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

Abstract

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 ${\mathrm{LaAlO}}_{3}/{\mathrm{SrTiO}}_{3}$ (111) interface where a critical thickness ${t}_{c}$ of nine epitaxial ${\mathrm{LaAlO}}_{3}$ ML is required to turn the interface from insulating to conducting and even superconducting. We show that ${t}_{c}$ 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 ${t}_{c}$ 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/${\mathrm{LaAlO}}_{3}/{\mathrm{SrTiO}}_{3}$ (111) interfaces, in contrast with the metal/${\mathrm{LaAlO}}_{3}/{\mathrm{SrTiO}}_{3}$ (001) interfaces where conductivity appears without superconductivity. We attribute this dissimilarity to the different orbital polarization of ${e}_{g}^{\ensuremath{'}}$ for the (111) versus ${d}_{xy}$ for the (001) interface.