Normal-state transport and vortex dynamics in thin films of two structural polymorphs of superconducting NbN

Abstract

The normal-state transport and the dynamic vortex response in simple cubic (sc) and face-centered cubic (fcc) polymorphs of NbN synthesized as epitaxial films of thickness d_F~2000 Å using the technique of pulsed laser ablation are reported. The nonequilibrium sc phase of NbN stabilizes on (100) MgO at elevated deposition temperatures T_D (gt;200 °C), and over a restricted range of nitrogen pressure (p_N2). The normal-state resistivity p_N(T), critical temperature (T_c), critical current density [ J_c(T)] and the temperature at which J_c goes to zero (T) depend largely on the extent of nonstoichiometry rather than the crystal symmetry of these films. The divergence of p_N(T) on cooling before the onset of T_c is analyzed in the framework of a grain boundary model in which electron transmittivity G across the grains is considered explicitly. The zero-field J_c deduced from screening measurements follows a temperature dependence of the type J_c(T)~(1-T/T_c)^β;, with β increasing from 0.75 to 1.2 as the dp_N(T)/dT changes sign from negative to positive. The films with negative dp_N(T)/dT are disordered and possibly nonstoichiometric. For stoichiometric films, the temperature dependence of J_c in the mixed state and the variation of the critical temperature T as a function of applied dc field are consistent with the model of thermally activated flux creep.