A proximity induced pseudogap - evidence for preformed pairs

TL;DR

This study uses scanning tunneling microscopy to show that proximity-induced gaps persist above the superconducting transition temperature, supporting the existence of preformed Cooper pairs in the pseudogap regime of cuprate superconductors.

Contribution

It provides evidence that incoherent Cooper pairs penetrate into the normal metal layer above $T_c$, challenging density-wave order explanations for the pseudogap.

Findings

Proximity-induced gaps persist above $T_c$ up to the pseudogap temperature.

Spectral features are independent of the normal metal layer details.

Results support preformed Cooper pairs existing above the superconducting transition.

Abstract

The temperature evolution of the proximity effect in Au/La$_{2-x}$Sr$_x$CuO$_4$ and La$_{1.55}$Sr$_{0.45}$CuO$_4$/La$_{2-x}$Sr$_x$CuO$_4$ bilayers was investigated using scanning tunneling microscopy. Proximity induced gaps, centered at the chemical potential, were found to persist above the superconducting transition temperature, $T_c$, and up to nearly the pseudogap crossover temperature in both systems. Such independence of the spectra on the details of the normal metal cap layer is incompatible with a density-wave order origin. However, our results can be accounted for by a penetration of incoherent Cooper pairs into the normal metal above $T_c$.