Tunneling spectroscopic study of the topological Kondo insulator SmB₆ via second harmonic measurements

In tunneling spectroscopy, it is well established that the first harmonic signal can be measured to determine the electronic density of states (DOS) in a material of interest. Naturally, one may wonder if the second harmonic signal has any physical meaning. In the case of strong-coupled Bardeen-Cooper-Schrieffer (BCS) superconductors, the second harmonic signal can be measured to determine the phonon modes involved in Cooper pairing [1]. This is because the BCS DOS is saliently modified due to a large self-energy correction caused by strong coupling. Our previous tunneling studies on the topological Kondo insulator SmB₆ have suggested that spin excitons, bosonic bulk excitations, interact with topological surface states to modify their DOS [2,3]. In this talk, we discuss the results of applying second harmonic measurement techniques to SmB₆ tunnel junctions. Of particular interest is an antisymmetric peak structure at ± 0.8 mV which persists up to ~5 K, accompanied by two distinct peaks at 4.5 meV and 8.5 meV persisting above 20 K and up to ~12 K, respectively.

[1] W. L. McMillan and J. M. Rowell, PRL 14,108 (1965).

[2] W.K Park et al., PNAS 113, 6599 (2016); PRB 103, 155125 (2021).

[3] G. A. Kapilevich et al., PRB 92, 085133 (2015).