Electronic properties of topological materials and NMR

• Applicant: Professor Dr. Jürgen Haase
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2020 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 442459148

• Final Report Year: 2023

Final Report Abstract

The aim of this proposal concerned the atomic scale understanding of fundamental bulk electronic properties of model topological insulators, which, for example, are responsible for their special topological surface states, but also for other properties. Concerning the electronic band inversion that sets the stage for the topological properties of these materials, it could be shown that the doping dependence of the NMR quadrupole splitting from the electric hyperfine interaction provides atomic scale measure of this non-trivial band inversion as induced by spin-orbit interaction in the model topological insulators Bi2Se3 and Bi2Te3 . Thus, NMR is one of the few (if not the only method) that can indeed investigate the band inversion in greater detail. Here, the comparison of the NMR results with first principal calculations is essential and shown to be very successful. The very unusual magnetic field induced change of local charge symmetry at nuclei remains an outstanding scientific problem that is not yet understood by theory. Here we proved that this unusual effect is present also in related materials with rather different NMR couplings. Furthermore, a strong doping dependence of this effect was established, which clearly relates it to the influence of the itinerant carriers. Interestingly, no dependence of the effect on the magnetic field could be found, at least down to about 2 Tesla. This observation points to fundamental electronic properties, as it should require low energy excitations that affect the electronic entropy and with it a number of macroscopic material properties, of interest for many applications, certainly thermo-electric properties, or as magnetic topological materials. Whether it can be important for the life time of topological surface states is not clear. Analytical theory is urgently needed. This effect is also of interest from a more general NMR point of view. As NMR approaches similar materials, be it in chemistry or physics, one cannot rely anymore on the classical interpretation of quadrupole splittings and thus as local symmetry residing with the crystal lattice. Connected to the effect of charge symmetry rotation is nuclear spin lattice relaxation, as it should be affected by corresponding electronic excitations. One expects contributions to the electric and magnetic hyperfine interaction, i.e. NMR quadrupolar and/or magnetic relaxation. Here we conducted a number of spin lattice relaxation studies to learn about these electronic degrees of freedom. We found unusual magnetic as well as quadrupolar relaxation, but we could not separate both effects with sufficient confidence given the quadrupolar split spectra. It is proposed to follow up on those measurements with doubleresonance techniques that enable the differentiation guided by the current results. Finally, there is the strong internuclear coupling from a special electronic susceptibility that we discovered earlier, but that is not understood in detail. It appears to be present even in the undoped materials so it cannot be RKKY interaction, rather it must rely on the Bloembergen Rowland mechanism. By investigating the field dependence of the NMR lines of the different nuclei in various materials, the general presence of the electronic coupling was verified, but there remain a number of inconsistencies, e.g. for Teout in Se2Te3 . We note that there is great interest in topological insulators doped with magnetic ions, and here this interaction determines the exchange interactions that are still controversially discussed in the literature. Therefore, further NMR studies of the indirect nuclear coupling appear mandatory.

Publications

• Field-induced charge symmetry revealed by nuclear magnetic resonance in the topological insulator Bi2Se3. Physical Review Research, 3(1).
  Guehne, R.; Haase, J.; Shekhar, C. & Felser, C.
  
• Moissanite anvil cell single crystal NMR at pressures of up to 4.4 GPa. Review of Scientific Instruments, 92(11).
  Kattinger, Carsten; Guehne, Robin; Tsankov, Stefan; Jurkutat, Michael; Erb, Andreas & Haase, Juergen
  
• Nuclear Magnetic Resonance of Three Dimensional Topological Insulators, Topological Quantum Science, Erice, Italy (2021).
  Robin Guehne
  
• Unusual Quadrupole NMR of Topological Insulator Bi2Te3. The Journal of Physical Chemistry C, 125(12), 6743-6748.
  Guehne, Robin; Williams, Grant V. M.; Chong, Shen V. & Haase, Jürgen
  
• 125Te NMR study of the bulk of topological insulators Bi2Te3 and Sb2Te3. Zeitschrift für anorganische und allgemeine Chemie, 648(21).
  Nachtigal, Jakob; Chong, Shen V.; Williams, Grant V. M.; Isaeva, Anna; Oeckler, Oliver; Haase, Jürgen & Guehne, Robin
  
• Electronic Properties of the Bulk of 3D Topological Insulators Investigated with Nuclear Magnetic Resonance, SUPERSTRIPES 2022, Frascati-Rome, Italy (2022).
  Robin Guehne
  
• Magnetic field induced charge symmetry in the bulk of the topological insulators Bi2 Se3 and Bi2 Te3 investigated with NMR, APS March Meeting 2022, Boston, USA (2022).
  Robin Guehne
  
• NMR Study of AgInTe2 at Normal and High Pressures. The Journal of Physical Chemistry C, 126(19), 8461-8466.
  Guehne, Robin; Kattinger, Carsten; Bertmer, Marko; Welzmiller, Simon; Oeckler, Oliver & Haase, Jürgen