Berezinskii-Kosterlitz-Thouless Transition in Ultrathin Niobium Films.

We use resistivity and current-voltage characteristics measurements to evaluate the impact of the disorder on the nature of the Berezinskii-Kosterlitz-Thouless transition in ultrathin niobium (Nb) films. The films, with thickness in the range of 3.6-8.5 nm, show the structural transition from polycrystalline to amorphous structure upon a decrease in the film thickness. We show that this transformation results in the smearing of the Berezinskii-Kosterlitz-Thouless transition, until eventually the Berezinskii-Kosterlitz-Thouless scenario breaks down due to film inhomogeneity. [ABSTRACT FROM AUTHOR]