Current–voltage characteristics of strained, highly underdoped LaSr x CuO4 thin films

Abstract

The temperature dependence of resistance and of current–voltage characteristics are studied for two underdoped La1.952Sr0.048CuO4 films with different thickness and different degrees of built-in compressive strain. Strain induces an inhomogeneous superconducting (SC) state in the films, with two-step SC transition, consistent with the presence of SC islands coupled by the proximity effect. On further cooling, superfluid density is suppressed and global phase coherence is never reached. 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 the blockade reported for Josephson junction (JJ) arrays. However, increasing magnetic field quickly diminishes the influence of Coulomb blockade on transport, which is distinct from the behavior observed for JJ arrays. The observation of Coulomb blockade indicates that Cooper pairs survive in the insulating state, suggesting the local character of pairing.