Andreev reflection in s-type superconductor proximized 3D topological insulator

TL;DR

This study examines Andreev reflection and shot noise in a topological insulator-superconductor junction, revealing a reduced Fano factor likely due to thermal conduction effects in the proximized TI.

Contribution

It provides the first detailed shot noise analysis of Andreev reflection in a 3D topological insulator with an s-type superconductor, highlighting unique thermal conduction effects.

Findings

Normal state Fano factor ~1/3 indicating diffusive transport

Effective charge doubling observed in AR regime

Reduced Fano factor (~0.22) in AR regime compared to normal state

Abstract

We investigate transport and shot noise in lateral N-TI-S contacts, where N is a normal metal, TI is a Bi-based three dimensional topological insulator (3D TI), and S is an s-type superconductor. In normal state, the devices are in the elastic diffusive transport regime, as demonstrated by a nearly universal value of the shot noise Fano factor $F_{\rm N}\approx1/3$ in magnetic field and in reference normal contact. In the absence of magnetic field, we identify the Andreev reflection (AR) regime, which gives rise to the effective charge doubling in shot noise measurements. Surprisingly, the Fano factor $F_{\rm AR}\approx0.22\pm0.02$ is considerably reduced in the AR regime compared to $F_{\rm N}$, in contrast to previous AR experiments in normal metals and semiconductors. We suggest that this effect is related to a finite thermal conduction of the proximized, superconducting TI owing to a residual density of states at low energies.