124 research outputs found
Path integrals approach to resisitivity anomalies in anharmonic systems
Different classes of physical systems with sizeable electron-phonon coupling
and lattice distortions present anomalous resistivity behaviors versus
temperature. We study a molecular lattice Hamiltonian in which polaronic charge
carriers interact with non linear potentials provided by local atomic
fluctuations between two equilibrium sites. We study a molecular lattice
Hamiltonian in which polaronic charge carriers interact with non linear
potentials provided by local atomic fluctuations between two equilibrium sites.
A path integral model is developed to select the class of atomic oscillations
which mainly contributes to the partition function and the electrical
resistivity is computed in a number of representative cases. We argue that the
common origin of the observed resistivity anomalies lies in the time retarded
nature of the polaronic interactions in the local structural instabilities.Comment: 4 figures, to appear in Phys.Rev.B, May 1st (2001
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
Charge Dynamics in Cuprate Superconductors
In this lecture we present some interesting issues that arise when the
dynamics of the charge carriers in the CuO planes of the high temperature
superconductors is considered. Based on the qualitative picture of doping, set
by experiments and some previous calculations, we consider the strength of
various inter and intra-cell charge transfer susceptibilities, the question of
Coulomb screening and charge collective modes. The starting point is the usual
p-d model extended by the long range Coulomb (LRC) interaction. Within this
model it is possible to examine the case in which the LRC forces frustrate the
electronic phase separation, the instability which is present in the model
without an LRC interaction. While the static dielectric function in such
systems is negative down to arbitrarily small wavevectors, the system is not
unstable. We consider the dominant electronic charge susceptibilities and
possible consequences for the lattice properties.Comment: 14 pages, 15 figures, latex, to be published in "From Quantum
Mechanics to Technology", Lecture Notes in Physics, Springe
Onset of dielectric modes at 110K and 60K due to local lattice distortions in non-superconducting YBa_{2}Cu_{3}O_{6.0} crystals
We report the observation of two dielectric transitions at 110K and 60K in
the microwave response of non-superconducting YBa_{2}Cu_{3}O_{6.0} crystals.
The transitions are characterized by a change in polarizability and presence of
loss peaks, associated with overdamped dielectric modes. An explanation is
presented in terms of changes in polarizability of the apical O atoms in the
Ba-O layer, affected by lattice softening at 110K, due to change in buckling of
the Cu-O layer. The onset of another mode at 60K strongly suggests an
additional local lattice change at this temperature. Thus microwave dielectric
measurements are sensitive indicators of lattice softening which may be
relevant to superconductivity.Comment: 5 pages, 3 ps format figure
Correlated local distortions of the TlO layers in TlBaCuO: An x-ray absorption study
We have used the XAFS (x-ray-absorption fine structure) technique to
investigate the local structure about the Cu, Ba, and Tl atoms in orthorhombic
Tl-2201 with a superconducting transition temperature T=60 K. Our results
clearly show that the O(1), O(2), Cu, and Ba atoms are at their ideal sites as
given by the diffraction measurements, while the Tl and O(3) atoms are more
disordered than suggested by the average crystal structure. The Tl-Tl distance
at 3.5 \AA{ } between the TlO layers does not change, but the Tl-Tl distance at
3.9 \AA{ } within the TlO layer is not observed and the Tl-Ba and Ba-Tl peaks
are very broad. The shorter Tl-O(3) distance in the TlO layer is about 2.33
\AA, significantly shorter than the distance calculated with both the Tl and
O(3) atoms at their ideal sites ( 0 or ). A model based
on these results shows that the Tl atom is displaced along the
directions from its ideal site by about 0.11 \AA; the displacements of
neighboring Tl atoms are correlated. The O(3) atom is shifted from the $4e$
site by about 0.53 \AA{ } roughly along the directions. A comparison of
the Tl L-edge XAFS spectra from three samples, with T=60 K, 76 K,
and 89 K, shows that the O environment around the Tl atom is sensitive to T
while the Tl local displacement is insensitive to T and the structural
symmetry. These conclusions are compared with other experimental results and
the implications for charge transfer and superconductivity are discussed. This
paper has been submitted to Phys. Rev. B.Comment: 20 pages plus 14 ps figures, REVTEX 3.
Two--magnon scattering and the spin--phonon interaction beyond the adiabatic approximation
We consider a model of Raman scattering for a two--dimensional
Heisenberg Anti-Ferromagnet which includes a {\it dynamical} spin--phonon
interaction. We observe a broadening of the line shape due to increased
coupling with excited high--energy spin states. Our results are close to a
model of random static exchange interactions, first introduced in this context
by Haas {\it et al.} [J. Appl. Phys. {\bf 75}, 6340, (1994)], which, when
extended to large numbers of spins, explains experiments in the parent
insulating compounds of high- superconductors.Comment: 14 pages (revtex format), 8 postscript figure
The nature of NV absorbers at high redshift
We present a study of NV absorption systems at 1.5 < z < 2.5 in the optical
spectra of 19 QSOs. Our analysis includes both absorbers arising from the
intergalactic medium as well as systems in the vicinity of the background
quasar. We construct detailed photoionization models to study the physical
conditions and abundances in the absorbers and to constrain the spectral
hardness of the ionizing radiation. The rate of incidence for intervening NV
components is dN/dz = 3.38 +/- 0.43, corresponding to dN/dX = 1.10 +/- 0.14.
The column density distribution function is fitted by the slope beta = 1.89 +/-
0.22, consistent with measurements for CIV and OVI. The narrow line widths
(b_NV ~ 6 km/s) imply photoionization rather than collisions as dominating
ionization process. The column densities of CIV and NV are correlated but show
different slopes for intervening and associated absorbers, which indicates
different ionizing spectra. Associated systems are found to be more metal-rich,
denser, and more compact than intervening absorbers. This conclusion is
independent of the adopted ionizing radiation. For the intervening NV systems
we find typical values of [C/H] ~ -0.6 and n_H ~ 10^-3.6 cm^-3, and sizes of a
few kpc, while for associated NV absorbers we obtain [C/H] ~ +0.7, n_H ~
10^-2.8 cm^-3, and sizes of several 10 pc. The abundance of nitrogen relative
to carbon [N/C] and alpha-elements like oxygen and silicon [N/alpha] is
correlated with [N/H], indicating the enrichment by secondary nitrogen. The
larger scatter in [N/alpha] in intervening systems suggests an inhomogeneous
enrichment of the IGM. There is an anti-correlation between [N/alpha] and
[alpha/C], which could be used to constrain the initial mass function of the
carbon- and nitrogen-producing stellar population.Comment: accepted by A&A, revised versio
Spatial distribution of photoelectrons participating in formation of x-ray absorption spectra
Interpretation of x-ray absorption near-edge structure (XANES) experiments is
often done via analyzing the role of particular atoms in the formation of
specific peaks in the calculated spectrum. Typically, this is achieved by
calculating the spectrum for a series of trial structures where various atoms
are moved and/or removed. A more quantitative approach is presented here, based
on comparing the probabilities that a XANES photoelectron of a given energy can
be found near particular atoms. Such a photoelectron probability density can be
consistently defined as a sum over squares of wave functions which describe
participating photoelectron diffraction processes, weighted by their normalized
cross sections. A fine structure in the energy dependence of these
probabilities can be extracted and compared to XANES spectrum. As an
illustration of this novel technique, we analyze the photoelectron probability
density at the Ti K pre-edge of TiS2 and at the Ti K-edge of rutile TiO2.Comment: Journal abstract available on-line at
http://link.aps.org/abstract/PRB/v65/e20511
Lattice dynamics effects on small polaron properties
This study details the conditions under which strong-coupling perturbation
theory can be applied to the molecular crystal model, a fundamental theoretical
tool for analysis of the polaron properties. I show that lattice dimensionality
and intermolecular forces play a key role in imposing constraints on the
applicability of the perturbative approach. The polaron effective mass has been
computed in different regimes ranging from the fully antiadiabatic to the fully
adiabatic. The polaron masses become essentially dimension independent for
sufficiently strong intermolecular coupling strengths and converge to much
lower values than those tradition-ally obtained in small-polaron theory. I find
evidence for a self-trapping transition in a moderately adiabatic regime at an
electron-phonon coupling value of .3. Our results point to a substantial
independence of the self-trapping event on dimensionality.Comment: 8 pages, 5 figure
Polarons and bipolarons in strongly interacting electron-phonon systems
The Holstein Hubbard and Holstein t--J models are studied for a wide range of
phonon frequencies, electron--electron and electron--phonon interaction
strengths on finite lattices with up to ten sites by means of direct Lanczos
diagonalization. Previously the necessary truncation of the phononic Hilbert
space caused serious limitations to either very small systems (four or even two
sites) or to weak electron--phonon coupling, in particular in the adiabatic
regime. Using parallel computers we were able to investigate the transition
from `large' to `small' polarons in detail. By resolving the low--lying
eigenstates of the Hamiltonian and by calculating the spectral function we can
identify a polaron band in the strong--coupling case, whose dispersion deviates
from the free--particle dispersion at low and intermediate phonon frequencies.
For two electrons (holes) we establish the existence of bipolaronic states and
discuss the formation of a bipolaron band. For the 2D Holstein t--J model we
demonstrate that the formation of hole--polarons is favoured by strong Coulomb
correlations. Analyzing the hole--hole correlation functions we find that hole
binding is enhanced as a dynamical effect of the electron--phonon interaction.Comment: 23 pages (Revtex) with 13 figures (ps, uuencoded
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