Vortices are topological defects of type-II superconductors in an external magnetic field. In a similar fashion to a quantum anomalous Hall insulator, quantum anomalous vortex (QAV) spontaneously nucleates due to orbital-and-spin exchange interaction between vortex core states and magnetic impurity moment, breaking time-reversal symmetry (TRS) of the vortex without an external field. Here, we used scanning superconducting quantum interference device microscopy (sSQUID) to search for its signatures in iron-chalcogenide superconductor Fe(Se,Te). Under zero magnetic field, we found a stochastic distribution of isolated anomalous vortices and antivortices with flux quanta Φ0. By applying a small local magnetic field under the coil of the nano-SQUID device, we observed hysteretic flipping of the vortices reminiscent of the switching of ferromagnetic domains, suggesting locally broken-TRS. We further showed vectorial rotation of a flux line linking a paired vortex-antivortex with the local field. These unique properties of the anomalous vortices satisfied the defining criteria of QAV. Our observation suggests a quantum vortex phase with spontaneous broken-TRS in a high-temperature superconductor.
@article{arxiv.2208.00642,
title = {Direct observation of quantum anomalous vortex in Fe(Se,Te)},
author = {Y. S. Lin and S. Y. Wang and X. Zhang and Y. Feng and Y. P. Pan and H. Ru and J. J. Zhu and B. K. Xiang and K. Liu and C. L. Zheng and L. Y. Wei and M. X. Wang and Z. K. Liu and L. Chen and K. Jiang and Y. F. Guo and Ziqiang Wang and Y. H. Wang},
journal= {arXiv preprint arXiv:2208.00642},
year = {2023}
}