Anderson localization of in-gap BdG quasiparticles in disordered Fe-based superconductor

The relationship of the pairing fluctuations and the Anderson transition of Bogoliubov-de Gennes (BdG) quasiparticles is critical in understanding the microscopic mechanism of Anderson localization in the presence of spontaneous symmetry breaking. However, the role of the superconducting order-parameter fluctuations in the localization of BdG quasiparticles have not been well understood. Our study, based on scanning tunneling microscopy measurements, investigates how BdG quasiparticles become Anderson-localized and delocalized as a function of energy and applied magnetic field in a quasi-two-dimensional Fe-based superconductor with sufficient zero-bias BdG quasiparticles. The anomalous multifractal spectra based on the spatial distributions of the pairing gaps and the coherent peak heights suggest that superconducting fluctuations play a key role in the delocalization of in-gap BdG quasiparticles. Our real-space Hartree-Fock-BCS-Anderson simulations and renormalization group analysis with potential and pairing fluctuations support strong general tendency towards quasiparticle localization and suggest a novel mechanism, i.e., pairing fluctuations, for delocalization of BdG quasiparticles and associated localization of phase-fluctuating Cooper pairs in quasi-two-dimensional disordered superconductors.