A proximity induced pseudogap - evidence for preformed pairs

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 $T_c$.