Fluctuation conductivity above upper critical field in ultrathin niobium films*

Fluctuating Cooper pairs surviving above upper critical field in two dimensional superconductors are predicted to introduce negative contribution to conductivity in the low temperature limit [1]. Here we report on the observation of such negative contribution in ultrathin Nb films [2]. The films, of thickness d ranging from 1.2 nm to 20 nm, undergo a transition from amorphous to polycrystalline structure at d about 3.3 nm. The superconductor-metal transition induced by magnetic field is observed, with the critical field approximately constant or decreasing as a power-law with the film conductance in polycrystalline or amorphous films, respectively. The scaling analysis indicates distinct scaling exponents in these two types of films. Negative contribution of the SC fluctuations to conductivity, well described by theoretical predictions, is particularly pronounced in amorphous films. Our observations suggest the development of local inhomogeneities in the amorphous films, in the form of proximity-coupled superconducting islands. [1] A. A. Varlamov, et al., Rev. Mod. Phys. 90, 015009 (2018). [2] I. Zaytseva et al., Sci. Rep. 10, 19062 (2020)