392 research outputs found
Reply to "Comment on 'Gravitating Magnetic Monopole in the Global Monopole Spacetime' "
In this Reply I present some arguments in favor of the stability of the
topological defect composed by global and magnetic monopoles.Comment: 1 page, no figures. Revised version improves the theoretical analysis
about electrostatic self-interaction in the global monopole spacetim
Angle-resolved photoemission in doped charge-transfer Mott insulators
A theory of angle-resolved photoemission (ARPES) in doped cuprates and other
charge-transfer Mott insulators is developed taking into account the realistic
(LDA+U) band structure, (bi)polaron formation due to the strong electron-phonon
interaction, and a random field potential. In most of these materials the first
band to be doped is the oxygen band inside the Mott-Hubbard gap. We derive the
coherent part of the ARPES spectra with the oxygen hole spectral function
calculated in the non-crossing (ladder) approximation and with the exact
spectral function of a one-dimensional hole in a random potential. Some unusual
features of ARPES including the polarisation dependence and spectral shape in
YBa2Cu3O7 and YBa2Cu4O8 are described without any Fermi-surface, large or
small. The theory is compatible with the doping dependence of kinetic and
thermodynamic properties of cuprates as well as with the d-wave symmetry of the
superconducting order parameter.Comment: 8 pages (RevTeX), 10 figures, submitted to Phys. Rev.
Singlet and triplet bipolarons on the triangular lattice
We study the Coulomb-Fr\"ohlich model on a triangular lattice, looking in
particular at states with angular momentum. We examine a simplified model of
crab bipolarons with angular momentum by projecting onto the low energy
subspace of the Coulomb-Fr\"ohlich model with large phonon frequency. Such a
projection is consistent with large long-range electron-phonon coupling and
large repulsive Hubbard . Significant differences are found between the band
structure of singlet and triplet states: The triplet state (which has a flat
band) is found to be significantly heavier than the singlet state (which has
mass similar to the polaron). We test whether the heavier triplet states
persist to lower electron-phonon coupling using continuous time quantum Monte
Carlo (QMC) simulation. The triplet state is both heavier and larger,
demonstrating that the heavier mass is due to quantum interference effects on
the motion. We also find that retardation effects reduce the differences
between singlet and triplet states, since they reintroduce second order terms
in the hopping into the inverse effective mass.Comment: Proceedings of SNS 200
Calculation of excited polaron states in the Holstein model
An exact diagonalization technique is used to investigate the low-lying
excited polaron states in the Holstein model for the infinite one-dimensional
lattice. For moderate values of the adiabatic ratio, a new and comprehensive
picture, involving three excited (coherent) polaron bands below the phonon
threshold, is obtained. The coherent contribution of the excited states to both
the single-electron spectral density and the optical conductivity is evaluated
and, due to the invariance of the Hamiltonian under the space inversion, the
two are shown to contain complementary information about the single-electron
system at zero temperature. The chosen method reveals the connection between
the excited bands and the renormalized local phonon excitations of the
adiabatic theory, as well as the regime of parameters for which the electron
self-energy has notable non-local contributions. Finally, it is shown that the
hybridization of two polaron states allows a simple description of the ground
and first excited state in the crossover regime.Comment: 12 pages, 9 figures, submitted to PR
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
Is the early reduction of fetal calf serum concentration in bovine in vitro embryo culture beneficial?
Relationship between work rate and oxygen uptake in mitochondrial myopathy during ramp-incremental exercise
We determined the response characteristics and functional correlates of the dynamic relationship between the rate (Δ) of oxygen consumption ( O2) and the applied power output (work rate = WR) during ramp-incremental exercise in patients with mitochondrial myopathy (MM). Fourteen patients (7 males, age 35.4 ± 10.8 years) with biopsy-proven MM and 10 sedentary controls (6 males, age 29.0 ± 7.8 years) took a ramp-incremental cycle ergometer test for the determination of the O2 on-exercise mean response time (MRT) and the gas exchange threshold (GET). The ΔO2/ΔWR slope was calculated up to GET (S1), above GET (S2) and over the entire linear portion of the response (S T). Knee muscle endurance was measured by isokinetic dynamometry. As expected, peak O2 and muscle performance were lower in patients than controls (P O2/ΔWR than controls, especially the S2 component (6.8 ± 1.5 vs 10.3 ± 0.6 mL·min-1·W-1, respectively; P O2/ΔWR (S T) and muscle endurance, MRT-O2, GET and peak O2 in MM patients (P O2/ΔWR below 8 mL·min-1·W-1 had severely reduced peak O2 values (O2) had lower ΔO2/ΔWR (P O2/ΔWR) is typically reduced in patients with MM, being related to increased functional impairment and higher cardiopulmonary stress
The Holstein Polaron
We describe a variational method to solve the Holstein model for an electron
coupled to dynamical, quantum phonons on an infinite lattice. The variational
space can be systematically expanded to achieve high accuracy with modest
computational resources (12-digit accuracy for the 1d polaron energy at
intermediate coupling). We compute ground and low-lying excited state
properties of the model at continuous values of the wavevector in
essentially all parameter regimes. Our results for the polaron energy band,
effective mass and correlation functions compare favorably with those of other
numerical techniques including DMRG, Global Local and exact diagonalization. We
find a phase transition for the first excited state between a bound and unbound
system of a polaron and an additional phonon excitation. The phase transition
is also treated in strong coupling perturbation theory.Comment: 24 pages, 11 figures submitted to PR
Wightman function and scalar Casimir densities for a wedge with two cylindrical boundaries
Wightman function, the vacuum expectation values of the field square and the
energy-momentum tensor are investigated for a massive scalar field with general
curvature coupling parameter inside a wedge with two coaxial cylindrical
boundaries. It is assumed that the field obeys Dirichlet boundary condition on
bounding surfaces. The application of a variant of the generalized Abel-Plana
formula enables to extract from the expectation values the contribution
corresponding to the geometry of a wedge with a single shell and to present the
interference part in terms of exponentially convergent integrals. The local
properties of the vacuum are investigated in various asymptotic regions of the
parameters. The vacuum forces acting on the boundaries are presented as the sum
of self-action and interaction terms. It is shown that the interaction forces
between the separate parts of the boundary are always attractive. The
generalization to the case of a scalar field with Neumann boundary condition is
discussed.Comment: 19 pages, 3 figure
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
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