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

Recently, with nuclear magnetic resonance (NMR) experiments we discovered unexpected signatures of the special electronic properties of topological materials. For example, the energy band inversion, responsible for the topological properties, affects the local charge symmetry, and for an inversion at the zone center it results in a NMR quadrupole splitting that we found to be in quantitative agreement with first principle calculations. Less well understood is the observation of a magnetic field induced change of the local charge symmetry, which can lead to orientation independent quadrupole effects if the crystal is rotated in a magnetic field - never reported with NMR experiments before. It is stipulated that electrons in total angular momentum states in these strongly spin-orbit coupled systems can follow the magnetic field direction to some extent, an effect that is known only in atomic physics. The life time of the involved electronic states must lead to a new nuclear relaxation mechanisms, of electric quadrupolar and magnetic dipolar origins. Finally, we found a strong indirect inter-nuclear coupling in these materials. While not unexpected for small bandgap systems, recent theory is only in qualitative agreement with experiment. But this mechanism will be vital for the behavior of magnetic impurities studied by many groups.We propose to investigate these effects in greater detail for a number of topological materials. This includes Bi2Se3 with various conduction electron densities, as well as the closely related systems Bi2Te3, Bi2Te2Se, Sb2Te3, and Bi(1.08)Sb(0.9)Sn(0.02)Te2S. Then, systems with more complex, non-trivial band topology from the Bi(x)Te(M) family (x=1,2,3, M= I, Br) will be studied. Collaboration with theory groups will advance our understanding of these new effects.

• DFG Programme: Research Grants