Direct Observation of Quantum Anomalous Vortex in Fe(Se,Te)

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 vortices (QAV) spontaneously nucleate due to orbital-and-spin exchange interaction between supercurrent and magnetic impurity moment without an external magnetic 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 Φ_{0}. 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 time-reversal symmetry. We further observed vectorial rotation of a flux line linking a vortex-antivortex pair by manipulating the local field. These unique properties of the anomalous vortices satisfy the defining criteria of QAV. Our observation suggests an emergent quantum phase with spontaneously nucleated vortex-antivortex matter in an iron-based superconductor with nontrivial topological band structure.