Pseudo-gap as a signature of inhomogeneous superconductivity in oxide interfaces

We present an explanation for recent tunneling experiments in LaAlO3/SrTiO3 interfaces which is based on the strongly inhomogeneous character of these interfaces. The measurements report signatures of superconductivity in the tunneling spectra while the global resistance of the sample is finite, i.e., a pseudo-gap state. In addition, even when the global resistance vanishes the zero-bias conductance remains finite. We show that these observations can be described by a model of superconducting (SC) islands embedded in a metallic background. The local critical temperatures of the SC island are randomly distributed, some of them necessarily exceeding the critical temperature for global percolation to the zero resistance state. Consequently, tunneling spectra display a suppression of the density of states and coherence peaks already well above the percolative transition. The temperature dependence of the spectra suggests that a sizable fraction of the metallic background becomes SC by proximity effect when the temperature is lowered.