Being the first rare earth mixed valence system and the first Kondo insulator, SmB₆ may very likely be the first topological Kondo insulator too. Recent studies have confirmed the existence of topological surface states (TSS) and surface chiral spin textures in single-crystal SmB₆ bulk materials, but the non-trivial nature and depth scale of the TSS remain unresolved. In this talk I will report our recent work on the use of spin pumping to study the non-trivial feature of the TSS in SmB₆ [1]. Our experiments used SmB₆ thin films grown on magnetic insulator Y₃Fe₅O₁₂ (YIG) bulk materials. Upon the excitation of magnetization precession in the YIG, a pure spin current is generated that flows from the YIG into the SmB₆ film and produces, via spin-orbit coupling, a lateral electrical voltage in the SmB₆ film. The spin-pumping voltage signal becomes considerably stronger as the temperature decreases from 150 K to 10 K. Such an enhancement most likely originates from the spin-momentum locking feature of the TSS of the SmB₆ film and may thereby serve as a strong evidence for the nontrivial nature of the TSS. The voltage data also show a unique film thickness dependence that suggests a TSS depth of about 32 nm for the TSS. The data also indicate a spin diffusion length of about 39 nm in the SmB₆ film. The spin-pumping results are supported by temperature-dependent transport measurements and theoretical analyses using a tight binding model.

[1] "Nontrivial nature and penetration depth of topological surface states in SmB6 thin films” Tao Liu, Yufan Li, Lei Gu, Junjia Ding, Houchen Chang, P. A. Praveen Janantha, Boris Kalinikos, Valentyn Novosad, Axel Hoffmann, Ruqian Wu, C. L. Chien, and Mingzhong Wu, Physical Review Letters 120, 207206 (2018).