Concomitant appearance of conductivity and superconductivity in (111)LaAlO3/SrTiO3 interface with metal capping

In polar-oxide interfaces, a certain number of monolayers (ML) is needed for conductivity to appear. This threshold for conductivity is explained by the accumulation of sufficient electric potential to initiate charge transfer to the interface. Here we study the (111) SrTiO3/LaAlO3 interface where a critical thickness of nine epitaxial LaAlO3 ML is required to turn the interface from insulating to conducting and even superconducting. We show that this critical thickness decreases to 3ML when depositing a cobalt over-layer (capping) and 6ML for platinum capping. The latter result contrasts with the (100) interface where platinum capping increases the critical thickness beyond that of the bare interface. These results suggest that the work function of the metallic capping plays an important role in both interfaces. Interestingly, for (111) SrTiO3/LaAlO3/Metal interfaces conductivity appears concomitantly with superconductivity in contrast with the SrTiO3/LaAlO3/Metal interface with LaAlO3 layer smaller than four ML (unit-cells), which are conducting but not superconducting. We suggest that this difference is related to the different sub-bands involved in conductivity for the (111) interfaces, comparing to the (100) interfaces. Our findings can be useful for superconducting devices made of such interfaces.