33 research outputs found
Nanoscale alpha-structural domains in the phonon-glass thermoelectric material beta-Zn4Sb3
A study of the local atomic structure of the promising thermoelectric material beta-Zn4Sb3, using atomic pair distribution function (PDF) analysis of x-ray- and neutron-diffraction data, suggests that the material is nanostructured. The local structure of the beta phase closely resembles that of the low-temperature alpha phase. The alpha structure contains ordered zinc interstitial atoms which are not long range ordered in the beta phase. A rough estimate of the domain size from a visual inspection of the PDF is <~10 nm. It is probable that the nanoscale domains found in this study play an important role in the exceptionally low thermal conductivity of beta-Zn4Sb3
Nanoscale clusters in the high performance thermoelectric AgPbmSbTem+2
The local structure of the AgPbmSbTem+2 series of thermoelectric materials
has been studied using the atomic pair distribution function (PDF) method.
Three candidate-models were attempted for the structure of this class of
materials using either a one-phase or a two-phase modeling procedure. Combining
modeling the PDF with HRTEM data we show that AgPbmSbTem+2 contains nanoscale
inclusions with composition close to AgPb3SbTe5 randomly embedded in a PbTe
matrix.Comment: 7 pages, 5 figures, 2 tables, submitted to PR
Neutron diffraction evidence of microscopic charge inhomogeneities in the CuO2 plane of superconducting La2-xSrxCuO4 (0 < x <0.30)
We present local structural evidence supporting the presence of charge
inhomogeneities in the CuO2 planes of underdoped La2-xSrxCuO4. High-resolution
atomic pair distribution functions have been obtained from neutron powder
diffraction data over the range of doping 0 < x < 0.30 at 10 K. Despite the
average structure getting less orthorhombic we see a broadening of the in-plane
Cu-O bond distribution as a function of doping up to optimal doping. Thereafter
the peak abruptly sharpens. Complementary evidence is also evident from the
observation of octahedral tilt disorder in the PDF at higher atomic separation.
This suggests a crossover from a charge inhomogeneous state at and below
optimal doping to a homogeneous charge state above optimal doping. The strong
response of the local structure to the charge-state implies a strong
electron-lattice coupling in these materials.Comment: 4 pages, 3 figures, submitted to Physical Review Letters (27-th of
June 1999) resubmitted to Phys. Rev. Lett. (8th of March 2000
Temperature Dependence of Spin Correlation and Charge Dynamics in the Stripe Phase of High-T_c Superconductors
We examine the temperature dependence of the electronic states in the stripe
phase of high-Tc cuprates by using the t-J model with a potential that
stabilizes vertical charge stripes. Charge and spin-correlation functions and
optical conductivity are calculated by using finite-temperature Lanczos method.
At zero temperature, the antiferromagnetic correlation between a spin in a
charge stripe and that in a spin domain adjacent to the stripe is weak, since
the charge stripe and the spin domain are almost separated. With increasing
temperature, the correlation increases and then decreases toward high
temperature. This is in contrast to other correlations that decrease
monotonically. From the examination of the charge dynamics, we find that this
anomalous temperature dependence of the correlation is the consequence of a
crossover from one-dimensional electronic states to two-dimensional ones.Comment: 7 pages in two-column format, 6 figures, to be published in Phys.
Rev.
Charge-Stripe Ordering From Local Octahedral Tilts: Underdoped and Superconducting La2-xSrxCuO4 (0 < x < 0.30)
The local structure of La2-xSrxCuO4, for 0 < x < 0.30, has been investigated
using the atomic pair distribution function (PDF) analysis of neutron powder
diffraction data. The local octahedral tilts are studied to look for evidence
of [110] symmetry (i.e., LTT-symmetry) tilts locally, even though the average
tilts have [010] symmetry (i.e., LTO-symmetry) in these compounds. We argue
that this observation would suggest the presence of local charge-stripe order.
We show that the tilts are locally LTO in the undoped phase, in agreement with
the average crystal structure. At non-zero doping the PDF data are consistent
with the presence of local tilt disorder in the form of a mixture of LTO and
LTT local tilt directions and a distribution of local tilt magnitudes. We
present topological tilt models which qualitatively explain the origin of tilt
disorder in the presence of charge stripes and show that the PDF data are well
explained by such a mixture of locally small and large amplitude tilts.Comment: 11 two-column pages, 11 figure
Investigation of the thermal expansion and heat capacity of the CaCu3Ti4O12 ceramics
The thermal expansion of the CaCu3Ti4O12 ceramics has been measured over a wide temperature
range 120–1200 K. The high quality of the samples under study has been confirmed by good agreement of
the results of measurements of the heat capacity in the range 2–300 K and in the vicinity of the phase transition of magnetic nature at 25 K with the data for the single crystal. No anomalies in the thermal expansion
that can be associated with the phase transition at 726–732 K assumed by other investigators have been found.
The influence exerted on the thermal expansion by the heat treatment of the sample in a helium atmosphere
and in air has been investigated
Possible charge inhomogeneities in the CuO2 planes of YBa2Cu3O6+x (x=0.25, 0.45, 0.65, 0.94) from pulsed neutron diffraction
The atomic pair distribution functions (PDF) of four powder samples of
YBa2Cu3O6+x (x=0.25, 0.45, 0.65, 0.94) at 15 K have been measured by means of
pulsed neutron diffraction. The PDF is modelled using a full-profile fitting
approach to yield structural parameters. In contrast to earlier XAFS work we
find no evidence of a split apical oxygen site. However, a slightly improved
fit over the average crystallographic model results when the planar Cu(2) site
is split along the z-direction. This is interpreted in terms of charge
inhomogeneities in the CuO2 planes.Comment: 8 pages, 3 figure
Phase Separation Models for Cuprate Stripe Arrays
An electronic phase separation model provides a natural explanation for a
large variety of experimental results in the cuprates, including evidence for
both stripes and larger domains, and a termination of the phase separation in
the slightly overdoped regime, when the average hole density equals that on the
charged stripes. Several models are presented for charged stripes, showing how
density waves, superconductivity, and strong correlations compete with quantum
size effects (QSEs) in narrow stripes. The energy bands associated with the
charged stripes develop in the middle of the Mott gap, and the splitting of
these bands can be understood by considering the QSE on a single ladder.Comment: significant revisions: includes island phase, 16 eps figures, revte
Flux Phase as a Dynamic Jahn-Teller Phase: Berryonic Matter in the Cuprates?
There is considerable evidence for some form of charge ordering on the
hole-doped stripes in the cuprates, mainly associated with the low-temperature
tetragonal phase, but with some evidence for either charge density waves or a
flux phase, which is a form of dynamic charge-density wave. These three states
form a pseudospin triplet, demonstrating a close connection with the E X e
dynamic Jahn-Teller effect, suggesting that the cuprates constitute a form of
Berryonic matter. This in turn suggests a new model for the dynamic Jahn-Teller
effect as a form of flux phase. A simple model of the Cu-O bond stretching
phonons allows an estimate of electron-phonon coupling for these modes,
explaining why the half breathing mode softens so much more than the full
oxygen breathing mode. The anomalous properties of provide a coupling
(correlated hopping) which acts to stabilize density wave phases.Comment: Major Revisions: includes comparisons with specific cuprate phonon
modes, 16 eps figures, revte
Effects of "Stripes" on the Magnetic Excitation Spectra of La1.48Nd0.4Sr0.12CuO4
The wave vector(q)- and energy(w)-dependent magnetic excitation spectra
c"(q,w), have been studied in the temperature(T) range of 10 K<T<150 K on
single crystals of La1.48Nd0.4Sr0.12CuO4, which has the static "stripe" order
in the low temperature phase. The incommensurability d of the peak of c"(q,w)
found by the q-scans and the peak width Dq exhibit characteristic
T-dependences. Observed profiles show clear incommensurate structure even at
rather high temperatures. The results indicate that effects of the slowly
fluctuating "stripes" exist not only in the low temperature tetragonal (LTT)
phase but also in the orthorhombic phase(LTO1) of the present system. The
persistence of the local structural characteristics of the low-T phase seems to
be important.Comment: 9 pages, 6 figure