Quantum anomalous vortex and Majorana zero mode induced by magnetic impurities in Fe-based superconductors.

In conventional superconductors, an external magnetic field is needed to generate magnetic vortices. We argue that the presence of strong spin-orbit coupling changes this folklore in a fundamental way. Topological defect excitations, dubbed quantum anomalous vortex (QAV), can nucleate around magnetic ions spontaneously in the absence of external magnetic fields [1]. With increasing exchange coupling, magnetic impurity states transition from the vortex-free Yu-Shiba-Rusinov states to the QAV core states. Several Fe-based superconductors exhibit Z₂ nontrivial band structures and superconducting topological surface states. We discuss recent experimental observations of QAV and candidate MZM at growth-induced interstitial magnetic Fe impurities [2], magnetic Fe adatoms deposited on the surface [3, 4], and the reversible transition between the Yu-Shiba-Rusinov states and integer quantized QAV core states including the MZM [4]. Other on-going experiments searching for the QAV and MZM will be discussed, as well as possible realizations of intrinsic chiral topological superconductivity in Fe-based superconductors.