Unconventional properties of superconductivity in SrTiO₃/LaAlO₃/SrTiO₃

The 2-Dimensional Electron Gases (2-DEGs) observed at the LaAlO₃/SrTiO₃ (LAO/STO) hetero-interface exhibits superconductivity, which has phase diagram as function of carrier density like high-temperature superconductor. However, the superconducting energy gap in LAO/STO has conventional superconductivity because the temperature dependence of superconducting energy gap follows Bardeen-Cooper-Schrieffer(BCS) theory. Here we report unconventional properties of superconductivity in STO/LAO/STO. We have fabricated vertical Josephson junction to measure the superconducting energy gap. We found that the superconducting energy gap estimated from the Andreev reflection of the Josephson junction did not follow BCS theory from the gap ratio, Δ/k_BT_C = 1.31. Additionally, we found evidence of unconventional superconductivity from the magnetic field dependence of superconducting critical current. It has opposite hysteresis, when it was compared with the hysteresis of magnetoresistance in STO/LAO/STO.