Quantum anomalous vortex and Majorana zero mode in iron-based superconductor Fe(Te,Se)

In conventional spin-singlet s-wave superconductors, a time-reversal symmetry breaking magnetic impurity creates a vortex-free defect hosting the Yu-Shiba-Rusinov (YSR) states inside the superconducting (SC) gap. We show that this folklore changes in s-wave superconductors with strong spin-orbit coupling (SOC). In this case, topological defect excitations can nucleate through a quantized phase winding of the SC order parameter around the magnetic ion without applying an external magnetic field. The role of the magnetic field is played by the exchange field and SOC as in the anomalous Hall effect. Such vortices, dubbed quantum anomalous vortices (QAVs), support robust Majorana zero-energy modes (MZMs) when superconductivity is induced in the topological surface states. We demonstrate that the zero-energy bound states observed in Fe(Te,Se) superconductors are possible realizations of the MZMs localized at the QAVs produced by the interstitial magnetic Fe. The quantum anomalous vortex matter can provide an advantageous platform for manipulating MZMs in quantum computing.