Emergent superconductivity in single-crystalline MgTi2O4 films via structural engineering

Spinel compounds have exhibited rich functionalities but have rarely shown superconductivity. Here, we report the emergence of superconductivity in the spinel ${\mathrm{MgTi}}_{2}{\mathrm{O}}_{4}$, known to be an insulator with a complicated order. The superconductivity is achieved by engineering a superlattice of ${\mathrm{MgTi}}_{2}{\mathrm{O}}_{4}$ and ${\mathrm{SrTiO}}_{3}$. The onset transition temperature in the ${\mathrm{MgTi}}_{2}{\mathrm{O}}_{4}$ layer can be tuned from 0 to 5 K in such a geometry, concurrently with a stretched out-of-plane lattice (from 8.51 to 8.53 \AA{}) compared to the bulk material. Such a positive correlation suggests ample room for further enhancement. Intriguingly, the superlattice exhibits an isotropic upper critical field ${B}_{\mathrm{c}2}$ that breaks the Pauli limit, distinct from the highly anisotropic feature of interface superconductivity. The origin of superconductivity in the ${\mathrm{MgTi}}_{2}{\mathrm{O}}_{4}$ layer is understood in combination with the electron energy loss spectra and first-principles electronic structure calculations, which point to the birth of superconductivity by suppressing orbital ordering. Our discovery not only provides a platform to explore the interplay between superconductivity and other exotic states, but also opens another window to realize superconductivity in spinel compounds as well as other titanium oxides.