96 research outputs found
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
Structure of Charge Density Waves in LaBaCuO
Although charge-density wave (CDW) correlations exist in several families of
cuprate supercon-ductors, they exhibit substantial variation in CDW wavevector
and correlation length, indicating a key role for CDW-lattice interactions. We
investigated this interaction in LaBaCuO using single
crystal x-ray diffraction to collect a large number of CDW peak intensities,
and determined the Cu and La/Ba atomic distortions induced by the formation of
CDW order. Within the CuO planes, the distortions involve a periodic
modulation of the Cu-Cu spacing along the direction of the ordering wave
vector. The charge ordering within the copper-oxygen layer induces an
out-of-plane breathing modulation of the surrounding lanthanum layers, which
leads to a related distortion on the adjacent copper-oxygen layer. Our result
implies that the CDW-related structural distortions do not remain confined to a
single layer but rather propagate an appreciable distance through the crystal.
This leads to overlapping structural modulations, in which CuO planes
exhibit distortions arising from the orthogonal CDWs in adjacent layers as well
as distortions from the CDW within the layer itself. We attribute this striking
effect to the weak c-axis charge screening in cuprates and suggest this effect
could help couple the CDW between adjacent planes in the crystal.Comment: 9 pages; Accepted in Phys. Rev.
Density functional electronic spectrum of the cluster and possible local Jahn-Teller distorsions in the La-Ba-Cu-O superconductor
We present a density functional theory (DFT) calculation in the generalized
gradient approximation to study the possibility for the existence of
Jahn-Teller (JT) or pseudo Jahn-Teller (PJT) type local distortions in the
La-Ba-Cu-O superconducting system. We performed the calculation and
correspondingly group theory classification of the electronic ground state of
the CuO elongated octahedra cluster, immersed in a background
simulating the superconductor. Part of the motivation to do this study is that
the origin of the apical deformation of the CuO cluster is not
due to a pure JT effect, having therefore a non {\it a priori} condition to
remove the degeneracy of the electronic ground state of the parent regular
octahedron. We present a comparative analysis of the symmetry classified
electron spectrum with previously reported results using unrestricted
Hartree-Fock calculations (UHF). Both the DFT and UHF calculations produced a
non degenerate electronic ground state, not having therefore the necessary
condition for a pure JT effect. However, the appearance of a degenerate E
state near to the highest occupied molecular orbital in the DFT calculation,
suggests the possibility for a PJT effect responsible for a local distortion of
the oxidized CuO cluster.Comment: 12 pages, 3 figures, submitted to International Journal of Modern
Physics B (IJMPB
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
Stacking disorder in -RuCl via x-ray three-dimensional difference pair distribution function analysis
The van der Waals layered magnet -RuCl offers tantalizing
prospects for the realization of Majorana quasiparticles. Efforts to understand
this are, however, hampered by inconsistent magnetic and thermal transport
properties likely coming from the formation of structural disorder during
crystal growth, postgrowth processing, or upon cooling through the first order
structural transition. Here, we investigate structural disorder in
-RuCl using x-ray diffuse scattering and three-dimensional
difference pair distribution function (3D-PDF) analysis. We develop a
quantitative model that describes disorder in -RuCl in terms of
rotational twinning and intermixing of the high and low-temperature structural
layer stacking. This disorder may be important to consider when investigating
the detailed magnetic and electronic properties of this widely studied
material.Comment: 6 pages; 3 figures; accepted in Physical Review
Peak positions and shapes in neutron pair correlation functions from powders of highly anisotropic crystals
The effect of the powder average on the peak shapes and positions in neutron
pair distribution functions of polycrystalline materials is examined. It is
shown that for highly anisotropic crystals, the powder average leads to shifts
in peak positions and to non-Gaussian peak shapes. The peak shifts can be as
large as several percent of the lattice spacing
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
- …