection of electrons from a graphene boundary we have discovered that it breaks the law of reflection and remains diffusive even when the wave length becomes arbitrarily large. In the talk I will explain under which conditions this happens, what is the reason for this breakdown of the law of reflection, and propose an experimental setup allowing to observe it.


Date: Thursday 16 May
Speaker: Cyrus Dreyer (Stony Brook and CCQ, NY, USA)

The flexoelectric (FxE) effect, where polarization is induced by a strain gradient, is universal in all insulators.

As devices shrink to the micro and nano scale, large strain gradients can occur, and therefore the FxE effect can play a significant role in their electrical and mechanical properties.

Also, the FxE effect can be exploited for novel device design paradigms such as piezoelectric ``meta-materials'' constructed from nonpiezoelectric constituents, or mechanical switching of ferroelectric polarization. One of the crucial limitations to understanding and exploiting the FxE effect has been the lack of an efficient first-principles methodology to calculate all of the components of the bulk FxE tensor; the clamped-ion transverse and shear components in particular are problematic. We have developed such a methodology based on density functional perturbation theory to calculate the full bulk, clamped-ion FxE tensor with unprecedented accuracy and efficiency.

In this talk I will review the microscopic aspects of the FxE effect, describe our computational methodology, and provide results for some simple systems including cubic perovskite oxides.

Date: Wednesday 8 May
Speaker: Pieter Gunnink (Twente University)
Title: Engineering a topological insulator in complex oxide heterostructures

The existence of topological insulators has been well known and studied for the past ten years, but so far only a limited selection of topological insulators have been found. With several growth methods now available which can grow materials with atomic-scale precision, it might be possible to engineer a topological insulator. Prime candidate are the complex oxides, which can easily be grown using pulsed laser deposition and exhibit a wide range of physical phenomena. In this talk I will discuss the possibility of engineering a topological insulator using Rashba type spin orbit coupled heterostructures. As a prototype system the quasi-2DEG present at the LaAlO3/SrTiO3 is considered.


Date: Thursday 2 May
Speaker: Slava Rychkov (IHES/ENS)
Title: Elementary introduction to the Conformal Bootstrap

Conformal field theories have been long known to describe the fascinating universal physics of scale invariant critical points: they describe continuous phase transitions in fluids, magnets, and numerous other materials. For decades it has been a dream to study these intricate strongly coupled theories nonperturbatively using symmetries and other consistency conditions. This idea, called the conformal bootstrap, saw some successes in two (or 1+1) dimensions but it is only in the last ten years that it has been fully realized in three (or 2+1) dimensions. This renaissance has been possible both due to significant analytical progress in understanding how to set up the bootstrap equations and the development of numerical techniques for finding or constraining their solutions. These developments have led to a number of groundbreaking results, including world record determinations of critical exponents and correlation function coefficients in the Ising and O(N) models in three dimensions. We will review these exciting developments for non-experts.

Thursday 25 April
Speaker: Konstantin Tikhonov (Karlsruhe Institute of Technology)
Title: Statistics of eigenstates near the localization transition on a random regular graph

We study spatially and frequency resolved correlations of wavefunctions (WF) and energy level statistics in Anderson model on the Random Regular Graph (RRG) at criticality and in the delocalized phase. We find a very good agreement between analytical results and numerical approaches, including exact diagonalization and numerical solution of the self-consistency equation for the probability distribution of the local Green functions. We observe correlation length directly in the spatial decay of WF correlations and in the spectral compressibility. Finally, we connect our results to properties of the Anderson model defined on finite dimensional (d) lattices at d >> 1, stressing interesting peculiarities of this limit.


Date: Thursday 14 March
Speaker: Achille Mauri (Radboud University)
Title: Fluctuations of membranes with dipole-dipole interactions

The mechanical and thermodynamic properties of membranes subject to small or vanishing external tension are determined in an essential way by thermal fluctuations of their shape. The statistics of fluctuations is controlled by anharmonic interactions, which lead to strong coupling phenomena and anomalous scaling of correlation functions.

In this seminar, fluctuations in membranes with a permanent out-of-plane polarization are discussed. We showed that the leading effect of electrostatic forces consists in renormalizations of the elastic moduli and the bending rigidity of the membrane. This suggests that the conventional scaling behaviour holds even in presence of long-range dipole-dipole interactions. These results are useful to understand the mechanical properties of two-dimensional materials with spontaneous polarization.

Date: Thursday 7 March
Speaker: Malte Rösner (Radboud University)
Title: Recent & Ongoing Projects on Correlated Nanostructures

Within the last years I was involved in various projects under the general topic of "correlated nanostructures" ranging from purely theoretical model and/or ab initio studies to larger collaborations with colleagues from the experiment. I will give a short summary of these efforts followed by some insights into ongoing projects which range from excitonic ground states and unconventional superconductivity in TNS and FeSe, to Coulomb engineering and spin-phonon coupling in WS2 and CrBr3. Instead of going into all detail, I will just briefly summarize these projects focusing on the main outstanding questions as well as on the methods used to answer them. With this, I hope to give a comprehensive overview of my work to layout the route for future collaborations.


Date: Thursday 14 February 2019
Speaker: Nikolay Gnezdilov (Leiden University)
SYK induced superconductivity

We show that a non-interacting quantum dot acquires odd-frequency Gor’kov anomalous averages in proximity to strongly-correlated Majorana zero-modes, described by the Sachdev-Ye-Kitaev (SYK) model. Despite the presence of finite anomalous pairing, superconducting gap vanishes for the intermediate coupling strength between the quantum dot and Majoranas. The increase of the coupling leads to smooth suppression of the original quasiparticles.

This effect might be used as a characterization tool for recently proposed tabletop realizations of the SYK model.


Date: Thursday 7 February 2019
Speaker: Shengjun Yuan (Wuhan University)
A New Approach for the Modeling of Complex Quantum Systems

The first step of the common approach in the study a quantum system is to obtain its eigenfunctions, from which the observable variables or physical properties can be derived directly. This is, in general, involving the diagonalization of the Hamiltonian matrix, and therefore has difficulties or even becomes unsolvable for complex quantum systems. Numerically, this is because the costs of the memory and CPU time in the diagonalization processes are not linearly dependent on the dimension of the Hamiltonian matrix. In this talk, I will show a new approach for the modeling of complex quantum systems without any diagonalization. As an example, I will focus on the problems in the theory of condensed matter and introduce the so-called tight-binding propagation method (TBPM). TBPM is developed for the modeling of quantum structures from mesoscopic to macroscopic level, without the requirement of the translational symmetry. I will give a general introduction of the method and show its applications in the study of two-dimensional materials, heterostructures, fractals and superstructures. I will also show how to combine TBPM with other well-known numerical methods such as the density functional theory and molecular dynamics, and discuss briefly the connection to many-body physics, such as the simulation of quantum spin systems and universal quantum computers.


Date: Thursday 20 December 2018
Speaker: Askarov Iliasov (Radboud University)
The level-spacing distribution of some fractals

Earlier it was suggested that the spectrum of some fractals with finite ramification number is limit set of dynamical systems. I will show that the energy-level distribution has power-law behavior for fractals, spectrum of which is the limit set of piece-wise linear functions.

Therefore one can assume that such a behavior is a general feature of fractals, and they are not described properly by random matrix theory. I will give several other arguments for the power-law behavior of energy level-spacing distribution.


Date: Tuesday 11 December 2018
Speaker: Evert van Nieuwenburg (Caltech)
How confused is my neural network?

There is information to be gained by studying how well a neural network is able to learn physics data. I will discuss the 'learning by confusion' method and its extension to 'discriminative cooperative networks'. Both of these methods rely on the ability to order physics data along one or multiple tuning parameters, which is a feat that is often impossible in non-physics data-sets. In the second part, I will describe how reinforcement learning methods may help in quantum error decoding/correction.


Date: Thursday 6 December 2018
Speaker: Andrei Lugovskoi (Radboud University)
Title: Superconductivity and electron-phonon properties of doped antimonene

Antimonene is a recently discovered two-dimensional semiconductor with exceptional environmental stability, high carrier mobility, and strong spin-orbit interactions. In combination with electric field, the latter provides an additional degree of control over the materials' properties because of induced spin splitting.

On the seminar I will report the results of computational study of electron-phonon coupling and superconductivity in n- and p-doped antimonene, paying a special attention to the role of electronic structure on both the superconductivity and overall stability of the system. Besides that, the effect of the perpendicular electric field would be discussed. Application of bias voltage leads to a considerable modification of the electronic structure, affecting the electron-phonon coupling in antimonene. While these effects are less obvious in case of electron-doping, field-effect in hole-doped antimonene results in a considerable variation of the critical temperature depending on bias voltage

Date: Thursday 29 November 2018
Speaker: Robert Sokolewicz (Radboud University)
Title: Diffusive spin-orbit torque at a surface of topological insulator

In this week's seminar, I will discuss the paper we recently submitted to PRL [1] (still under review). We consider an insulating ferromagnet/topological insulator bilayer and study the effect of conducting electrons on the magnetization of the ferromagnet. The spin polarization of the conducting electrons induces a torque on the magnetization in the ferromagnet. This system was already studied in Refs. [2-3], however, the non-local properties of the out-of-plane spin-polarization of conducting electrons was overlooked in these publications. We show that there exists also a new torque (that we call "diffusive spin-orbit torque"), which is shown to emerge in the presence of a spatially inhomogeneous low-frequency ac electric field. The required electric field configuration can be created, by a grated top gate.

[1]  R. J. Sokolewicz, I. A. Ado, M. I. Katsnelson, P. M. Ostrovsky, and M. Titov, arXiv:1810.05828 [2]  A.  Sakai  and  H.  Kohno,  Physical  Review  B, 89 (2014) [3]  P. B. Ndiaye, C. A. Akosa, M. H. Fischer, A. Vaezi, E.-A. Kim,  and A. Manchon, Physical Review B 96 (2017)


Date: Thursday 22 November
Speaker: David Soriano (Radboud University)
Title: 2D Ferromagnetic Materials: The case of CrI3

The recent discovery of 2D ferromagnetic materials offers new possibilities for the design of magnetic devices based on van der Waals interactions and proximity effects. In this seminar, I will discuss the origin of magnetic exchange anisotropy in this type of materials (more specifically, CrI3) and will also show recent experimental and theoretical results on CrI3 tunnel barriers. These results demonstrate the possibility to probe the magnetic order of few-layers CrI3 via electron tunneling and to explore the magnon spectrum via inelastic tunneling[1]. To conclude, I will introduce a new type of device consisting of CrI3-encapsulated bilayer Graphene. This device behaves as a van der Waals spin valve where a spin polarized current is carried by the graphene layers when the adjacent CrI3 layers are ferromagnetically aligned[2].

[1] D. Klein et al. Science 360, 1218 (2018) [2] C. Cardoso et al. Phys. Rev. Lett. 121, 067701 (2018)

Seminars 2016-2018