We suggest a simple model describing the temperature-driven crossover between Abrikosov vortex lattice and superconducting droplet state in dirty superconductors with fluctuations either in the impurity concentration or in the crystal axes orientation. Our analysis is based on the Usadel-type theory with a spatially modulated diffusion coefficient. This modulation appears to break a regular vortex lattice into a random set of weakly coupled superconducting droplets emerging below the fluctuating upper critical field $H_{c2}\left(T\right)$. These droplets cause the resistivity drop at the onset of superconducting transition, being responsible for the increasing broadening of the resistive transition in the increasing magnetic field. The above crossover reveals itself in a positive curvature of the $H_{c2}\left(T\right)$ curves, allowing us, thus, to explain the phase diagrams observed in a wide class of disordered superconducting materials.

• Received 1 February 2016
• Revised 27 January 2017

DOI:https://doi.org/10.1103/PhysRevB.95.104520