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.

No abstract

The diffusion of vortices down a thermal gradient produces a Josephson signal which is detected as a vortex Nernst effect. In a recent report by Xu et al. ͓Nature 406, 486 ͑2000͔͒, an enhanced Nernst signal identified with vortex-like excitations was observed in a series of La 2Ϫx Sr x CuO 4 ͑LSCO͒ crystals at temperatures 50-100 K above T c . To pin down the onset temperature T of the vortexlike signal in the lightly doped regime (0.03рxр0.07), we have reanalyzed the carrier contribution to the Nernst signal in detail. By supplementing Nernst measurements with thermopower and Hall-angle data, we isolate the off-diagonal Peltier conductivity ␣ xy and show that its profile provides an objective determination of T . With the results, we revise the phase diagram for the fluctuation regime in LSCO to accommodate the lightly doped regime. In the cuprate Bi 2 Sr 2Ϫy La y CuO 6 , we find that the carrier contribution is virtually negligible for y in the range 0.4 -0.6. The evidence of an extended temperature interval with vortexlike excitations is even stronger in this system. Finally, we discuss how T relates to the pseudogap temperature T* and the implications of strong fluctuations between the pseudogap state and the d-wave superconducting state.

No abstract

We have performed a systematic angle-resolved photoemission spectroscopy (ARPES) study of the high-T c cuprates La 2−x Sr x CuO 4 , ranging from the underdoped insulator to the superconductor to the overdoped metal. We have revealed a systematic doping evolution of the band dispersions and (underlying) Fermi surfaces, pseudogap and quasi-particle features under the influence of strong electron-electron interaction and electron-phonon interaction. The unusual transport and thermodynamic properties are explained by taking into account the pseudogap opening and the Fermi arc formation, due to which the carrier number decreases as the doped hole concentration decreases.

Lightly doped La2-xSrxCuO4 in the so-called "insulating" spin-glass phase has been studied by angle-resolved photoemission spectroscopy. We have observed that a "quasiparticle" (QP) peak crosses the Fermi level in the node direction of the d-wave superconducting gap, forming an "arc" of Fermi surface, which explains the metallic behavior at high temperatures of the lightly doped materials. The QP spectral weight of the arc smoothly increases with hole doping, which we attribute to the n approximately x behavior of the carrier number in the underdoped and lightly doped regions.

The electronic structure of the La2−xSrxCuO4 (LSCO) system has been studied by angle-resolved photoemission spectroscopy (ARPES). We report on the evolution of the Fermi surface, the superconducting gap and the band dispersion around the extended saddle point k = (π, 0) with hole doping in the superconducting and metallic phases. As hole concentration x decreases, the flat band at (π, 0) moves from above the Fermi level (EF) for x > 0.2 to below EF for x < 0.2, and is further lowered down to x = 0.05. From the leading-edge shift of ARPES spectra, the magnitude of the superconducting gap around (π, 0) is found to monotonically increase as x decreases from x = 0.30 down to x = 0.05 even though Tc decreases in the underdoped region, and the superconducting gap appears to smoothly evolve into the normal-state gap at x = 0.05. It is shown that the energy scales characterizing these low-energy structures have similar doping dependences. For the heavily overdoped sample (x = 0.30), the band dispersion and the ARPES spectral lineshape are analyzed using a simple phenomenological self-energy form, and the electronic effective mass enhancement factor m * /m b ≃ 2 has been found. As the hole concentration decreases, an incoherent component that cannot be described within the simple self-energy analysis grows intense in the high-energy tail of the ARPES peak. Some unusual features of the electronic structure observed for the underdoped region (x < ∼ 0.10) are consistent with the numerical works on the stripe model.

The stripe order in La 2Ϫx Sr x NiO 4ϩ␦ with 0.289ՇxՇ0.5 was studied with neutron-scattering technique. At low temperatures, all samples exhibit hole stripe order. Incommensurability ⑀ of the stripe order is approximately linear in the hole concentration n h ϭxϩ2␦ up to xϭ1/2, where ␦ denotes the off stoichiometry of oxygen atoms. The charge and spin ordering temperatures exhibit maxima at n h ϭ