Speaker
Description
We perform scanning tunneling microscopy/spectroscopy (STM/STS) study on the layered iron based superconductor KCa2Fe4As4F2 with a critical temperature of about 33.5 K. The commonly obtained surface shows full-gap feature with energy gap values close to 4.6 meV. This type of spectrum presents a uniform full gap in space indicating the absence of gap nodes in this superconductor. Quasiparticle interference patterns have also been measured, which show an intra-band scattering pattern possibly due to the hole-like pocket. In addition, we observe discrete bound states with energy levels deviating from the widely believed ratio of 1:3:5 in vortices. 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, i.e.,T/T_c≪Δ/E_F, 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. Our STM measurements on KCa2Fe4As4F2 reveal its nodeless superconducting gap nature and generalized feature of vortex bound states, which provide a comprehensive understanding in this highly layered iron-based superconductor.
