233 research outputs found
Jet confinement by magneto-torsional oscillations
Many quasars and active galactic nuclei (AGN) appear in radio, optical, and
X-ray maps, as a bright nuclear sources from which emerge single or double
long, thin jets. When observed with high angular resolution these jets show
structure with bright knots separated by relatively dark regions. Nonthermal
nature of a jet radiation is well explained as the synchrotron radiation of the
relativistic electrons in an ordered magnetic field. We consider magnetic
collimation, connected with torsional oscillations of a cylinder with elongated
magnetic field, and periodically distributed initial rotation around the
cylinder axis. The stabilizing azimuthal magnetic field is created here by
torsional oscillations, where charge separation is not necessary. Approximate
simplified model is developed. Ordinary differential equation is derived, and
solved numerically, what gives a possibility to estimate quantitatively the
range of parameters where jets may be stabilized by torsional oscillations.Comment: accepted for publication in Astrophysics and Space Scienc
Self-Trapping of Polarons in the Rashba-Pekar Model
We performed quantum Monte Carlo study of the exciton-polaron model which
features the self-trapping phenomenon when the coupling strength and/or
particle momentum is varied. For the first time accurate data for energy,
effective mass, the structure of the polaronic cloud, dispersion law, and
spectral function are available throughout the crossover region. We observed
that self-trapping can not be reduced to hybridization of two states with
different lattice deformation, and that at least three states are involved in
the crossover from light- to heavy-mass regimes.Comment: 5 pages, 5 figures, Accepted to Phys. Rev. B Rapid Communication
Effect of screening of the electron-phonon interaction on the temperature of Bose-Einstein condensation of intersite bipolarons
Here we consider an interacting electron-phonon system within the framework
of extended Holstein-Hubbard model at strong enough electron-phonon interaction
limit in which (bi)polarons are the essential quasiparticles of the system. It
is assumed that the electron-phonon interaction is screened and its potential
has Yukawa-type analytical form. An effect of screening of the electron-phonon
interaction on the temperature of Bose-Einstein condensation of the intersite
bipolarons is studied for the first time. It is revealed that the temperature
of Bose-Einstein condensation of intersite bipolarons is higher in the system
with the more screened electron-phonon interaction.Comment: 6 pages, 4 figure
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.
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
Spectroscopy of vibrational modes in metal nanoshells
We study the spectrum of vibrational modes in metal nanoparticles with a
dielectric core. Vibrational modes are excited by the rapid heating of the
particle lattice that takes place after laser excitation, and can be monitored
by means of pump-probe spectroscopy as coherent oscillations of transient
optical spectra. In nanoshells, the presence of two metal surfaces results in a
substantially different energy spectrum of acoustic vibrations than for solid
particles. We calculated the energy spectrum as well as the damping of
nanoshell vibrational modes. The oscillator strength of fundamental breathing
mode is larger than that in solid nanoparticles. At the same time, in very thin
nanoshells, the fundamental mode is overdamped due to instantaneous energy
transfer to the surrounding medium
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
New Longitudinal Waves in Electron-Positron-Ion Quantum Plasmas
A general quantum dispersion equation for electron-positron(hole)-ion quantum
plasmas is derived and studied for some interesting cases. In an
electron-positron degenerate Fermi gas, with or without the Madelung term, a
new type of zero sound waves are found. Whereas in an electron-hole plasmas a
new longitudinal quantum waves are revealed, which have no analogies in quantum
electron-ion plasmas. The excitation of these quantum waves by a low-density
monoenergetic straight electron beam is examined. Furthermore, the KdV equation
for novel quantum waves is derived and the contribution of the Madelung term in
the formation of the KdV solitons is discussed
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