Quasiclassical theory of coherent charge transport into multi-band superconductors

We formulate a quasiclassical theory of coherent charge transport in junctions involving multi-band iron-based superconductors (FeBSs), explicitly taking into account the complex excitation spectrum and unconventional nature of superconducting pairing in FeBSs. We perform calculations assuming intraorbital superconducting pairing in FeBSs described by the s± model. Further, taking into account that the Fermi energy in FeBSs is much larger than the superconducting gaps, we formulate the quasiclassical approximation. This formulation makes our theory suitable for straightforward numerical calculations and more practical compared to previous approaches. Based on this quasiclassical theory, we calculated the conductance of a junction between a single band normal metal and an FeBS.