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

TL;DR

This study observes unconventional vortex bound states and Friedel oscillations in an iron-based superconductor, revealing deviations from expected energy ratios due to quantum effects and impurity influences, supported by experimental STM data and theoretical calculations.

Contribution

It provides the first observation of Friedel oscillations of vortex bound states and explains their origin through self-consistent Bogoliubov-de Gennes calculations in the extreme quantum limit.

Findings

Vortex bound states deviate from the 1:3:5 energy ratio.

Friedel oscillations of vortex states are observed for the first time.

Discrepancies at high energies are attributed to impurity effects.

Abstract

We report the observation of discrete vortex 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 microcopy (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 the vortex bound states in type-II superconductors.