Unconventional superconductivity in SrTiO₃/LaAlO₃/SrTiO₃ trilayer system

A trilayer structure, composed of 7 unit cell (uc) of SrTiO₃ (STO) and one uc of LaAlO₃ (LAO) deposited on an STO substrate, was fabricated and its electrical transport properties at low temperatures were studied. We have found that the trilayer become superconducting at temperature below 180 mK. We have measured the magnetic field dependence of the critical current of the trilayer. The critical current exhibits a hysteresis with the magnetic field. Surprisingly, the direction of the hysteresis is opposite to that of the conventional magnetoresistance hysteresis. The superconductivity and magnetism coexist at this trilayer system and the superconducting order parameter of the system can have a helical ordering. We have also fabricated vertical Josephson junction by depositing Al on top of the trilayer. Superconducting energy gap is estimated from the Andreev reflection peak of the Josephson junction. Measured gap ratio, Δ/k_BT_C = 1.31, suggests that this system is not a conventional BCS-type superconductor.