Zero-bias conductance peak observed in Au/FeSe0.3Te0.7 ramp-type junctions and its implication on the superconducting order parameter

Abstract

Based upon the currently predominant theory of Fe-based superconductors, the order parameter symmetry is the nodaless s\pm-wave. Andreev conductance involving normal metals and such superconductors does not have a zero bias conductance peak (ZBCP) except maybe for a very narrow range of a junction-dependent fitting parameter. We have measured differential conductance of the in-plane boundary junctions between Au and superconducting FeSe0.3Te0.7 films. The Au/FeSe0.3Te0.7 ramp-type junctions are fabricated with the bias current direction along the <110> axis of the epitaxial FeSe0.3Te0.7 film. We have always observed a pronounced ZBCP as well as some gap-like features in the conductance spectra of all samples studied, thus our experimental results suggest not the s\pm-wave theory but the possible existence of a nodal point around 45{\deg} in at least one of the gaps in FeSe0.3Te0.7 superconductor.