346 research outputs found
Polarons in the harmonic lattice. I. Standing polaron
We obtain analytical expressions for the large- and small-radius polarons on
the one-dimensional lattice in the TBA approximation. The equations of motion
for this model are treated classically for the oscillator subsystem, while a
quantum description is used for the electron. The electron-phonon interaction
is considered in the linear Su--Schrieffer--Heeger approximation. Good
agreement between analytical formulae and accurate numerical simulation is
obtained. The dynamics of polaron formation from different initial conditions
is considered. Some features of the wave function evolution, governed by the
finite lattice length, are elucidated.Comment: 18 pages, 8 figure
"Ping-pong" electron transfer. I. First reflection of the Loschmidt echo
Quantum dynamics of the electron wave function on one-dimensional lattice is
considered. The lattice consists of equal sites and one impurity site. The
impurity site differs from other sites by the on-site electron energy and
the hopping integral . The wave function is located on the impurity site at
. The wave packet is formed which travels along the lattice, and
reflects from its end. Reflections happen many times (Loschmidt echo) and this
phenomenon is considered in the second part of the paper. Analytical
expressions for the wave packet front propagation at different values and
are derived, and they are in excellent agreement with the numerical
simulation. The obtained results can help in interpretation of recent
experiments on highly efficient charge transport in synthetic olygonucleotides.Comment: 22 pages, 11 figure
"Ping-pong" electron transfer. II. Multiple reflections of the Loschmidt echo and the wave function trapping by an acceptor
This paper continues the preceding paper on the problem of quantum dynamics
on the lattice. Firstly we consider the multiple reflections of the wave
function (Loschmidt echo). The phenomenon of wave function concentration on the
impurity site after reflections is found. The solution representing the total
amplitude is obtained as the series in terms of partial amplitudes
. The contribution of th partial amplitude becomes dominant only
after th reflection from the lattice end. An excellent agreement between
analytical and accurate numerical results is obtained. Next problem, -- wave
packet trapping by defects, is solved by numerical simulation. Analytical
expressions are derived in few cases allowing to estimate the quantum
efficiency of charge transfer. Obtained results can qualitatively explain
recent experiments on the highly efficient charge transport in olygonucleotides
and polypeptides.Comment: 18 pages, 12 figure
Heat conductivity in the beta-FPU lattice. Solitons and breathers as energy carriers
This paper consists of two parts. The first part proposes a new
methodological framework within which the heat conductivity in 1D lattices can
be studied. The total process of heat conductivity is decomposed into two
contributions where the first one is the equilibrium process at equal
temperatures T of both lattice ends and the second -- non-equilibrium process
with the temperature \Delta T of one end and zero temperature of the other. The
heat conductivity in the limit \Delta T \to 0 is reduced to the heat
conductivity of harmonic lattice. A threshold temperature T_{thr} scales
T_{thr}(N) \sim N^{-3} with the lattice size N. Some unusual properties of heat
conductivity can be exhibited on nanoscales at low temperatures. The
thermodynamics of the \beta-FPU lattice can be adequately approximated by the
harmonic lattice. The second part testifies in the favor of the soliton and
breather contribution to the heat conductivity in contrast to [N. Li, B. Li, S.
Flach, PRL 105 (2010) 054102]. In the continuum limit the \beta-FPU lattice is
reduced to the modified Korteweg - de Vries equation with soliton and breather
solutions. Numerical simulations demonstrate their high stability. New method
for the visualization of moving solitons and breathers is suggested. An
accurate expression for the dependence of the sound velocity on temperature is
also obtained. Our results support the conjecture on the solitons and breathers
contribution to the heat conductivity.Comment: 29 pages, 14 figure
Polarons on one-dimensional lattice. II. Moving polaron
In the present study we revise the possible polaron contribution to the
charge and energy transfer over long distances in biomolecules like DNA. The
harmonic and the simple inharmonic () lattices are
considered. The systems of PDEs are derived in the continuum approximation. The
PDEs have the one-soliton solution for polarons on the harmonic lattice. It
describes a moving polaron, the polaron velocity lies in the region from zero
to the sound velocity and depends on the polaron amplitude. The PDEs describing
polarons on the inharmonic lattice also have the one-soliton solution only in
the case of special relation between parameters (parameter of inharmonicity
and parameter of electron-phonon interaction ). Polaron
dynamics is numerically investigated in the wide range of parameters, where the
analytical solutions are not available. Supersonic polarons are observed on
inharmonic lattice with high inharmonicity. There is the range of parameters
and where exists a family of unusual stable moving polarons
with the envelope consisting of several peaks (polarobreather solution). The
results are in qualitative agreement with recent experiments on the charge
transport in DNA.Comment: 20 pages, 10 figure
Heat conductance in nonlinear lattices at small temperature gradients
This paper proposes a new methodological framework within which the heat
conductance in 1D lattices can be studied. The total process of heat
conductance is separated into two parts where the first one is the equilibrium
process at equal temperatures of both ends and the second one --
non-equilibrium with the temperature of one end and zero temperature
of the other. This approach allows significant decrease of computational time
at . The threshold temperature is found which
scales with the lattice size and by convention
separates two mechanisms of heat conductance: phonon mechanism dominates at . Solitons and breathers are directly visualized in numerical
experiments. The problem of heat conductance in non-linear lattices in the
limit can be reduced to the heat conductance of harmonic
lattice with time-dependent stochastic rigidities determined by the equilibrium
process at temperature . The detailed analysis is done for the -FPU
lattice though main results are valid for one-dimensional lattices with
arbitrary potentials.Comment: 29 pages, 17 figure
Software correlator for Radioastron mission
In this paper we discuss the characteristics and operation of Astro Space
Center (ASC) software FX correlator that is an important component of
space-ground interferometer for Radioastron project. This project performs
joint observations of compact radio sources using 10 meter space radio
telescope (SRT) together with ground radio telescopes at 92, 18, 6 and 1.3 cm
wavelengths. In this paper we describe the main features of space-ground VLBI
data processing of Radioastron project using ASC correlator. Quality of
implemented fringe search procedure provides positive results without
significant losses in correlated amplitude. ASC Correlator has a computational
power close to real time operation. The correlator has a number of processing
modes: "Continuum", "Spectral Line", "Pulsars", "Giant Pulses","Coherent".
Special attention is paid to peculiarities of Radioastron space-ground VLBI
data processing. The algorithms of time delay and delay rate calculation are
also discussed, which is a matter of principle for data correlation of
space-ground interferometers. During 5 years of Radioastron space radio
telescope (SRT) successful operation, ASC correlator showed high potential of
satisfying steady growing needs of current and future ground and space VLBI
science. Results of ASC software correlator operation are demonstrated.Comment: 28 pages, 17 figure
Deformation effects in low-momentum distributions of heavy nuclei
Momentum distributions (MD) of deep hole proton states in U are
studied paying particular attention to the influence of deformation. Two
essentially different mean-field models, Woods-Saxon (WS) and
Skyrme-Hartree-Fock with the SkM force, are used. Noticeable deviations
between the WS and SkM results are found. They are mainly due to the
difference in effective nucleon mass. In particular, SkM gives much weaker
deformation effects at low momenta than WS. It is shown that, in spite of the
deformation mixing, MD at low momenta can serve for identification of
hole states originating from spherical sub-shells.
Moreover, following the WS calculations, the deformation results in additional
states with strong contributions. A possibility to probe
such states in knock-out experiments is discussed.Comment: 9 pages, 3 figures. In the replaced version the figures were
optimized without changing their conten
High-Resolution X-Ray Studies of the Direct Spin Contact of EuO with Silicon
Ferromagnetic semiconductor europium monoxide (EuO) is believed to be an
effective spin injector when directly integrated with silicon. Injection
through spin-selective ohmic contact requires superb structural quality of the
interface EuO/Si. Recent breakthrough in manufacturing free-of-buffer-layer
EuO/Si junctions calls for structural studies of the interface between the
semiconductors. Ex situ high-resolution X-ray diffraction and reflectivity
accompanied by in situ reflection high-energy electron diffraction reveal
direct coupling at the interface. A combined analysis of XRD and XRR data
provides a common structural model. The structural quality of the EuO/Si spin
contact by far exceeds that of previous reports and thus makes a step forward
to the ultimate goals of spintronics
Vibrational resonances in 1D Morse and FPU lattices
In the present paper the resonances of vibrational modes in one-dimensional
random Morse lattice are found and analyzed. The resonance energy exchange is
observed at some values of elongation. Resonance is
investigated in details. The interacting modes are inequivalent: the
higher-frequency mode is much more stable in the excited state, i.e. its
life-time is larger than the life-time of lower-frequency mode under the
resonance conditions. Simple model of two nonlinearly coupled harmonic
oscillators is also considered. It allows to get analytical description and to
investigate the kinetics and the energy exchange degree vs. such parameters as
the resonance detuning and specific energy. The very similar behavior is found
in the Morse and the two-oscillatory models, and an excellent agreement between
analytical and numerical results is obtained. Analogous resonance phenomena are
also found in the random Fermi-Pasta-Ulam lattice under contraction.Comment: 10 pages, 5 figure
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