10 research outputs found
Pressure Studies on a High- Superconductor Pseudogap and Critical Temperatures
We report simultaneous hydrostatic pressure studies on the critical
temperature and on the pseudogap temperature performed through
resistivity measurements on an optimally doped high- oxide
. The resistivity is measured as
function of the temperature for several different applied pressure below 1GPa.
We find that both and increases linearly with the pressure. This
result demonstrate that the well known intrinsic pressure effect on is
also present at and both temperatures are originated by the same
superconducting mechanism.Comment: 4 pages and 2 figures in eps, final versio
Structural and Magnetic Instabilities of LaSrCaCuO
A neutron scattering study of nonsuperconducting
LaSrCaCuO (x=0 and 0.2), a bilayer copper oxide without CuO
chains, has revealed an unexpected tetragonal-to-orthorhombic transition with a
doping dependent transition temperature. The predominant structural
modification below the transition is an in-plane shift of the apical oxygen. In
the doped sample, the orthorhombic superstructure is strongly disordered, and a
glassy state involving both magnetic and structural degrees of freedom develops
at low temperature. The spin correlations are commensurate.Comment: published versio
Theory of the Diamagnetism Above the Critical Temperature for Cuprates
Recently experiments on high critical temperature superconductors has shown
that the doping levels and the superconducting gap are usually not uniform
properties but strongly dependent on their positions inside a given sample.
Local superconducting regions develop at the pseudogap temperature () and
upon cooling, grow continuously. As one of the consequences a large diamagnetic
signal above the critical temperature () has been measured by different
groups. Here we apply a critical-state model for the magnetic response to the
local superconducting domains between and and show that the
resulting diamagnetic signal is in agreement with the experimental results.Comment: published versio
Pinning of stripes by local structural distortions in cuprate high-Tc superconductors
We study the spin-density wave (stripe) instability in lattices with mixed
low-temperature orthorhombic (LTO) and low-temperature tetragonal (LTT) crystal
symmetry. Within an explicit mean-field model it is shown how local LTT regions
act as pinning centers for static stripe formation. We calculate the
modulations in the local density of states near these local stripe regions and
find that mainly the coherence peaks and the van Hove singularity (VHS) are
spatially modulated. Lastly, we use the real-space approach to simulate recent
tunneling data in the overdoped regime where the VHS has been detected by
utilizing local normal state regions.Comment: Conference proceedings for Stripes1
A Theory for High- Superconductors Considering Inhomogeneous Charge Distribution
We propose a general theory for the critical and pseudogap
temperature dependence on the doping concentration for high- oxides,
taking into account the charge inhomogeneities in the planes. The well
measured experimental inhomogeneous charge density in a given compound is
assumed to produce a spatial distribution of local . These differences
in the local charge concentration is assumed to yield insulator and metallic
regions, possibly in a stripe morphology. In the metallic region, the
inhomogeneous charge density yields also spatial distributions of
superconducting critical temperatures and zero temperature gap
. For a given sample, the measured onset of vanishing gap
temperature is identified as the pseudogap temperature, that is, , which
is the maximum of all . Below , due to the distribution of
's, there are some superconducting regions surrounded by insulator or
metallic medium. The transition to a superconducting state corresponds to the
percolation threshold among the superconducting regions with different
's. To model the charge inhomogeneities we use a double branched
Poisson-Gaussian distribution. To make definite calculations and compare with
the experimental results, we derive phase diagrams for the BSCO, LSCO and YBCO
families, with a mean field theory for superconductivity using an extended
Hubbard Hamiltonian. We show also that this novel approach provides new
insights on several experimental features of high- oxides.Comment: 7 pages, 5 eps figures, corrected typo
Upper critical field calculations for the high critical temperature superconductors considering inhomogeneities
We perform calculations to obtain the curve of high temperature
superconductors (HTSC). We consider explicitly the fact that the HTSC possess
intrinsic inhomogeneities by taking into account a non uniform charge density
. The transition to a coherent superconducting phase at a critical
temperature corresponds to a percolation threshold among different
superconducting regions, each one characterized by a given .
Within this model we calculate the upper critical field by means of an
average linearized Ginzburg-Landau (GL) equation to take into account the
distribution of local superconducting temperatures . This
approach explains some of the anomalies associated with and why
several properties like the Meissner and Nernst effects are detected at
temperatures much higher than .Comment: Latex text, add reference
Study of the negative thermal expansion of cuprite-type structures by means of temperature-dependent pair distribution function analysis : Preliminary results
Copper (I) and silver (I) oxides crystallize with the same structure, and both show a wide range of negative thermal expansion (NTE): Cu2O contracts with temperature up to about 200 K and then expands, while Ag2O has a NTE up to its decomposition temperature at about 450 K. Here we report a careful temperature-dependent pair distribution function (PDF) analysis that showed that copper oxide, at about 200 K, exhibits geometric distortions of the tetrahedral units, probably related to a change in the solid angle of the polyhedra. Silver oxide, on the other hand, showed the same distortions even at the lowest temperature measured (10 K): structural refinements of the PDF confirmed the presence of local distortions (below 10 \uc5) at all temperatures