118 research outputs found
Nominal doping and partition of doped holes between planar and apical orbitals in La2-xSrxCuO4
By considering both the average structural parameters obtained from Rietveld
refinement of neutron powder diffraction data, and the local structural
parameters obtained from the atomic pair distribution function, we have tested
the recent hypothesis of Perry et al [Perry et al., Phys. Rev. B v.65, 144501
(2002)] that doping in La2-xSrxCuO4 system occurs as localized defects of
predominantly Cud3z^2-r^2--Opz character associated with the Sr dopants
accompanied by a local destruction of the Jahn-Teller distortion. While the
structural parameters behave qualitatively according to the prediction of this
model, a quantitative analysis indicates that doped holes predominantly appear
in the planar Cudx^2-y^2--Opx,y band as is normally assumed. However, a small
amount of the doped charge does enter the Cud3z^2-r^2--Opz orbitals and this
should be taken into account when theoretical phase diagrams are compared to
experiment. We present a calibration curve, p = x (1.00(1) - 0.45(7) x), for
the planarcharge doping, p, vs strontium content, x, for the La2-xSrxCuO4
system.Comment: 6 pages, 4 figures, submitted to Phys. Rev.
Structural response to local charge order in underdoped but superconducting La2-x(Sr,Ba)xCuO4
We report an anomalous local structural response in the CuO2 planes
associated with the appearance of charge inhomogeneities at low temperature in
underdoped but superconducting La2-x(Sr,Ba)xCuO4. We used pair distribution
function analysis of neutron powder diffraction data. The increase in the Cu-O
bond length distribution at low temperature has an onset temperature which
correlates with observations of charge and spin freezing seen by other probes.Comment: 4 pages, 3 figure
Microscopic Charge Inhomogeneities in Underdoped La_2-xSr_xCuO_4: Local Structural Evidence
We present local structural evidence for the existence of charge
inhomogeneities at low temperature in underdoped and optimally doped
La_2-xSr_xCuO_4. The inhomogeneities disappear for x > 0.2. The evidence for
the charge inhomogeneities comes from an anomalous increase in the in-plane
Cu-O bond length distribution in the underdoped samples as well as evidence for
CuO_6 octahedral tilt inhomogeneities in the intermediate range structure.
Preliminary analysis of the temperature dependence of this phenomenon indicates
that the inhomogeneities set in at temperatures in the range 60 K < Tco < 130 K
which depends on doping.Comment: 8 pages, 7 figures, to appear in J. Supercond., proceedings of the
conference "Major Trends in Superconductivity in the New Millenium, Klosters,
Switzerland, April 1st-6th 2000
Cu(IrCr)S: a model system for studying nanoscale phase coexistence at the metal-insulator transition
Increasingly, nanoscale phase coexistence and hidden broken symmetry states
are being found in the vicinity of metal-insulator transitions (MIT), for
example, in high temperature superconductors, heavy fermion and colossal
magnetoresistive materials, but their importance and possible role in the MIT
and related emergent behaviors is not understood. Despite their ubiquity, they
are hard to study because they produce weak diffuse signals in most
measurements. Here we propose Cu(IrCr)S as a model system,
where robust local structural signals lead to key new insights. We demonstrate
a hitherto unobserved coexistence of a Ir charge-localized dimer phase
and Cr-ferromagnetism. The resulting phase diagram that takes into account the
short range dimer order, is highly reminiscent of a generic MIT phase diagram
similar to the cuprates. We suggest that the presence of quenched strain from
dopant ions acts as an arbiter deciding between the competing ground states
Phase separation and nanostructuring in the thermoelectric material PbTe1-xSx
The average and local structures of the (PbTe)1-x(PbS)x system of
thermoelectric materials has been studied using the Rietveld and atomic pair
distribution function (PDF) methods. Samples with 0:25 < x are macroscopically
phase separated. Phase separation was suppressed in a quenched x = 0:5 sample
which, nonetheless, exhibited a partial spinodal decomposition. The promising
thermoelectric material with x = 0:16 showed intermediate behavior. Combining
TEM and bulk scattering data suggests that the sample is a mixture of PbTe rich
material and a partially spinodally decomposed phase similar to the quenched
50% sample. This results in a nano-meter scale inhomogeneous material that
accounts for its very low thermal conductivity.Comment: 9 pages, 9 figures, 4 tables, submitted to PR
Charge-screening role of -axis atomic displacements in YBaCuO and related superconductors
The importance of charge reservoir layers for supplying holes to the CuO
planes of cuprate superconductors has long been recognized. Less attention has
been paid to the screening of the charge transfer by the intervening ionic
layers. We address this issue in the case of YBaCuO, where CuO
chains supply the holes for the planes. We present a simple
dielectric-screening model that gives a linear correlation between the relative
displacements of ions along the axis, determined by neutron powder
diffraction, and the hole density of the planes. Applying this model to the
temperature dependent shifts of ions along the axis, we infer a charge
transfer of 5-10% of the hole density from the planes to the chains on warming
from the superconducting transition to room temperature. Given the significant
coupling of -axis displacements to the average charge density, we point out
the relevance of local displacements for screening charge modulations and note
recent evidence for dynamic screening of in-plane quasiparticles. This line of
argument leads us to a simple model for atomic displacements and charge
modulation that is consistent with images from scanning-tunneling microscopy
for underdoped BiSrCaCuO.Comment: 12 pages, 10 figures; final version, accepted for publication in
Phys. Rev.
Neutron powder diffraction atomic pair distribution function analysis using the ARCS chopper spectrometer at the Spallation Neutron Source
The frst neutron powder diffraction based atomic pair distribution functions
(PDFs) are reported from the new wide angular-range chopper spectrometer ARCS
at the Spallation Neutron Source at Oak Ridge National Laboratory. The
spectrometer was run in white-beam mode, with no Fermi chopper. The PDF
patterns of Si, Ni, and Al2O3 were refined using the PDFfit method and the
results compared to data collected at the NPDF diffractometer at Los Alamos
National Laboratory. The resulting fits are of high quality demonstrating that
quantitatively reliable powder diffraction data can be obtained from ARCS when
operated in this configuration.Comment: 4 pages, 5 figures, submitted to Journal of Applied Crystallograph
Room temperature local nematicity in FeSe superconductor
We report pair distribution function measurements of the iron-based
superconductor FeSe above and below the structural transition temperature.
Structural analysis reveals a local orthorhombic distortion with a correlation
length of about 4 nm at temperatures where an average tetragonal symmetry is
observed. The analysis further demonstrates that the local distortion is larger
than the distortion at temperatures where the average observed symmetry is
orthorhombic. Our results suggest that the low-temperature macroscopic nematic
state in FeSe forms from an imperfect ordering of orbital-degeneracy-lifted
nematic fluctuations which persist up to at least 300 K.Comment: 6 pages, 4 figure
Quantum liquid with strong orbital fluctuations: the case of a pyroxene family
We discuss quasi one-dimensional magnetic Mott insulators of the pyroxene
family where spin and orbital degrees of freedom remain tightly bound. We
analyze their excitation spectrum and outline the conditions under which the
orbital degrees of freedom become liberated so that the excitations become
dispersive and the spectral weight shifts to energies much smaller than the
exchange integral.Comment: 5 pages, 5 figures, Supplemental Material, revised versio
Insulating and metallic spin glass in KFeNiSe (0.06 1.44) single crystals
We report electron doping effects by Ni in
KFeNiSe (0.06 1.44) single
crystal alloys. A rich ground state phase diagram is observed. Small amount of
Ni ( 4\%) suppressed superconductivity below 1.8 K, inducing insulating
spin glass magnetic ground state for higher Ni content. With further Ni
substitution, metallic resistivity is restored. For high Ni concentration in
the lattice the unit cell symmetry is high symmetry with no phase
separation whereas both space groups were detected in the phase
separated crystals when concentration of Ni Fe. The absence of
superconductivity coincides with the absence of crystalline Fe vacancy order.Comment: 6 pages, 4 figure
- …