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 hightemperature superconductors. Here we report the fabrication of superconducting Mg-Ti-O films on MgAl2O4 substrates with three principal orientations by ablating a MgTi2O4 target. Strikingly, a single-crystalline film with an unexpected structure has been found on the (011)-oriented substrate, with a zero resistance transition temperature (Tc0) 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 Ti9O10-like structure (denoted as Mg: Ti9O10). Such a structure is unstable in bulk but is favorable on the (011)-surface of MgAl2O4 due to the interfacial strain. Similar to other superconducting titanium oxides, the Mg: Ti9O10 film exhibits a relatively large quasi-isotropic upper critical field (Bc2 - 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.