Title: Pair tunneling with negative U at the LAO/STO complex oxide interface

Abstract: 

The conducting interface at the LaAlO$_{3}$/SrTiO$_{3}$ (LAO/STO) interface has sparked large interest due to its many coexisting properties. Interfaces based on non-crystalline top layers show similar characteristics to their crystalline equivalent. Recently, we reported superconductivity in patterned devices of the disordered-LAO/STO interface where strong evidence was found for a superconducting phase originating from a Josephson-coupled array of superconducting puddles, possibly induced by inhomogeneous doping. We studied this system with nano-patterned gates to form a nano-scale constriction in a Hall bar, in which we were able to pinch off current flow. Close to pinch-off, a dot regime is formed with peaks of conductivity, that form diamonds of suppressed current when applying a bias voltage across the constriction. The conductance peaks split up when applying a large in-plane or out-of-plane magnetic field, indicating pairing of electrons without superconductivity (as reported in Cheng et al., Nature 2015). We study the system as a function of gate voltage, bias voltage, magnetic field and temperature and compare with a negative-U model that describes the behavior of pair tunneling, which differs from the conventional Coulomb-blockade scenario.