T4toT2resistivity transition and superconducting fluctuations in disordered VN films

Thin films of Vn have been prepared with a wide range of structural disorder as characterized by residual resistivities from the clean value of ${\rho }_0$=5.0 μΩ cm up to 100 μΩ cm. The disorder is primarily a consequence of nitrogen vacancies in as-made films or α-particle damage of clean, stoichiometric VN. We have observed a transition in the power-law dependence of the low-temperature resistivity from $T^4$ to $T^2$ as disorder increases, consistent with the universal behavior of strong-coupled superconductors proposed by Gurvitch. Comparison of this transition with the universal plot leads to an estimate of the electron-phonon coupling ${\lambda }_p$ between 1.0 and 1.1. Disordered films also display an enhanced conductivity several degrees above $T_c$ which is well described by a model of three-dimensional (3D) superconducting fluctuations. This represents one of the few cases where fluctuation enhanced conductivity has been observed in 3D films, other cases being the high-$T_c$ oxides such as ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$.