Coulomb blockade in strongly underdoped, strained LaSrCuO films

Many recent studies reveal the development of electronic granularity on the approach to superconductor-insulator transition in two dimensional superconductors. The role of inhomogeneity is particularly important for the understanding of the emergence of superconductivity in underdoped cuprates, which appear to be inherently inhomogeneous systems, possibly as a result of local character of pairing.

In this work we study the temperature dependence of resistance and of current-voltage characteristics for underdoped La_1.952Sr_0.048CuO₄ films with different thickness and different degree of built-in compressive strain, induced by the lattice mismatch between the film and the substrate. Strain produces inhomogeneous superconducting (SC) state in the films, evidenced by two-step SC transition, and lack of  the global phase coherence on cooling towards T=0. Instead, a reentrant increase of resistance appears, reaching several orders of magnitude in the thinner, more strongly strained film, and saturating at lowest temperatures. The saturation is accompanied by Coulomb blockade, which resembles blockade reported for Josephson junction arrays. The observation of Coulomb blockade indicates that Cooper pairs survive in the insulating state, suggesting the local character of pairing.