Vortex glass transition and thermal creep in niobium films

The evolution of the vortex glass (VG) phase transition and vortex creep with decreasing film thickness is studied in ultrathin, polycrystalline niobium films, with thickness in the range 7.4 nm to 44 nm, using current-voltage characteristics measurements in perpendicular magnetic field. Standard methods, including scaling laws, allow to identify VG transition in the thickest film, while in thinner films creep produces large uncertainty in the putative VG transition temperature and scaling exponents. Using strong pinning theory we perform analysis of the creep, and extract the dependence of the activation energy for vortex pinning on temperature, magnetic field, and film thickness. This analysis provides more information on vortex dynamics than the standard evaluation of critical current density. The results reveal two distinct regimes of pinning, which we propose to identify with $\delta l$ or $\delta T_c$-types of pinning (due to spacial fluctuation of mean free path $l$ or spacial fluctuation of superconducting transition temperature $T_c$, respectively). In the thickest film $\delta l$ pinning is observed, but with the decrease of film thickness the second pinning regime appears, and becomes dominant in the thinnest film. We link these pinning regimes with the structural disorder due to grain boundaries, which produce charge carrier scattering in the thickest film, but with decreasing film thickness gradually evolve into amorphous inclusions, producing fluctuations in the $T_c$.