Emergent superconductivity in single-crystalline MgTi2 O4 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 MgTi2O4, known to be an insulator with a complicated order. The superconductivity is achieved by engineering a superlattice of MgTi2O4 and SrTiO3. The onset transition temperature in the MgTi2O4 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 Å) compared to the bulk material. Such a positive correlation suggests ample room for further enhancement. Intriguingly, the superlattice exhibits an isotropic upper critical field Bc2 that breaks the Pauli limit, distinct from the highly anisotropic feature of interface superconductivity. The origin of superconductivity in the MgTi2O4 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.