The charge frustration with the mixed-valence state inherent to LiTi2O4, 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 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 Eg protected by Oh point group in LiTi2O4. 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.
@article{arxiv.2110.13397,
title = {Evidence for unconventional superconductivity in a spinel oxide},
author = {Huanyi Xue and Lijie Wang and Zhongjie Wang and Guanqun Zhang and Wei Peng and Shiwei Wu and Chunlei Gao and Zhenghua An and Yan Chen and Wei Li},
journal= {arXiv preprint arXiv:2110.13397},
year = {2023}
}