Control of nonreciprocal charge transport in topological insulator/superconductor heterostructures with Fermi level tuning and superconducting-layer thickness

TL;DR

This study demonstrates control over nonreciprocal charge transport in topological insulator/superconductor heterostructures by tuning Fermi level and superconductor thickness, revealing the role of topological surface states and symmetry breaking.

Contribution

It introduces a method to reversibly control the magnitude and sign of NCT via Fermi level tuning and layer thickness adjustment in TI/SC heterostructures, highlighting the importance of topological surface states.

Findings

Reversal of NCT direction by Fermi level tuning.

Enhanced NCT magnitude with reduced superconductor thickness.

NCT controlled by symmetry breaking at the TI/SC interface.

Abstract

Nonreciprocal charge transport (NCT) is defined as a phenomenon where electrical resistance depends on the current direction. It has been drawing much attention because it sensitively reflects the symmetry breaking of material systems. A topological insulator (TI)/superconductor (SC) heterostructure where the topological surface state (TSS) of the TI layer is proximitized with the SC layer is one such system that presents a sizable NCT due to a large spin-orbit coupling and superconductivity. Here, we report a control of the magnitude and sign of NCT: reversal of the direction of NCT by tuning the Fermi energy of TSS of the TI layer with respect to the charge neutral point by systematic regulation of Sb composition $x$ in a TI/SC heterostructures of (Bi$_{1-x}$Sb$_x$)$_2$Te$_3$/FeSe$_{0.1}$Te$_{0.9}$. The result is consistent with the model of a TSS proximitized with superconductivity. Furthermore, we find a significant enhancement of the magnitude of NCT in the TI/SC heterostructures by reducing the thickness of the SC layer. The enhancement can be ascribed to the inversion-symmetry breaking of the FeSe$_{0.1}$Te$_{0.9}$ SC-layer itself adjacent to the TI layer. Our results highlight the essential role of the TSS for exhibiting NCT and offer new knobs to control the direction and magnitude of NCT.