Spin-dependent conductivity of iron-based superconductors in a magnetic field

Abstract

We study dc conductivity of iron-based superconductors α-FeSe and LaOFFeAs by measuring the conductance of point-contact heterojunctions in NS and NN modes of transport (N and S denote normal and superconducting states, respectively). In the NS regime, measurements were performed in case of defect-free NS boundary due to shifting it inside the superconductor by the transport current. Under these conditions, we observed the contact conductance to increase at the NS→NN transition driven either by temperature or by magnetic field, and to decrease at the reverse transition. We attribute this effect to the manifestation of spin-dependent nature of the Andreev reflection (spin accumulation) in consequence of the magnetism at the normal side of the NS boundary. Investigating normal conductance in a magnetic field we revealed the nonpersistent hysteresis and square-law dependence of positive magnetoresistance on the magnetic field which fact confirmed this conclusion and pointed to the leading role of itinerant magnetism in the normal ground state of the superconductors studied. Based on the experimental findings and analysis we conclude that there exists a long-range magnetic order in the normal ground state of investigated iron-based superconductors with nematic ferromagnetic exchange interaction between band conduction electrons and local magnetic moments of the ions. © 2015 Elsevier B.V.