Charge transfer between a superconductor and a hopping insulator

TL;DR

This paper develops a theory describing how charge transfer occurs between a superconductor and a hopping insulator at low temperatures, focusing on a process similar to Andreev reflection involving Cooper pairs.

Contribution

It introduces a theoretical framework for charge transfer via two-electron processes at superconductor-insulator interfaces, including calculation of interface resistance.

Findings

Time reversal reflection governs charge transfer.

Interface resistance can be experimentally measured.

In mesoscopic structures, resistance can surpass bulk hopping resistance.

Abstract

We develop a theory of the low-temperature charge transfer between a superconductor and a hopping insulator. We show that the charge transfer is governed by the coherent two-electron -- Cooper pair conversion process, time reversal reflection, where electrons tunnel into superconductor from the localized states in the hopping insulator located near the interface, and calculate the corresponding interface resistance. This process is an analog to conventional Andreev reflection process. We show that the time reversal interface resistance is accessible experimentally, and that in mesoscopic structures it can exceed the bulk hopping resistance.