Evidence for unconventional superconductivity in a spinel oxide

TL;DR

This study provides evidence of unconventional d-wave superconductivity in LiTi₂O₄, a spinel oxide, characterized by fourfold symmetry in its superconducting state, revealing insights into its pairing mechanism.

Contribution

The paper reports the discovery of fourfold rotational symmetry in the superconductivity of LiTi₂O₄, indicating unconventional d-wave pairing protected by crystal symmetry.

Findings

Fourfold oscillations in magnetoresistivity and upper critical field deep inside the superconducting state.

Anisotropy vanishes in the normal state, confirming it as an intrinsic property of the superconducting phase.

Unconventional d-wave pairing symmetry with E_g irreducible representation in LiTi₂O₄.

Abstract

The charge frustration with the mixed-valence state inherent to LiTi$_2$O$_4$, which is found to be a unique spinel oxide superconductor, is the impetus for paying special attention to reveal the existence of intriguing superconducting properties. Here, we report a pronounced fourfold rotational symmetry of the superconductivity in high-quality single-crystalline LiTi$_2$O$_4$ (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 $O_h$ point group in LiTi$_2$O$_4$. Our findings demonstrate the unconventional character of the pairing interaction in a three-dimensional spinel oxide superconductor and shed new light on the pairing mechanism of unconventional superconductivity.