Charge Segregation and Antiferromagnetism in High-Tc Superconductors

Local antiferromagnetism coexists with superconductivity in the cuprates. Charge segregation provides a way to reconcile these properties. Direct evidence for modulated spin and charge densities has been found in neutron and X-ray scattering studies of Nd-doped La(2-x)Sr(x)CuO(4). Here we discuss the nature of the modulation, and present some new results for a Zn-doped sample. Some of the open questions concerning the connections between segregation and superconductivity are described.Comment: 9 pp using elsart.sty, 3 eps figures included with psfig.sty, for Proc. of ISSP-7, to be published in J. Phys. Chem. Solid

Neutron scattering studies of Zn-doped La2-xSrxCuO4

This paper reviews studies of the spatial modulation of spin in La2-xSrxCuO4 (LSCO) through neutron-scattering measurements of the elastic magnetic peaks, with emphasis on Zn-doped LSCO. The elastic incommensurate magnetic peaks can be categorized in two classes with respect to the temperature dependence. Nd-doped LSCO, La2CuO4+y and LSCO with x ~ 1/8 exhibit an ordinary power law behavior. On the other hand, Zn-doped LSCO and LSCO with x = 0.02-0.07 show an exponential temperature dependence, and the correlation lengths stay short. This discrepancy can be attributed to the difference in the mechanism of pinning spin fluctuations. The former systems have pinning centers which is coherent to the lattice. In the latter, however, pinning centers are randomly scattered in the CuO2 planes, thus incoherent. The incoherent pinning model is discussed with referring to recent mu-SR studies.Comment: 6 pages, including 4 figures and 1 table. Proceedings for ISS2000 (Tokyo, October 2000

Effect of spatial variations of superconducting gap on suppression of the transition temperature by impurities

We calculate correction to the critical temperature of a dirty superconductor, which results from the local variations of the gap function near impurity sites. This correction is of the order of T_c/E_F and becomes important for short-coherence length superconductors. It generally reduces a pair-breaking effect. In s-wave superconductors small amounts of nonmagnetic impurities can increase the transition temperature.Comment: 5 pages, ReVTE

The Superconductivity, Intragrain Penetration Depth and Meissner Effect of RuSr2(Gd,Ce)2Cu2O10+delta

The hole concentration (p)(delta), the transition temperature Tc, the intragrain penetration depth lambda, and the Meissner effect were measured for annealed RuSr2(Gd,Ce)2Cu2O10+delta samples. The intragrain superconducting transition temperature Tc} varied from 17 to 40 K while the p changed by only 0.03 holes/CuO2. The intragrain superfluid-density 1/lambda^2 and the diamagnetic drop of the field-cooled magnetization across Tc (the Meissner effect), however, increased more than 10 times. All of these findings are in disagreement with both the Tc vs. p and the Tc vs. 1/lambda^2 correlations proposed for homogeneous cuprates, but are in line with a possible phase-separation and the granularity associated with it.Comment: 7 pages, 6 figures, accepted for publication in Phys. Rev. B (May 2, 2002

Microscopic phase separation in the overdoped region of high-Tc cuprate superconductors

We propose a phenomenological model for high-$T_{c}$ superconductors (HTSC) assuming: (1) a microscopic phase separation between superconducting and normal-metal areas in the overdoped region; and (2) existence of a homogeneous superconducting phase only below the pseudo-gap $T^{*}$ line, which shows a sharp reduction towards $T^{*}\sim 0$ at a mildly overdoped critical concentration $x_{c}$. This model explains anomalous doping and temperature dependences of $n_{s}/m^{*}$ (superconducting carrier density / effective mass) observed in several overdoped HTSC systems. We point out an analogy to superfluid $^{4}$He/$^{3}$He films, and discuss an energetic origin of microscopic phase separation.Comment: 5 pages, 4 figure

Simple Model for the Variation of Superfluid Density with Zn Concentration in YBCO

We describe a simple model for calculating the zero-temperature superfluid density of Zn-doped YBa_2Cu_3O_{7-\delta} as a function of the fraction x of in-plane Cu atoms which are replaced by Zn. The basis of the calculation is a ``Swiss cheese'' picture of a single CuO_2 layer, in which a substitutional Zn impurity creates a normal region of area $\pi\xi_{ab}^2$ around it as originally suggested by Nachumi et al. Here $\xi_{ab}$ is the zero-temperature in-plane coherence length at x = 0. We use this picture to calculate the variation of the in-plane superfluid density with x at temperature T = 0, using both a numerical approach and an analytical approximation. For $\delta = 0.37$, if we use the value $\xi_{ab}$ = 18.3 angstrom, we find that the in-plane superfluid decreases with increasing x and vanishes near $x_c = 0.01$ in the analytical approximation, and near $x_c = 0.014$ in the numerical approach. $x_c$ is quite sensitive to $\xi_{ab}$, whose value is not widely agreed upon. The model also predicts a peak in the real part of the conductivity, Re$\sigma_e(\omega, x)$, at concentrations $x \sim x_c$, and low frequencies, and a variation of critical current density with x of the form $J_c(x) \propto n_{S,e}(x)^{7/4}$ near percolation, where $n_{S,e}(x)$ is the in-plane superfluid density.Comment: 19 pages including 6 figures, submitted to Physica

Hole-Pairs in a Spin Liquid: Influence of Electrostatic Hole-Hole Repulsion

The stability of hole bound states in the t-J model including short-range Coulomb interactions is analyzed using computational techniques on ladders with up to $2 \times 30$ sites. For a nearest-neighbors (NN) hole-hole repulsion, the two-holes bound state is surprisingly robust and breaks only when the repulsion is several times the exchange $J$. At $\sim 10$ hole doping the pairs break only for a NN-repulsion as large as $V \sim 4J$. Pair-pair correlations remain robust in the regime of hole binding. The results support electronic hole-pairing mechanisms on ladders based on holes moving in spin-liquid backgrounds. Implications in two dimensions are also presented. The need for better estimations of the range and strength of the Coulomb interaction in copper-oxides is remarked.Comment: Revised version with new figures. 4 pages, 5 figure

Superfluid density of high-Tc cuprate systems: implication on condensation mechanisms, heterogeneity and phase diagram

Extensive muon spin relaxation measurements have been performed to determine the magnetic field penetration depth in high-Tc cuprate superconductors with simple hole doping, Zn-doping, overdoping, and formation of static SDW nano islands. System dependence of $n_{s}/m^{*}$ (superconducting carrier density / effective mass) reveals universal correlations between Tc and $n_{s}/m^{*}$ in all these cases with / without perturbation. Evidence for spontaneous and microscopic phase separation was obtained in the cases with strong perturbation, i.e., Zn-doping. overdoping and SDW nano-islands. The length scale of this heterogeneity is shown to be comparable to the in-plance coherence length. We discuss implications of these results on condensation mechanisms of HTSC systems, resorting to an analogy with He films, on regular and porous media, reminding essential features of Bose-Einstein, BCS and Kosterlitz-Thouless condensation/transition in 2-d and 3-d systems, and comparing models of BE-BCS crossover and phase fluctuations. We propose a new phase diagram for HTSC systems based on distinction between pair formation and superconducting phase fluctuations in the pseudogap region and spontaneous phase separation in the overdoped region. We also remind anomaly in BEDT and A3C60 systems similar to that in overdoped cuprates, seen in the evolution from superconducting to metallic ground state.Comment: 21 pages, 18 figures, invited papter presented at the HTSC Workshop, Williamsburg, Virginia, June 7-8, 2002, to appear in Solid State Communications (Special Issue edited by A.J. Millis, S. Uchida, Y.J. Uemura): contact [email protected]

Dimensionality of spin modulations in 1/8-doped lanthanum cuprates from the perspective of NQR and muSR experiments

We investigate the dimensionality of inhomogeneous spin modulation patterns in the cuprate family of high-temperature superconductors with particular focus on 1/8-doped lanthanum cuprates. We compare one-dimensional stripe modulation pattern with two-dimensional checkerboard of spin vortices in the context of nuclear quadrupole resonance(NQR) and muon spin rotation(muSR) experiments. In addition, we also consider the third pattern, a two-dimensional superposition of spin spirals. Overall, we have found that none of the above patterns leads to a consistent interpretation of the two types of experiments considered. This, in particular, implies that the spin vortex checkerboard cannot be ruled out on the basis of available NQR/muSR experimental results.Comment: 6 pages, 2 figure