100 research outputs found
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- 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 line, which shows a
sharp reduction towards at a mildly overdoped critical
concentration . This model explains anomalous doping and temperature
dependences of (superconducting carrier density / effective mass)
observed in several overdoped HTSC systems. We point out an analogy to
superfluid He/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 around it as
originally suggested by Nachumi et al. Here 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 ,
if we use the value = 18.3 angstrom, we find that the in-plane
superfluid decreases with increasing x and vanishes near in the
analytical approximation, and near in the numerical approach.
is quite sensitive to , whose value is not widely agreed upon.
The model also predicts a peak in the real part of the conductivity,
Re, at concentrations , and low frequencies,
and a variation of critical current density with x of the form near percolation, where 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 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 . At hole doping the
pairs break only for a NN-repulsion as large as . 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 (superconducting carrier density /
effective mass) reveals universal correlations between Tc and 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
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