Phase-fluctuating superconductivity in overdoped La2−xSrxCuO4

Abstract

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 incoherent 1-4 . 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 state 5-10 . 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 pseudogap 11 , in the slightly overdoped region of the phase diagram where the superfluid pair density and correlation energy are both maximal 12 . Here we show, by tracking the restoration of the normal-state magnetoresistance in overdoped La 2−x Sr x CuO 4 , 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 density 1 , a characteristic of both underdoped and overdoped cuprates 12-14 . The formation of the pseudogap, by inference, is therefore both independent of and distinct from superconductivity.In underdoped cuprates, an energy gap (pseudogap), of as yet unknown origin, appears in the electronic density of states well before superconductivity develops. The continuous evolution 6 of the pseudogap into the superconducting gap, combined with similarities in their gap magnitudes and symmetries 5 , has led to suggestions that the pseudogap is a precursor superconducting state 7-10 characterized by a broad temperature region over which the superconducting order parameter is finite but the phase fluctuates 3,4 . This picture of precursor pairing remains controversial however and is challenged by measurements indicating that the pseudogap itself closes at a critical doping concentration 11 , slightly beyond optimal doping, where superconductivity is most robust 12 . As it stands, there have been very few studies to date of the fluctuating superconductivity in overdoped, superconducting cuprates to establish the precise relationship between the pseudogap and phase fluctuation regimes.To address this, we focus here on the evolution of the upper critical field H c2 with temperature, as inferred by measurements of the in-plane resistivity ρ ab (T ,H ) in pulsed and d.c. magnetic fields. Previous transport studies in cuprates identified H c2 (T ) with the 'knee' at or near the top of the ρ ab (T ) curve or the temperature at which ρ ab (H ) attained a certain fraction of the normal-state value 15 . Either technique would lead to a H c2 (T ) line with a markedly different shape from that determined by the Nernst effect, for example. In this study, we adopt an alternative approach 16,17 , using the evolution of the transverse magnetoresistance ρ ab (H )(= ρ ab (H )− ρ ab (0) with H c) with temperature, field and doping, as ...

dc Josephson Current Between an Isotropic and a d-Wave or Extended s-Wave Partially Gapped Charge Density Wave Superconductor

“…Another viewpoint consists in relating pseudogaps to those competing orderings, but treating them, on the equal footing with superconductivity, as well-developed states that can be made allowance for in the mean field approximation, fluctuation effects being non-crucial. We believe that the available observations support the latter viewpoint (see, e.g., recent experimental evidences of CDW formation in various cuprates [202][203][204][205]). Moreover, although undoped cuprates are antiferromagnetic insulators [206], the CDW seems to be a more suitable candidate responsible for the pseudogap phenomena, which competes with Cooper pairing in doped high-T c oxide samples [123][124][125][126][127], contrary to what is the most probable for iron-based pnictides and chalcogenides [78,207].…”

Disorder and superfluid density in overdoped cuprate superconductors

“…19 The mean-field model considered here does not include fluctuation effects, which are known to be important in the cuprates [11][12][13][14][15][16][17][18][19][20][21][22] and are probably responsible for the downturns in the experimentally observed ρ s (T ) on the approach to T c . 64 Nevertheless, one way in which the underlying T c0 might become visible in experiments would be as rare regions in which the local disorder level is lower than average.…”

Transport near a quantum critical point in BaFe2(As1−xPx)2

“…At low fields, this fit deviates from the quadratic dependence in the near-optimally doped samples, even at temperatures T > T c , although the deviation ostensibly disappears at sufficiently high T (see Supplementary Section A). This may be due to a complex balance of mobilities between electron and hole pockets or due to superconducting fluctuations persisting at T > T c , as seen in cuprate materials 3,4 . Thus, ρ 0,T is slightly overestimated, causing an offset in the zero-field resistance estimate δ R ≡ (ρ exptl − ρ fit )| 0,T .…”

“…1 Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA, 2 Geballe Laboratory for Advanced Materials and Department of Applied Physics, Stanford University, California 94305, USA, 3 National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310, USA, 4 Department of Physics, University of California, Berkeley, California 94720, USA, 5 Department of Materials Science and Engineering, Stanford University, California 94305, USA, 6 National High Magnetic Field Laboratory, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA, 7 H. H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK, 8 High Field Magnet Laboratory, Institute of Molecules and Materials, Radboud University Nijmegen, Toernooiveld 7, 6525 ED Nijmegen, Netherlands. *e-mail: analytis@berkeley.edu of contributions from all Fermi surfaces.…”