Superconducting transition, fluctuation, and vortex motion in a two-dimensional single-crystal Nb film

We report a detailed study on the transport properties of an ultrathin single-crystal Nb film, whose thickness (d≊20 Å) is less than all other relevant length scales except the lattice constant. The two-dimensional (2D) nature of the sample in the superconducting state is reflected in the characteristic distinction between its parallel and perpendicular critical fields, as well as in the temperature dependence of the fluctuation conductivity. Because of the long mean free path of this single-crystal film, we find it necessary to include both Maki-Thompson and Aslamazov-Larkin terms in the fluctuation-conductivity analysis. The logarithmic temperature dependence of the resistance, signature of the 2D electron-electron interaction effect, has been observed when the external magnetic fields exceed the upper critical field, indicating that the sample is also two dimensional in its normal state. In addition, dissipation below ${\mathit{T}}_{\mathit{c}}$ under small applied current is attributed to vortex motion; activation energy as a function of magnetic field has been obtained via the application of the Anderson-Kim model extended to two dimensions. The characteristics of the Kosterlitz-Thouless transition, which has been observed only in high-resistance 2D superconducting films, have been seen in our relatively low-resistance Nb film, although they are greatly affected by the presence of strong pinning. This work provides insight into the non- equilibrium properties of clean 2D superconducting films.