Fourfold Symmetric Superconductivity in Spinel Oxide LiTi₂O₄(001) Thin Films

The charge frustration with the mixed-valence state inherent to LiTi₂O₄, which is found to be the only oxide superconductor with spinel structure, is the impetus for paying special attention to unveil the underlying intriguing superconducting properties. Here, we report a pronounced fourfold rotational symmetry of the superconductivity in high-quality single-crystalline LiTi₂O₄(001) thin films. Both the magnetoresistivity and upper critical field under an applied magnetic field manifest striking fourfold oscillations deep inside the superconducting state, whereas the anisotropy vanishes in the normal state, demonstrating that it is an intrinsic property of the superconducting phase. We attribute this behavior to the unconventional d-wave superconducting Cooper pairs with the irreducible representation of E_g protected by the O_h point group in cubic LiTi₂O₄. Our findings show the nontrivial character of the pairing interaction in a three-dimensional spinel oxide superconductor.