Vortex-like excitations and the onset of superconducting phase fluctuation in underdoped La2-xSrxCuO4

Abstract

Two general features of a superconductor, which appear at the critical temperature, are the formation of an energy gap and the expulsion of magnetic flux (the Meissner effect). In underdoped copper oxides, there is strong evidence that an energy gap (the pseudogap) opens up at a temperature significantly higher than the critical temperature (by 100-220 K). Certain features of the pseudogap suggest that it is closely related to the gap that appears at the critical temperature (for example, the variation of the gap magnitudes around the Fermi surface and their maximum amplitudes are very similar). However, the Meissner effect is absent in the pseudogap state. The nature of the pseudogap state, and its relation (if any) to the superconducting state are central issues in understanding copper oxide superconductivity. Recent evidence suggests that, in the underdoped regime, the Meissner state is destroyed above the critical temperature by strong phase fluctuations (as opposed to a vanishing of the superfluid density). Here we report evidence for vortices (or vortex-like excitations) in La(2-x)Sr(x)CuO4 at temperatures significantly above the critical temperature. A thermal gradient is applied to the sample in a magnetic field. Vortices are detected by the large transverse electric field produced as they diffuse down the gradient (the Nernst effect). We find that the Nernst signal is anomalously enhanced at temperatures as high as 150 K.

Bose–Einstein condensation of strongly correlated electrons and phonons in cuprate superconductors

“…In particular, the bipolaron theory accounts for the anomalously large Nernst signal, the thermopower and the insulating-like in-plane low temperature resistance [47,48] as observed [51,64,65,66].…”

“…Since the Hall mobility of carriers localised below µ, σ l yx , has the sign opposite to that of carries in the extended states above µ, σ ext yx , the sign of the off-diagonal Peltier conductivity α l yx should be the same as the sign of α ext yx . Then neglecting the magneto-orbital effects in the resistivity (since Θ ≪ 1 [51]) we obtain…”

“…Clearly the model, Eqs. (15,16) can account for a low value of S tan Θ compared with a large value of e y in some underdoped cuprates [51,65] due to the sign anomaly. Even in the case when localised bosons contribute little to the conductivity their contribution to the thermopower S = ρ(α ext xx − |α l xx |)) could almost cancel the opposite sign contribution of itinerant carriers [47].…”

Condensation, excitation, pairing, and superfluid density in high-Tc superconductors: the magnetic resonance mode as a roton analogue and a possible spin-mediated pairing

“…2, we include a point (green star) having T F of La 1.9 Sr 0.1 CuO 4 in the horizontal and the Nernst onset T on of the same system [13,14] on the vertical axes, respectively. Interestingly, this point for "hypothetical underdoped LSCO" lies very close to the T BE line, similar to those of bulk 4 He and ultracold 40 K. This suggests that the effect of pair overlapping in the very underdoped region may be modest, and/or the coherence length could be comparable to the intercarrier distance.…”

Study on Unconventional Superconductivity after the BCS Paradigm

“…[15] While recent Nernst measurements reveal a strong signal which may correspond to the existence of the simultaneous vortex excitations. [16] The more direct evidence for the pseudogap is coming from the angle resolved photoemission spectroscopy (ARPES) [17] One appealing picture for the normal state is the gapped feature near the anti-nodal area due to the correlation effect, while the electrons near the nodal point have still finite life-time recovering the meaningful term quasiparticles in this region. This truncated "Fermi surface" is shown in Fig.5.…”