Research on iron-based superconductor $Sr_{2}VO_{3}FeAs$ using scanning tunneling microscopy

Abstract

Forward focused interfacial phonons between iron-based superconductors (FeSCs) and perovskite substrates have received considerable attention due to the possibility of enhancing preexisting superconductivity. However, a direct real-space evidence of interfacial-phonon enhanced superconductivity are currently lacking. Using scanning tunneling spectroscopy, we studied the correlation between superconductivity and electron-phonon interaction in a single crystal iron-based superconductor $Sr_{2}VO_{3}FeAs$ ($T_c \approx 33 K$). We observed the replica bands in the QPI data as a spectroscopic evidence of forward-scattering phonons. Using gap-map-based masked QPI analysis and self-consistent QPI calculations, we observed a number of subtle changes in the renormalized bands consistent with the positive correlation between the electron-phonon coupling and the superconducting gap. We also confirmed that the source of the gap inhomogeneity could be explained not by charge doping effect but by change in local electron-phonon coupling strength in the presence of O vacancies in the $VO_2$ layer. Our results provide a unique demonstration of phonon-induced enhancement of unconventional superconductivity in a bulk heterostructure system comprised of FeAs/oxide layers. We also report the spin-polarized scanning tunneling microscopy (SPSTM) study in the identical sample. From our SPSTM measurement, we observed the $C_4$ symmetric (2×2) magnetic domains and its phase domain walls. This magnetic order is perfectly consistent with the plaquette antiferromagnetic order in tetragonal Fe spin lattice expected from theories based on the Heisenberg exchange interaction of local Fe moments and the quantum order by disorder. The temperature dependent topograph clearly shows that the magnetic phase transition begins at $T_N \approx 50 K$, which is consistent with the experimental results of nuclear magnetic resonance. Furthermore, from our large scale time lapse topographs, we found the existence of phase domain walls with three types of spin configuration. We confirmed that these domain walls are pulled by spin-polarized tip under the influence of spin-polarized tunneling current which entails the spin transfer torque. The domain wall dynamics are consistent with theoretical simulation based on the extended Landau-Lifshitz-Gilbert equation.