Abstract dr. Pavel Jelinek
Quantum spin rings represent an intriguing platform for studying unconventional magnetic order and exotic quantum phases, and they are also promising materials for emerging quantum technologies. Conventional spin systems consist of a set of weakly interacting spins, which are well represented by the Heisenberg spin model [1].
Here, we demonstrate that a strong interaction between radical centers in spin rings of different sizes leads to fluctuation of the total number of unpaired electrons and non-trivial antiferromagnetic order that extends beyond the Heisenberg picture. We will show that the electronic structure of these spin rings is governed by the concept of 4n/4n+2 Hückel (anti)aromaticity for even-membered rings, while for odd-membered rings, it possesses a highly degenerate frustrated magnetic ground state. The strongly coupled spin rings are experimentally realized through the on-surface synthesis of magnetic carbon-based macrocycles consisting of phenalyne units. The tight correlation between electronic structure and Hückel rules of aromaticity is revealed by scanning tunneling spectroscopy and multireference calculations. This work proposes a novel design principle of coupled spin systems with strongly entangled electronic structure, which can also be extended to spin chains.
References
1. Sh. Mishra et al., Nature 598, 287 (2021); J. Hieulle et al., Angewandte Chemie Int. Ed. 60, 25224 (2021).
2. M. Kumar et al. arXiv:2603.17854 (2026)
Abstract prof. Peter Liljeroth
Conventional materials hosting exotic quantum phases typically have complex atomic structures, inhomogeneities from defects, impurities, and dopants making it difficult to rationally engineer their electronic properties. This can be overcome using van der Waals (vdW) materials and their heterostructures. I will discuss how low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS) can be used to probe magnetic excitations in vdW quantum materials down to
the monolayer limit and at the atomic scale. I will illustrate these capabilities through our recent results on understanding multiferroicity in van der Waals materials in the monolayer limit. Our findings provide atomic-scale evidence on the mechanism of multiferroicity in NiI2 and related compounds and we are also able to visualize the collective excitations of the multiferroic order, electromagnons. These examples demonstrate how complex magnetic structures and excitations can
be probed at the atomic scale and how the versatility of vdW heterostructures enables designing emergent quantum states beyond those found in nature.
