Emergent mystery in the Kondo insulator samarium hexaboride

Abstract

Samarium hexaboride (SmB6) is an example of a Kondo insulator, in which strong electron correlations cause a band gap to open. SmB6 hosts both a bulk insulating state and a conductive surface state. Within a Fermi-liquid framework, the strongly correlated ground-state electronic structure can be mapped to a simple state resembling a topological insulator. Although uncertainties remain, many experiments provide compelling evidence that the conductive surface states have a topological origin. However, the bulk behaviour is less well understood and some experiments indicate bulk in-gap states. This has inspired the development of many theories that predict the emergence of new bulk quantum phases beyond Landau’s Fermi-liquid model. We review the current progress on understanding both the surface and the bulk states, especially the experimental evidence for each. A mystery centres on the existence of the bulk in-gap states and why they appear in some experiments but not others. Adding to the mystery is why quantum oscillations in SmB6 appear only in magnetization but not in resistivity. We conclude by elaborating on three questions: why SmB6 is worth studying, what can be done to move forwards and what other correlated insulators could give additional insight. The Kondo insulator samarium hexaboride is the first experimentally demonstrated example of a strongly correlated topological insulator. This article reviews the topological theory and experimental evidence, including a mystery as to the origin of quantum oscillations and their relation to possible unconventional bulk in-gap states.