1,062 research outputs found
Nonlinear tunneling in two-dimensional lattices
We present thorough analysis of the nonlinear tunneling of Bose-Einstein
condensates in static and accelerating two-dimensional lattices within the
framework of the mean-field approximation. We deal with nonseparable lattices
considering different initial atomic distributions in the highly symmetric
states. For analytical description of the condensate before instabilities are
developed, we derive several few-mode models, analyzing both essentially
nonlinear and quasi-linear regimes of tunneling. By direct numerical
simulations, we show that two-mode models provide accurate description of the
tunneling when either initially two states are populated or tunneling occurs
between two stable states. Otherwise a two-mode model may give only useful
qualitative hints for understanding tunneling but does not reproduce many
features of the phenomenon. This reflects crucial role of the instabilities
developed due to two-body interactions resulting in non-negligible population
of the higher bands. This effect becomes even more pronounced in the case of
accelerating lattices. In the latter case we show that the direction of the
acceleration is a relevant physical parameter which affects the tunneling by
changing the atomic rates at different symmetric states and by changing the
numbers of bands involved in the atomic transfer
Low-temperature thermal conductivity in polycrystalline graphene
The low-temperature thermal conductivity in polycrystalline graphene is
theoretically studied. The contributions from three branches of acoustic
phonons are calculated by taking into account scattering on sample borders,
point defects and grain boundaries. Phonon scattering due to sample borders and
grain boundaries is shown to result in a -behaviour in the thermal
conductivity where varies between 1 and 2. This behaviour is found to
be more pronounced for nanosized grain boundaries.
PACS: 65.80.Ck, 81.05.ue, 73.43.C
Electronic screening and damping in magnetars
We calculate the screening of the ion-ion potential due to electrons in the
presence of a large background magnetic field, at densities of relevance to
neutron star crusts. Using the standard approach to incorporate electron
screening through the one-loop polarization function, we show that the magnetic
field produces important corrections both at short and long distances. In
extreme fields, realized in highly magnetized neutron stars called magnetars,
electrons occupy only the lowest Landau levels in the relatively low density
region of the crust. Here our results show that the screening length for
Coulomb interactions between ions can be smaller than the inter-ion spacing.
More interestingly, we find that the screening is anisotropic and the screened
potential between two static charges exhibits long range Friedel oscillations
parallel to the magnetic field. This long-range oscillatory behavior is likely
to affect the lattice structure of ions, and can possibly create rod-like
structures in the magnetar crusts. We also calculate the imaginary part of the
electron polarization function which determines the spectrum of electron-hole
excitations and plays a role in damping lattice phonon excitations. We
demonstrate that even for modest magnetic fields this damping is highly
anisotropic and will likely lead to anisotropic phonon heat transport in the
outer neutron star crust.Comment: 14 pages, 5 Figure
Heat capacity and phonon mean free path of wurtzite GaN
We report on lattice specific heat of bulk hexagonal GaN measured by the heat
flow method in the temperature range 20-300 K and by the adiabatic method in
the range 5-70 K. We fit the experimental data using two temperatures model.
The best fit with the accuracy of 3 % was obtained for the temperature
independent Debye's temperature {\rm K} and Einstein's
temperature {\rm K}. We relate these temperatures to the
function of density of states. Using our results for heat conduction
coefficient, we established in temperature range 10-100 K the explicit
dependence of the phonon mean free path on temperature . Above 100 K, there is the evidence of contribution of the Umklapp
processes which limit phonon free path at high temepratures. For phonons with
energy {\rm K} the mean free path is of the order 100
{\rm nm}Comment: 5 pages, 4 figure
Restricted Wiedemann-Franz law and vanishing thermoelectric power in one-dimensional conductors
In one-dimensional (1D) conductors with linear E-k dispersion (Dirac systems)
intrabranch thermalization is favored by elastic electron-electron interaction
in contrast to electron systems with a nonlinear (parabolic) dispersion. We
show that under external electric fields or thermal gradients the carrier
populations of different branches, treated as Fermi gases, have different
temperatures as a consequence of self-consistent carrier-heat transport.
Specifically, in the presence of elastic phonon scattering, the Wiedemann-Franz
law is restricted to each branch with its specific temperature and is
characterized by twice the Lorenz number. In addition thermoelectric power
vanishes due to electron-hole symmetry, which is validated by experiment.Comment: 10 pages, 2 figure
Lattice thermal conductivity of graphene with conventionally isotopic defects
The thermal conductivity of doped graphene flake of finite size is
investigated with emphasis on the influence of mass of substituting atoms on
this property. It is shown that the graphene doping by small concentrations of
relatively heavy atoms results in a disproportionately impressive drop of
lattice thermal conductivity.Comment: 12 pages, 3 figure
Adiabatic-Nonadiabatic Transition in the Diffusive Hamiltonian Dynamics of a Classical Holstein Polaron
We study the Hamiltonian dynamics of a free particle injected onto a chain
containing a periodic array of harmonic oscillators in thermal equilibrium. The
particle interacts locally with each oscillator, with an interaction that is
linear in the oscillator coordinate and independent of the particle's position
when it is within a finite interaction range. At long times the particle
exhibits diffusive motion, with an ensemble averaged mean-squared displacement
that is linear in time. The diffusion constant at high temperatures follows a
power law D ~ T^{5/2} for all parameter values studied. At low temperatures
particle motion changes to a hopping process in which the particle is bound for
considerable periods of time to a single oscillator before it is able to escape
and explore the rest of the chain. A different power law, D ~ T^{3/4}, emerges
in this limit. A thermal distribution of particles exhibits thermally activated
diffusion at low temperatures as a result of classically self-trapped polaronic
states.Comment: 15 pages, 4 figures Submitted to Physical Review
Optical conductivity of metal nanofilms and nanowires: The rectangular-box model
The conductivity tensor is introduced for the low-dimensional electron
systems. Within the particle-in-a-box model and the diagonal response
approximation, components of the conductivity tensor for a quasi-homogeneous
ultrathin metal film and wire are calculated under the assumption (where is the characteristic small dimension of the
system, is the Fermi wavelength for bulk metal). We find the
transmittance of ultrathin films and compare these results with available
experimental data. The analytical estimations for the size dependence of the
Fermi level are presented, and the oscillations of the Fermi energy in
ultrathin films and wires are computed. Our results demonstrate the strong size
and frequency dependences of the real and imaginary parts of the conductivity
components in the infrared range. A sharp distinction of the results for Au and
Pb is observed and explained by the difference in the relaxation time of these
metals.Comment: 13 pages, 8 figure
Topological Degeneracy and Vortex Manipulation in Kitaev's Honeycomb Model
The classification of loop symmetries in Kitaev's honeycomb lattice model provides a natural framework to study the Abelian topological degeneracy. We derive a perturbative low-energy effective Hamiltonian that is valid to all orders of the expansion and for all possible toroidal configurations. Using this form we demonstrate at what order the system's topological degeneracy is lifted by finite size effects and note that in the thermodynamic limit it is robust to all orders. Further, we demonstrate that the loop symmetries themselves correspond to the creation, propagation, and annihilation of fermions. We note that these fermions, made from pairs of vortices, can be moved with no additional energy cost
Spin-glass instability of short-range spherical ferromagnet
In structurally disordered ferromagnets the weak random dipole-dipole
exchange may transform the polydomain state into a spin-glass one. To some
extent the properties of such phase in disordered isotropic ferromagnet can be
qualitatively described by the spherical model with the short-range
ferromagnetic interaction and weak frustrated infinite-range random-bond
exchange. This model is shown to predict that spin-glass phase substitute the
ferromagnetic one at the arbitrary small disorder strength and that its
thermodynamics has some similarity to that of polydomain state along with some
significant distinctions. In particular, the longitudinal susceptibility at
small fields becomes frozen below transition point at a constant value
depending on the disorder strength, while the third order nonlinear magnetic
susceptibilitiy exhibits the temperature oscillations in small field near the
transition point. The relation of these predictions to the experimental data
for some disordered isotropic ferromagnets is discussed.Comment: 7 pages, 5 figure
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