Possible routes to superconductivity in the surface layers of V-doped Mg1−δTi2O4 through multiple charge transfers and suppression of Jahn-Teller activity

Abstract

Superconductivity in the family of spinel oxides is very rare owing to their robust Mott-insulating nature. About half a century ago, ${\mathrm{LiTi}}_{2}{\mathrm{O}}_{4}$ became the first reported spinel compound to show superconductivity with a 12 K transition temperature. Since then, several unsuccessful attempts were made to enhance the ${T}_{\mathrm{c}}$ of this family of materials. However, a very recent experiment [A. Rahaman et al., arXiv:2209.02053] has reported superconductivity at a higher temperature (below $\ensuremath{\sim}16$ K) in the V-doped ${\mathrm{Mg}}_{1\ensuremath{-}\ensuremath{\delta}}{\mathrm{Ti}}_{2}{\mathrm{O}}_{4}$ thin surface layer while its bulk counterpart remains Mott insulating. The superconducting ${T}_{\mathrm{c}}$ of this material is significantly higher compared to other engineered ${\mathrm{MgTi}}_{2}{\mathrm{O}}_{4}$ 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 ${\mathrm{Mg}}_{1\ensuremath{-}\ensuremath{\delta}}{\mathrm{Ti}}_{2}{\mathrm{O}}_{4}$ 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.