Non-BCS-type superconductivity and critical thickness of SrTiO3/LaAlO3/SrTiO3 trilayer interface system

[Display omitted] •When thin LaAlO3 (1-unit-cell)/SrTiO3 is capped by SrTiO3, conduction is induced.•The critical thickness condition for conduction is consistent with DFT calculation.•SrTiO3/LaAlO3/SrTiO3 shows non-BCS-type superconductivity. As a quest for two-dimensional conducting interface with exotic functionalities for future electronic devices, the perovskite heterointerface of LaAlO3/SrTiO3 (LAO/STO) has been intensively studied. For the LAO/STO heterostructure, the critical thickness of the LAO layer for metallic conduction is 4 unit cells (uc) according to the polar catastrophe scenario. However, we find that metallic conduction can also be induced at the LAO/STO interface with only 1 uc of LAO, as long as it is capped by 3 or more uc of STO. Consistent results are obtained from density functional theory calculations. For this STO/LAO/STO trilayer, we also confirmed a peculiar non-BCS-type superconductivity with a suppressed superconducting gap, which may imply a superconducting coupling distinct from previously reported BCS-type superconductivity in LAO/STO heterointerface. These observations suggest that the STO/LAO/STO trilayer can be another testing board to explore carrier conduction in two-dimensional electron systems for electronic device applications utilizing exotic functionalities.