Observation of the superconducting proximity effect from surface states in SmB${}_6$/YB${}_6$ thin film heterostructures via terahertz spectroscopy

Abstract

The AC conduction of epitaxially-grown SmB${}_6$ thin films and superconducting heterostructures of SmB${}_6$/YB${}_6$ are investigated via time domain terahertz spectroscopy. A two-channel model of thickness-dependent bulk states and thickness-independent surface states accurately describes the measured conductance of bare SmB${}_6$ thin films, demonstrating the presence of surface states in SmB${}_6$. While the observed reductions in the simultaneously-measured superconducting gap, transition temperature, and superfluid density of SmB${}_6$/YB${}_6$ heterostructures relative to bare YB${}_6$ indicate the penetration of proximity-induced superconductivity into the SmB${}_6$ overlayer; the corresponding SmB${}_6$-thickness independence between different heterostructures indicates that the induced superconductivity is predominantly confined to the interface surface state of the SmB${}_6$. This study demonstrates the ability of terahertz spectroscopy to probe proximity-induced superconductivity at an interface buried within a heterostructure, and our results show that SmB${}_6$ behaves as a predominantly insulating bulk surrounded by conducting surface states in both the normal and induced-superconducting states in both terahertz and DC responses, which is consistent with the topological Kondo insulator picture.

arXiv comment7 pages, 3 figures. Accepted to Phys. Rev. Lett

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