DRIVING SPIN ORBIT COUPLING TO THE EXTREME (exSOC)
What is spin-orbit coupling (SOC) and why is it so important in condensed matter? Generally, an electron orbiting the nucleus in an atom creates a small magnetic field (which is larger for a heavier atom). As the electron also possesses an intrinsic "magnetic bar" called spin, this spin now interacts with the induced magnetic field of the orbital motion. We call this interaction spin-orbit coupling. The result is that, depending on the orientation of the spin with respect to the induced magnetic field, the electron experiences an additional energy boost or a small energy decrease. SOC is also present when electrons move through a crystal lattice. Some materials do not conduct a current, like glass or wood – these are insulators. The physical origin of this non-conductivity lies in the fact that, in an insulator, the energy bands associated with the motion of the electrons are separated by a gap. And here it becomes interesting: only recently it was found that in some insulators, SOC is strong enough to lead to the phenomenon of band inversion, with dramatic consequences. The inversion creates robust surface (conducting) states within the gap, turning the material's surface into a metal while the interior remains insulating. Such materials are named topological insulators.
The Austrian-Czech Bilateral Project exSOC concentrates on Ce- and U-based Kondo insulators (KI). An example material is Ce3Bi4Pt3. The name KI refers to the fact that in these intermetallic compounds the gap between the valence and the conduction bands is created by strong correlations (Kondo effect of the f electron from Ce and conduction electrons from Bi and Pt). Due to the heavy elements constituting them, these materials also exhibit a large SOC. As before, sufficiently strong SOC may lead to band inversion, potentially turning the material into a topological Kondo insulator (TKI). Yet, in a strongly correlated setting, the role of SOC is poorly understood. In fact, entirely new states of matter may result from the interplay of strong SOC and strong correlations, making this a vastly open new field. Questions we address are: Can a KI be driven into a TKI state upon enhancing SOC? Do Ce- or U-based TKIs exist? Which SOC is more relevant, the SOC related to the f electrons from Ce or U or the SOC originating from the Bi and Pt electrons? We intend to answer these questions by a series of chemical substitutions which tune the SOC strength (for example, replacing Ce by much heavier U), and a series of experiments to detect and characterize the new emerging states.