Phase-fluctuating superconductivity in overdoped La2−xSrxCuO4

Abstract

Superconducting phase fluctuations are often associated with the pseudogap phase of the copper-oxide superconductors. However, the same fluctuations exist in the overdoped part of the phase diagram where the pseudogap is absent, suggesting that phase fluctuations are independent of the pseudogap In underdoped cuprate superconductors, phase stiffness is low and long-range superconducting order is destroyed readily by thermally generated vortices (and anti-vortices), giving rise to a broad temperature regime above the zero-resistive state in which the superconducting phase is incoherent1,2,3,4. It has often been suggested that these vortex-like excitations are related to the normal-state pseudogap or some interaction between the pseudogap state and the superconducting state5,6,7,8,9,10. However, to elucidate the precise relationship between the pseudogap and superconductivity, it is important to establish whether this broad phase-fluctuation regime vanishes, along with the pseudogap11, in the slightly overdoped region of the phase diagram where the superfluid pair density and correlation energy are both maximal12. Here we show, by tracking the restoration of the normal-state magnetoresistance in overdoped La2−xSrxCuO4, that the phase-fluctuation regime remains broad across the entire superconducting composition range. The universal low phase stiffness is shown to be correlated with a low superfluid density1, a characteristic of both underdoped and overdoped cuprates12,13,14. The formation of the pseudogap, by inference, is therefore both independent of and distinct from superconductivity.