On the possibility of fast vortices in the cuprates: A vortex plasma model analysis of THz conductivity and diamagnetism in La2−xSrx

Abstract

We present measurements of the fluctuation superconductivity in an underdoped thin film of La1.905Sr0.095CuO4 using time-domain THz spectroscopy. We compare our results with measurements of diamagnetism in a similarly doped crystal of La2−xSrxCuO4. We show through a vortexplasma model that if the fluctuation diamagnetism solely originates in vortices, then they must necessarily exhibit an anomalously large vortex diffusion constant, which is more than two orders of magnitude larger than the Bardeen-Stephen estimate. This points to either the extremely unusual properties of vortices in the under-doped d-wave cuprates or a contribution to the diamagnetic response that is not superconducting in origin.Nearly 25 years after the demonstration of hightemperature superconductivity in the cuprate superconductors and more than 15 years since the discovery of the anomalous pseudogap in underdoped compounds, the microscopic physics of the superconducting phase and its relationship to the pseudogap remain hotly debated. Due to their low superfluid densities, it is generally agreed that superconducting fluctuations will be large and prominent in these materials [1]. What is less agreed upon is the temperature range above T c in which superconducting correlations are truly significant and their contributions to the physics of the pseudogap. Experimental probes such as photoemission, tunneling, NMR spin relaxation, heat capacity, the Nernst effect, and diamagnetic susceptibility have shown evidence for a gaplike structure reminiscent of d-wave superconductivity in the density of states implying a strong connection of the pseudogap to superconductivity and/or superconducting correlations at temperatures well above T c [2-6]. However, other mechanisms exist that can create such structures in the density of states [7,8].Interestingly, perhaps the most essential probe of the electronic properties -charge transport -does not show an extended range of superconducting fluctuations in temperature or field. [9][10][11]. In La 2−x Sr x CuO 4 the region of enhanced diamagnetism extends almost 100 K above T c [6] while the THz fluctuation conductivity has an extent limited to 10 -20 K above T c [12]. This is surprising as one might expect a close correspondence between these quantities [13]. Similarly, it has been argued from Nernst and diamagnetism measurements that H c2 may be as high as 150 T [6], while the resistive transition is essentially complete in optimally and underdoped LSCO by 45 T [10,11].In this Rapid Communication we present results of our detailed THz time-domain spectroscopy (TTDS) study of the fluctuation superconductivity in LSCO. The THz fluctuation conductivity shows an onset approximately only 10 K above T c , which contrasts strongly with measurements like diamagnetism in which the onset is approximately 100K above T c . We analyze our data in the context of a vortex plasma model and show, however that it is not the functional dependences of these data that are in strongest contrast, but their overall scales. Conve...

Evolution of superconducting correlations within magnetic-field-decoupled La2−xBaxCuOStegen1, Han2, Wu3 et al. 2013Phys. Rev. B338320View full textAdd to dashboardCite

“…11,78 Along with other recent work, 14,15 our results suggest that such a scenario is overly optimistic. The rise of ρ on warming through T 2D c indicates a loss of uniform superconducting correlations in the normal state.…”

“…Josephson coupling between the layers leads to the onset of three-dimensional (3D) superconductivity as soon as the correlation length for superconducting order within the layers becomes sizable. 18,19 The recent observations [12][13][14][15][16] that strong superconducting correlations appear only in a regime that is reasonably close to T c are consistent with the expectation that 3D order should appear as soon as 2D superconducting correlations become substantial. 18 In fact, in a previous study of La 2−x Ba x CuO 4 (LBCO) with x = 0.095, evidence was found indicating that superconducting correlations between layers start to develop locally before superconducting correlations diverge within the layers.…”

“…2-8 This work motivated theoretical suggestions 9-11 of a possible phase-disordered twodimensional (2D) superconducting state (2D vortex liquid) that might exist above T c . In contrast, recent studies of superconducting contributions to magnetoresistance 12 and low-frequency optical conductivity, 13,14 as well as further torque magnetometry studies, 15,16 indicate that strong superconducting correlations are found only within a relatively narrow range (∼10 K) above T c ; the response at higher temperatures is quite weak relative to expectations for a 2D vortex-liquid state. 17 There is general agreement that pairing interactions within the CuO 2 layers are responsible for the development of superconducting correlations in the cuprates.…”

Current-current Fermi-liquid corrections to the superconducting fluctuations on conductivity and diamagnetism

“…Since one would expect that the same length scale is involved in both cases, ξ 2 σ /ξ 2 χ d = 1, right above T c , δσ/δχ d should be of order ∼10 5 ( A/T) −1 , while experimentally it turns out to be two orders of magnitude smaller than this 1,13 [see also the discussion below Eq. (36)].…”

“…If we did not consider the interactions in the p-h channel leading to the HF correction of the band dispersion, we would get ξ k = ξ 0 k [see Eq. (13)] and the velocity would coincide with the current, so that c = c . In this case, as mentioned above, the electromagnetic potential A is coupled only with the pairing field via a term ∼cAp 2 p , so that the fluctuation correction to the current-current response function (20) is given by the usual AL contribution 9 (see Fig.…”