Possible routes to superconductivity in the surface layers of V-doped Mg$_{1-\delta}$Ti$_2$O$_4$ through multiple charge transfers and suppression of Jahn-Teller activity

TL;DR

This paper investigates how charge transfer and suppression of Jahn-Teller activity in V-doped MgTi$_2$O$_4$ surface layers can induce superconductivity at higher temperatures, contrasting with the Mott-insulating bulk.

Contribution

It provides a first-principles explanation for the emergence of superconductivity in V-doped MgTi$_2$O$_4$ surface layers through charge transfer and Jahn-Teller suppression.

Findings

Charge transfer reduces Jahn-Teller activity.

Degraded antiferromagnetic order weakens Mottness.

Superconductivity emerges at higher temperatures in surface layers.

Abstract

Superconductivity in the family of spinel oxides is very rare owing to their robust Mott-insulating nature. About half a century ago, LiTi$_2$O$_4$ became the first reported spinel compound to show superconductivity with a 12K transition temperature. Since then, several unsuccessful attempts were made to enhance the T$_c$ of this family of materials. However, a very recent experiment [arXiv:2209.02053] has reported superconductivity at a higher temperature (below 16K), in the V-doped Mg$_{1-\delta}$Ti$_2$O$_4$ thin surface layer while its bulk counterpart remains Mott insulating. The superconducting T$_c$ of this material is significantly higher compared to other engineered MgTi$_2$O$_4$ thin films grown on different substrates. From our first-principles analysis, we have identified that Mg-depletion significantly reduces Jahn-Teller (JT) activity and antiferromagnetic superexchange at the surface layer of V-doped MgTi$_2$O$_4$ due to considerable charge transfer between various ions. The combined effect of a degraded antiferromagnetic order and reduced JT activity weakens the Mottness of the system, leading to the emergence of superconductivity at higher temperatures.