Bound states in the core of ordered vortices in stoichiometric iron-based superconductor CaKFe4As4

We report scanning tunneling microscopy/spectroscopy (STM/S) observations of Majorana zero modes (MZMs) and Caroli de Genne--Matricon bound states in the vortex core of stoichiometric iron-based superconductor $\mathrm{CaKF}{\mathrm{e}}_{4}\mathrm{A}{\mathrm{s}}_{4}$. Previous STS study on Fe(Se,Te) showed that topologically trivial and nontrivial vortices coexist, possibly due to the inhomogeneities in Fe(Se,Te) as a doped material. In the present study, we demonstrate the presence of a much more regular vortex lattice on a cleaved surface of $\mathrm{CaKF}{\mathrm{e}}_{4}\mathrm{A}{\mathrm{s}}_{4}$ single crystal compared with that in Fe(Se,Te). We found that despite the presence of a very regular vortex lattice, the topologically trivial and nontrivial vortices still coexist, and the ratio of the nontrivial vortices is not high. We checked the distribution of the MZMs and found that they are not influenced by the regularity of the vortex arrangements. Consistent with the previous work, we confirmed that higher magnetic field reduces the ratio of topologically nontrivial vortices, highlighting the necessity of limiting the magnetic field to achieve a stable and practical MZM lattice. Other sources of the suppression of a MZM in the vortex core of $\mathrm{CaKF}{\mathrm{e}}_{4}\mathrm{A}{\mathrm{s}}_{4}$ are suggested.