Distribution functions and current-correlations in normal-metal--superconductor hetero-structures

TL;DR

This paper introduces new distribution functions for electrons and holes in normal-metal--superconductor heterostructures, linking them to current fluctuations and correlations, and highlights mesoscopic effects in current correlations.

Contribution

It defines partial densities of states for heterostructures and analyzes how the superconductor interface acts as an Andreev mirror affecting current and noise characteristics.

Findings

Voltage measurements show no contact resistance to the superconductor.

Positive current correlations are a mesoscopic effect.

Interface acts as an Andreev mirror influencing current fluctuations.

Abstract

We introduce electron-like and hole-like distribution functions, which determine the currents and the fluctuation spectra of the currents measured at a normal-conductor--superconductor hetero-structure. These distribution functions are expressed with the help of newly defined partial densities of states for hetero-structures. Voltage measurements using a weakly coupled contact on such a structure show the absence of a contact resistance to the superconducting reservoir and illustrate how the interface to the superconductor acts as an Andreev mirror. We also discuss the current-current correlations measured at two normal contacts and argue that the appearance of positive correlations is a purely mesoscopic effect, which vanishes in the limit of a large number of channels and in the average over an ensemble.