Epitaxial stabilization of an orthorhombic Mg-Ti-O superconductor

The titanium oxide superconductors exhibit many intriguing phenomena analogous to cuprates and iron pnictides/chalcogenides and thus provide an ideal platform for studying the pairing mechanism of high-temperature superconductors. Here we report the fabrication of superconducting Mg-Ti-O films on ${\mathrm{MgAl}}_{2}{\mathrm{O}}_{4}$ substrates with three principal orientations by ablating a ${\mathrm{MgTi}}_{2}{\mathrm{O}}_{4}$ target. Strikingly, a single-crystalline film with an unexpected structure has been found on the (011)-oriented substrate, with a zero resistance transition temperature (${T}_{\mathrm{c}0}$) up to 5.0 K which is the highest among the three principal orientations. Compositional and structural characterizations confirm that the film has a highly reduced Mg/Ti ratio and an orthorhombic ${\mathrm{Ti}}_{9}{\mathrm{O}}_{10}$-like structure (denoted as Mg: ${\mathrm{Ti}}_{9}{\mathrm{O}}_{10}$). Such a structure is unstable in bulk but is favorable on the (011)-surface of ${\mathrm{MgAl}}_{2}{\mathrm{O}}_{4}$ due to the interfacial strain. Similar to other superconducting titanium oxides, the Mg: ${\mathrm{Ti}}_{9}{\mathrm{O}}_{10}$ film exhibits a relatively large quasi-isotropic upper critical field (${B}_{\mathrm{c}2}\ensuremath{\sim}13.7$ T). The similarity points to a common origin for the superconductivity in the family, providing valuable insights for the mechanism of unconventional superconductivity in transition metal compounds.