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

TL;DR

This study uses terahertz spectroscopy to observe the superconducting proximity effect from surface states in SmB$_6$/YB$_6$ heterostructures, revealing interface-confined induced superconductivity and confirming SmB$_6$ as a topological Kondo insulator.

Contribution

It demonstrates the use of terahertz spectroscopy to probe proximity-induced superconductivity at buried interfaces in topological insulator heterostructures.

Findings

Surface states in SmB$_6$ are confirmed by conductance modeling.

Superconductivity penetrates into SmB$_6$ mainly at the interface surface state.

Induced superconductivity is confined to the surface state, independent of SmB$_6$ thickness.

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.