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 dF∼2000Å using the technique of pulsed laser ablation are reported. The nonequilibrium sc phase of NbN stabilizes on (100) MgO at elevated deposition temperatures TD (>200°C), and over a restricted range of nitrogen pressure (pN2). The normal-state resistivity ρN(T), critical temperature (Tc), critical current density [Jc(T)] and the temperature at which Jc goes to zero (T*) depend largely on the extent of nonstoichiometry rather than the crystal symmetry of these films. The divergence of ρN(T) on cooling before the onset of Tc is analyzed in the framework of a grain boundary model in which electron transmittivity Γ across the grains is considered explicitly. The zero-field Jc deduced from screening measurements follows a temperature dependence of the type Jc(T)∼(1−T∕Tc)β, with β increasing from 0.75 to 1.2 as the dρN(T)∕dT changes sign from negative to positive. The films with negative dρN(T)∕dT are disordered and possibly nonstoichiometric. For stoichiometric films, the temperature dependence of Jc 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.

Source-reported materials — not catalogue approval

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