The ac conduction of epitaxially grown ${\mathrm{SmB}}_6$ thin films and superconducting heterostructures of ${\mathrm{SmB}}_6/{\mathrm{YB}}_6$ 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 ${\mathrm{SmB}}_6$ thin films, demonstrating the presence of surface states in ${\mathrm{SmB}}_6$. While the observed reductions in the simultaneously measured superconducting gap, transition temperature, and superfluid density of ${\mathrm{SmB}}_6/{\mathrm{YB}}_6$ heterostructures relative to bare ${\mathrm{YB}}_6$ indicate the penetration of proximity-induced superconductivity into the ${\mathrm{SmB}}_6$ overlayer; the corresponding ${\mathrm{SmB}}_6$-thickness independence between different heterostructures indicates that the induced superconductivity is predominantly confined to the interface surface state of the ${\mathrm{SmB}}_6$. 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 ${\mathrm{SmB}}_6$ 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.

• Received 10 February 2022
• Revised 12 August 2022
• Accepted 26 January 2023

DOI:https://doi.org/10.1103/PhysRevLett.130.096901