Freezing out of a low-energy bulk spin exciton in SmB6

TL;DR

This study uses muon spin rotation to investigate low-energy spin excitons in SmB6, revealing their freeze-out at low temperatures and suggesting their role in the material's anomalous bulk properties and surface conductivity.

Contribution

It provides experimental evidence for the freezing out of bulk spin excitons in SmB6 and links this to its low-temperature electronic behavior.

Findings

Spin excitons freeze out below 20 K.

Muon Knight shift decreases below 5-6 K.

Bulk properties are affected by exciton behavior.

Abstract

The Kondo insulator SmB6 is purported to develop into a robust topological insulator at low temperature. Yet there are several puzzling and unexplained physical properties of the insulating bulk. It has been proposed that bulk spin excitons may be the source of these anomalies and may also adversely affect the topologically-protected metallic surface states. Here, we report muon spin rotation measurements of SmB6 that show thermally-activated behavior for the temperature dependences of the transverse-field (TF) relaxation rate below 20 K and muon Knight shift below 5-6 K. Our data are consistent with the freezing out of a bulk low-energy (~ 1 meV) spin exciton concurrent with the appearance of metallic surface conductivity. Furthermore, our results support the idea that spin excitons play some role in the anomalous low-temperature bulk properties of SmB6.