Orbiting the Dirac nodal line: unconventional quantum oscillations in ZrSiS

High magnetic fields provide new insights on electronic interactions in topological materials

Researchers from the High Field Magnet Laboratory have observed anomalous oscillations in the electrical resistance of the semimetal ZrSiS when subjected to high magnetic fields, providing evidence of strong interaction between the charge carriers . Until now, the remarkable properties of topological materials have been realized without the presence of strong interactions or correlations. These observations are a milestone  as they represent a first example of topological phenomena dressed with strong electron correlations. In the future, such correlation effects may induce wholesale changes in the physical state of the material in question (insulating, superconducting, magnetic, etc…) and thus provide significant potential for future technological applications. . These findings have been reported in Nature Physics as an Article on 6th of November 2017 (advance online publication).

Semi-metals are solid state materials which have a small
overlap between the conduction band andPicture 1 the valence band. In case the conduction and valence bands touch at the Fermi energy, semi-metals are referred to as topological semi-metals, and are classified in Weyl, Dirac and nodal-line semi-metals depending in which form this touching occurs. The nodal-line semi-metal ZrSiS possesses a layered tetragonal structure and is one example where the bands touch each other along a three-dimensional loop, referred to as the nodal line.

“ZrSiS, compared to many other topological semi-metals, has an ideal band structure to investigate the physics of Dirac quasi-particles” says Steffen Wiedmann, senior scientist at HFML - “since all the energy bands crossing the Fermi level possess a linear energy-momentum relation in a large range up to 2 eV”.

The researchers have performed magneto-transport experiments on ZrSiS crystals and observed quantum oscillations in the resistance at low temperatures and up to 33 T at the HFML. Quantum oscillations in the magneto-resistance are a powerful tool to access the intrinsic properties of charge carriers in materials such as the mass of the quasi-particles and the degree of electronic correlations. When applying a high enough magnetic field to ZrSiS in a specific crystallographic orientation, the electrons, rather than following conventional orbits tied to single “pockets” of the Fermi surface (green and violet sections in the upper panel of the figure), perform complete revolutions along the Dirac line, the so-called breakdown orbits, which manifest themselves in fast oscillations in the resistance of the material (bottom panel of the figure). Subsequent measurements of the resistance as a function of the magnetic field at different values of temperature and field intervals, have demonstrated an unconventional evolution of the oscillations’ amplitude, revealing an enhancement in the mass of the quasi-particles which is commensurate with electron-electron interactions.

“Correlation effects have been predicted to arise in the presence of an extended Dirac loop” - says Sergio Pezzini, postdoctoral research associate at the HFML - “Our experiment demonstrates for the first time this scenario. Moreover, the breakdown orbits can selectively enclose the vertexes of the loop, providing a sort of “diagnostic tool” for the topological character of the electronic states”. “We are very excited about the upcoming studies we are already preparing for” - states Maarten van Delft, PhD student at HFML - “We aim at identifying new materials and routes to magnify and drive these effects”.

These results are reported in an Article published in the latest issue of the journal Nature Physics. The study is the result of the collaboration between scientists at HFML and IMM, together with researchers at the MPI of Solid State Research in Stuttgart and the University of Bristol. “Our collaboration presents a unique combination of expertise.” - says Nigel Hussey, director of the HFML. Hussey and Wiedmann emphasize that the high quality ZrSiS crystals grown in Stuttgart, calculations of the electronic structure carried out at Bristol University and their close collaboration with the Condensed Matter Theory group at the IMM are decisive for the interpretation of their data. “At HFML, we are able to perform experiments in some of the most extreme physical conditions available on our planet, without which these new phenomena could have never been observed”.

Related publication (advance online publication):

Unconventional mass enhancement around the Dirac nodal loop in ZrSiS, S. Pezzini, M.R. van Delft, L.M. Schoop, B.V. Lotsch, A. Carrington, M. I. Katsnelson, N.E. Hussey and S. Wiedmann, Nature Physics (2017)
DOI: http://dx.doi.org/10.1038/NPHYS4306

Contact: Nigel.Hussey@ru.nl