Abstract

In underdoped cuprate superconductors, phase stiffness is low and long-range superconducting order is easily wiped out by thermally-generated vortices. This gives rise to a broad temperature regime above the zero-resistive state in which the superconducting phase is incoherent.

It has often been suggested that these vortex-like excitations are related to the normal-state pseudogap, with the pseudogap described as a kind of precursor superconducting pairing. However, to determine the precise relationship between the pseudogap and superconductivity, it is important to establish whether the broad phase-fluctuation regime vanishes, along with the pseudogap, in the slightly overdoped region of the phase diagram.

I will present measurements tracking the restoration of the normal-state magnetoresistance in overdoped La_(2-x)Sr_(x)CuO_(4), that show that the phase-fluctuation regime remains broad across the entire superconducting composition range [1]. This universal low phase stiffness is correlated with a low superfluid density, a characteristic of both underdoped and overdoped cuprates. The formation of the pseudogap, by inference, is therefore both independent of and distinct from superconductivity.

[1] P.M.C. Rourke et al., Nature Physics 7, 455 (2011).