Superconducting Energy Gap Features of FeSe Investigated by Tunneling Spectroscopy on Planar Junctions

Planar tunneling junctions of the presumably unconventional superconductor FeSe were prepared and investigated. The junctions consisted of rf-sputtered FeSe/AlO x/Ag multilayers patterned lithographically into mesa structures. Bias voltage-dependent differential conductivities dI/dV(V) of junctions in the tunneling, intermediate barrier strength, and metallic regimes were investigated. Depending on the barrier type between two to four features of the conductivity were obtained, which are discussed in the framework of multiple superconducting energy gaps. Specifically, we reproduced with all barrier types two established energy gaps and identified another feature as presumably originating from an additional large superconducting gap. A further conductivity feature could be associated with a resonant state.