Current-current correlations in hybrid superconducting and normal metal multiterminal structures

TL;DR

This paper investigates current correlations in a hybrid superconductor-normal metal setup, showing how noise measurements reveal details of the superconducting wave function and can be tuned to change correlation signs.

Contribution

It provides a theoretical analysis of current-current correlations in hybrid structures, highlighting the role of crossed Andreev reflection and elastic cotunneling in different regimes.

Findings

Crossed Andreev reflection and elastic cotunneling dominate cross-correlations.

Noise measurements can probe the superconducting wave function directly.

Sign of cross-correlation can be tuned by adjusting voltages.

Abstract

We consider a hybrid system consisting of two normal metal leads weakly connected to a superconductor. Current-current correlations of the normal leads are studied in the tunneling limit at subgap voltages and temperatures. We find that only two processes contribute to the cross-correlation: the crossed Andreev reflection (emission of electrons in different leads) and the elastic cotunneling. Both processes are possible due to the finite size of the Cooper pair. Noise measurements can thus be used to probe directly the superconducting wave function without the drawbacks appearing in average current measurements where the current is dominated by direct Andreev reflection processes. By tuning the voltages it is possible to change the sign of the cross correlation. Quantitative predictions are presented both in the diffusive and ballistic regimes.