Anomalously sharp superconducting transitions in overdoped La_2−xSr_xCuO_4 films

Abstract

We present measurements of ab-plane resistivity ρ_ab(T) and superfluid density (n_s∝λ^−2, λ=magnetic penetration depth) in La_2−xSr_xCuO_4 (LSCO) films. As Sr concentration x exceeds about 0.22, the superconducting transition sharpens dramatically, becoming as narrow as 200 mK near the super-to-normal-metal quantum critical point. At the same time, λ^−2(0) and transition temperature T_c decrease, and upward curvature develops in λ^−2(T). Given the sharp transitions, we interpret these results in the context of a homogeneous d-wave superconducting state, not a phase-separated state, with elastic scattering that is enhanced relative to underdoped LSCO due to weaker electron correlations.