1,276 research outputs found
Impurity states in graphene with intrinsic spin-orbit interaction
We consider the problem of electron energy states related to strongly
localized potential of a single impurity in graphene. Our model simulates the
effect of impurity atom substituting the atom of carbon, on the energy spectrum
of electrons near the Dirac point. We take into account the internal spin-orbit
interaction, which can modify the structure of electron bands at very small
neighborhood of the Dirac point, leading to the energy gap. This makes possible
the occurrence of additional impurity states in the vicinity of the gap.Comment: 10 pages, 5 figure
Low-temperature electrical resistivity in paramagnetic spinel LiV2O4
The 3d electron spinel compound LiV2O4 exhibits heavy fermion behaviour below
30K which is related to antiferromagnetic spin fluctuations strongly enhanced
in an extended region of momentum space. This mechanism explains enhanced
thermodynamic quantities and nearly critical NMR relaxation in the framework of
the selfconsistent renormalization (SCR) theory. Here we show that the low-T
Fermi liquid behaviour of the resistivity and a deviation from this behavior
for higher T may also be understood within that context. We calculate the
temperature dependence of the electrical resistivity \rho(T) assuming that two
basic mechanisms of the quasiparticle scattering, resulting from impurities and
spin-fluctuations, operate simultaneously at low temperature. The calculation
is based on the variational principle in the form of a perturbative series
expansion for \rho(T). A peculiar behavior of \rho(T) in LiV2O4 is related to
properties of low-energy spin fluctuations whose T-dependence is obtained from
SCR theory.Comment: 10 pages, 3 figures, to appear in Phys. Rev.
Dynamics of the Formation of Bright Solitary Waves of Bose-Einstein Condensates in Optical Lattices
We present a detailed description of the formation of bright solitary waves
in optical lattices. To this end, we have considered a ring lattice geometry
with large radius. In this case, the ring shape does not have a relevant effect
in the local dynamics of the condensate, while offering a realistic set up to
implement experiments with conditions usually not available with linear
lattices (in particular, to study collisions). Our numerical results suggest
that the condensate radiation is the relevant dissipative process in the
relaxation towards a self-trapped solution. We show that the source of
dissipation can be attributed to the presence of higher order dispersion terms
in the effective mass approach. In addition, we demonstrate that the stability
of the solitary solutions is linked with particular values of the width of the
wavepacket in the reciprocal space. Our study suggests that these critical
widths for stability depend on the geometry of the energy band, but are
independent of the condensate parameters (momentum, atom number, etc.).
Finally, the non-solitonic nature of the solitary waves is evidenced showing
their instability under collisions.Comment: 7 pages, 7 figures, to appear in PR
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
Raduga experiment: Multizonal photographing the Earth from the Soyuz-22 spacecraft
The main results of the scientific research and 'Raduga' experiment are reported. Technical parameters are presented for the MKF-6 camera and the MSP-4 projector. Characteristics of the obtained materials and certain results of their processing are reported
Correlation effects in sequential energy branching: an exact model of the Fano statistics
Correlation effects in in the fluctuation of the number of particles in the
process of energy branching by sequential impact ionizations are studied using
an exactly soluble model of random parking on a line. The Fano factor F
calculated in an uncorrelated final-state "shot-glass" model does not give an
accurate answer even with the exact gap-distribution statistics. Allowing for
the nearest-neighbor correlation effects gives a correction to F that brings F
very close to its exact value. We discuss the implications of our results for
energy resolution of semiconductor gamma detectors, where the value of F is of
the essence. We argue that F is controlled by correlations in the cascade
energy branching process and hence the widely used final-state model estimates
are not reliable -- especially in the practically relevant cases when the
energy branching is terminated by competition between impact ionization and
phonon emission.Comment: 11 pages, 4 figures. Submitted to Physical Review
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
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