Surface-phase superconductivity in Mg-deficient V-doped MgTi$_2$O$_4$ spinel

TL;DR

This study discovers a surface-phase V-doped MgTi2O4 spinel exhibiting high-temperature superconductivity at around 16 K with a very high critical magnetic field, surpassing previous spinel superconductors.

Contribution

It reports the first observation of high-temperature surface-phase superconductivity in V-doped MgTi2O4 spinel, with a T_c of approximately 16 K and a critical field of about 25 Tesla.

Findings

Superconductivity observed at T_c ≈ 16 K in V-doped MgTi2O4 surface layer.

Superconducting transition confirmed by diamagnetic response.

High critical magnetic field of approximately 25 Tesla.

Abstract

Around fifty years ago, LiTi$_2$O$_4$ was reported to be first spinel oxide to exhibit a superconducting transition with highest T$_c$ $\approx$ 13.7 K. Recently, MgTi$_2$O$_4$ has been found to be the only other spinel oxide to reveal a superconducting transition with a T$_c$ $\approx$ 3 K, however, its superconducting state is realized only in thin film superlattices involving SrTiO$_3$. We find that a V-doped Mg$_{1-x}$Ti$_2$O$_4$ phase, which gets stabilized as a thin surface layer on top of stoichiometric and insulating V-doped MgTi$_2$O$_4$ bulk sample, exhibits high-temperature superconductivity with T$_c$ $\approx$ 16 K. The superconducting transition is also confirmed through a concomitant sharp diamagnetic transition immediately below T$_c$. The spinel phase of the superconducting surface layer is elucidated through grazing-incidence X-ray diffraction and Micro-Raman spectroscopy. A small shift of the sharp superconducting transition temperature ($\sim$ 4 K) with application of a high magnetic field (upto 9 Tesla) suggests a very high critical field for the system, $\sim$ 25 Tesla. Thus, V-doped Mg$_{1-x}$Ti$_2$O$_4$ exhibits the highest T$_c$ among spinel superconductors and also possesses a very high critical field.