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 ${d}_{F}\ensuremath{\sim}2000\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ using the technique of pulsed laser ablation are reported. The nonequilibrium sc phase of NbN stabilizes on (100) MgO at elevated deposition temperatures ${T}_{D}$ $(g200\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C})$, and over a restricted range of nitrogen pressure $({p}_{{\mathrm{N}}_{2}})$. The normal-state resistivity ${\ensuremath{\rho}}_{N}(T)$, 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 ${\ensuremath{\rho}}_{N}(T)$ on cooling before the onset of ${T}_{c}$ is analyzed in the framework of a grain boundary model in which electron transmittivity $\ensuremath{\Gamma}$ across the grains is considered explicitly. The zero-field ${J}_{c}$ deduced from screening measurements follows a temperature dependence of the type ${J}_{c}(T)\ensuremath{\sim}{(1\ensuremath{-}T∕{T}_{c})}^{\ensuremath{\beta}}$, with $\ensuremath{\beta}$ increasing from 0.75 to 1.2 as the $d{\ensuremath{\rho}}_{N}(T)∕dT$ changes sign from negative to positive. The films with negative $d{\ensuremath{\rho}}_{N}(T)∕dT$ 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.