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

In epitaxial polar-oxide interfaces, conductivity sets in beyond a finite number of monolayers (ML). This threshold for conductivity is explained by accumulating sufficient electric potential to initiate charge transfer to the interface. Here we experimentally and theoretically study the LaAlO3/SrTiO3 (111) interface where a critical thickness tc of nine epitaxial LaAlO3 ML is required to turn the interface from insulating to conducting and even superconducting. We show that tc decreases to 3 ML when depositing a cobalt overlayer (capping) and 6 ML for platinum capping. The latter result contrasts with the (001) interface, where platinum capping increases tc beyond the bare interface. Our density functional theory calculations with a Hubbard U term confirm the observed threshold for conductivity for the bare and the metal-capped interfaces. Interestingly, conductivity appears concomitantly with superconductivity for metal/LaAlO3/SrTiO3 (111) interfaces, in contrast with the metal/LaAlO3/SrTiO3 (001) interfaces where conductivity appears without superconductivity. We attribute this dissimilarity to the different orbital polarization of eg′ for the (111) versus dxy for the (001) interface.