Friedel Oscillations of Vortex Bound States under Extreme Quantum Limit in KCa2Fe4As4F2

We report the observation of discrete bound states with the energy levels deviating from the widely believed ratio of 1 :3 :5 in the vortices of an iron-based superconductor KCa2Fe4As4F2 through scanning tunneling microscopy (STM). Meanwhile Friedel oscillations of vortex bound states are also observed for the first time in related vortices. By doing self-consistent calculations of Bogoliubov-de Gennes equations, we find that at extreme quantum limit, the superconducting order parameter exhibits a Friedel-like oscillation, which modifies the energy levels of the vortex bound states and explains why it deviates from the ratio of 1 :3 :5. The observed Friedel oscillations of the bound states can also be roughly interpreted by the theoretical calculations, however some features at high energies could not be explained. We attribute this discrepancy to the high energy bound states with the influence of nearby impurities. Our combined STM measurement and the self-consistent calculations illustrate a generalized feature of vortex bound states in type-II superconductors.