C‐axis transport of pnictide superconductors

Abstract

The generic structure of most high Tc superconductors is a stacking sequence of superconducting planes separated by so‐called charge reservoir layers. It is well known that carrier doping of these materials is achieved either by substitution of atoms or by nonstoichiometry in the charge reservoir layer. The alternating type of stacking causes yet another two important consequences. First, the transport anisotropy of these materials can be so high, that in the superconducting state the c‐axis transport is governed by the intrinsic Josephson effect. Whereas the anisotropy of our investigated LaO1–xFxFeAs samples was not sufficient, optimum‐doped Ca10(FeAs)10Pt4As8 showed hysteretic c‐axis current–voltage characteristics. Together with the Ambegaokar–Baratoff like temperature dependence of the critical current, this is a strong indication of intrinsic Josephson effects. Second, it is possible to deposit in the charge reservoir layers a substantial amount of charge only by injecting large c‐axis currents. By charge compensation, this decreases the concentration of mobile electrons in the conducting layers of electron‐doped materials. We were able to verify this in all details by c‐axis transport measurements of LaO1−xFxFeAs single crystals and pure and Pt doped (CaFe1–xPtxAs)10Pt4As8 single crystals. After current injection, we observed a decrease of Tc for doping levels at or below the Tc maximum, and a Tc increase for doping levels beyond the maximum. In all cases, the resistivity of the samples increased significantly. In both material classes, heavily overdoped samples showed a spectacular Tc increase by more than 10 K only accomplished by carrier injection.Generic stacking sequence of layered superconductors and crystal structure of the pnictide LaOFeAs.