Proximity-Induced Pseudogap: Evidence for Preformed Pairs

The temperature evolution of the proximity effect in Au/La2-xSrxCuO4 and La1.55Sr0.45CuO4/La2-xSrxCuO4 bilayers was investigated using scanning tunneling microscopy. Proximity-induced gaps, centered at the chemical potential, were found to persist above the superconducting transition temperature, Tc, 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 Tc.