Abstract
The Kondo insulator SmB6 is purported to develop into a robust topological insulator at low temperatures. 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 dependence of the transverse-field relaxation rate below 20 K and a decreasing contact hyperfine field contribution to the positive 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.
Highlights
Due to a combination of spin–orbit coupling and time reversal symmetry, a topological insulator (TI) supports protected metallic edge and surface states in two-dimensional (2D) and threedimensional (3D) systems, respectively.[1]
Bc may instead originate from direct overlap of the μ+ with the wavefunction of localized magnetic electrons, or from bonding of the μ+ to an ion that is covalently bonded to a local atomic magnetic moment.[38]
The μ+ sometimes forms a bound state with an electron, known as a muonium atom (Mu).[38]
Summary
Due to a combination of spin–orbit coupling and time reversal symmetry, a topological insulator (TI) supports protected metallic edge and surface states in two-dimensional (2D) and threedimensional (3D) systems, respectively.[1]. Experimental evidence for SmB6 being a TI is provided by transport measurements that have demonstrated predominant surface electrical conduction below ~5–7 K,4–6 and the detection of in-gap surface states by angle-resolved photoemission spectroscopy (ARPES).[7,8,9,10,11] recent high-resolution ARPES results suggest that the surface conductivity is not associated with topological surface states.[12] at low T, there is a sizable metallic-like T-linear specific heat of bulk origin,[13] and significant bulk ac-conduction.[14] Quantum oscillations are observed in the magnetization of SmB6 as expected for 2D metallic surface states,[15] but subsequent measurements suggest that the origin is a bulk 3D Fermi surface.[16] These findings have raised the possibility of charge-neutral fermions in the insulating bulk.[17,18,19]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
