Epitaxial stabilization of an orthorhombic Mg-Ti-O superconductor

TL;DR

This study reports the epitaxial stabilization of an orthorhombic Mg-Ti-O superconductor with a transition temperature of 5.0 K, revealing insights into unconventional superconductivity in titanium oxides.

Contribution

It demonstrates the epitaxial stabilization of a previously unstable Mg-Ti-O phase with superconductivity, highlighting the role of interface strain in stabilizing novel structures.

Findings

Achieved epitaxial growth of Mg-Ti-O films with high-quality crystallinity.

Discovered a superconducting orthorhombic Mg: Ti9O10 phase with Tc of 5.0 K.

Observed an isotropic upper critical field exceeding the Pauli limit.

Abstract

The family of titanium oxide superconductors exhibits many intriguing phenomena comparable to cuprates and iron pnictides/chalcogenides, and thus provides an ideal platform to contrastively study the unconventional pairing mechanism of high-temperature superconductors. Here, we successfully deposit superconducting Mg-Ti-O films on MgAl$_2$O$_4$ substrates with three principal orientations by ablating a MgTi$_2$O$_4$ target. Particularly, it is striking to observed that a single-crystalline film of an unintended structure has been grown on the (011)-oriented substrate, with the highest zero resistance transition temperature ($T_{\mathrm{c}0}$) of 5.0 K among them. The film has a highly reduced Mg/Ti ratio and an orthorhombic Ti$_9$O$_{10}$-like structure (denoted as Mg: Ti$_9$O$_{10}$), demonstrated by further characterizations of chemical composition and structure. Such a structure is unstable in bulk but favorable to be epitaxially stabilized on the (011)-surface of MgAl$_2$O$_4$ due to a relatively small strain at the formed interface. An isotropic upper critical field ($B_{\mathrm{c}2}$) up to 13.7 T that breaks the Pauli limit is observed in the Mg: Ti$_9$O$_{10}$ film, analogous to other superconducting titanium oxides. The similarity points to a common origin for the superconductivity in the family, which will provide valuable opinions for the mechanism of unconventional superconductivity in transition metal compounds.