4,250 research outputs found
Multi-orbital bosons in bipartite optical lattices
We study interacting bosons in a two dimensional bipartite optical lattice.
By focusing on the regime where the first three excited bands are nearly
degenerate we derive a three orbital tight-binding model which captures the
most relevant features of the bandstructure. In addition, we also derive a
corresponding generalized Bose-Hubbard model and solve it numerically under
different situations, both with and without a confining trap. It is especially
found that the hybridization between sublattices can strongly influence the
phase diagrams and in a trap enable even appearances of condensed phases
intersecting the same Mott insulating plateaus.Comment: Minor change
A Comment On Gintis's "The Dynamics of General Equilibrium"
This is a comment on Gintis (2007, 'The Dynamics of General Equilibrium', Economic Journal 117 (523) , 1280\u20131309), who provides an agent-based model of a Walrasian economy where the t\ue2tonnement is replaced by imitation. His simulations show that the economy converges to the Walrasian equilibrium. Gintis concludes that 1) his stability results provide some justification for the importance placed upon the Walrasian model, and 2) models allowing agents to imitate successful others lead to an economy with a reasonable level of stability and efficiency. Since these conclusions appear to be intended as general, we caution that Gintis's findings can only be accepted for Walrasian models without capital goods; in models with capital goods imitation-based adjustments alter the equilibrium's data (which makes the demonstration of stability impossible) and raise other important problems (absent from Gintis's simulations) still awaiting exploration
Long term ordering kinetics of the two dimensional q-state Potts model
We studied the non-equilibrium dynamics of the q-state Potts model in the
square lattice, after a quench to sub-critical temperatures. By means of a
continuous time Monte Carlo algorithm (non-conserved order parameter dynamics)
we analyzed the long term behavior of the energy and relaxation time for a wide
range of quench temperatures and system sizes. For q>4 we found the existence
of different dynamical regimes, according to quench temperature range. At low
(but finite) temperatures and very long times the Lifshitz-Allen-Cahn domain
growth behavior is interrupted with finite probability when the system stuck in
highly symmetric non-equilibrium metastable states, which induce activation in
the domain growth, in agreement with early predictions of Lifshitz [JETP 42,
1354 (1962)]. Moreover, if the temperature is very low, the system always gets
stuck at short times in a highly disordered metastable states with finite life
time, which have been recently identified as glassy states. The finite size
scaling properties of the different relaxation times involved, as well as their
temperature dependency are analyzed in detail.Comment: 10 pages, 17 figure
Electric field generation by the electron beam filamentation instability: Filament size effects
The filamentation instability (FI) of counter-propagating beams of electrons
is modelled with a particle-in-cell simulation in one spatial dimension and
with a high statistical plasma representation. The simulation direction is
orthogonal to the beam velocity vector. Both electron beams have initially
equal densities, temperatures and moduli of their nonrelativistic mean
velocities. The FI is electromagnetic in this case. A previous study of a small
filament demonstrated, that the magnetic pressure gradient force (MPGF) results
in a nonlinearly driven electrostatic field. The probably small contribution of
the thermal pressure gradient to the force balance implied, that the
electrostatic field performed undamped oscillations around a background
electric field. Here we consider larger filaments, which reach a stronger
electrostatic potential when they saturate. The electron heating is enhanced
and electrostatic electron phase space holes form. The competition of several
smaller filaments, which grow simultaneously with the large filament, also
perturbs the balance between the electrostatic and magnetic fields. The
oscillations are damped but the final electric field amplitude is still
determined by the MPGF.Comment: 14 pages, 10 plots, accepted for publication in Physica Script
U.S. Congress prosopographer - A tool for prosopographical research of legislators
Peer reviewe
Fast dictionary-based compression for inverted indexes
Dictionary-based compression schemes provide fast decoding operation, typically at the expense of reduced compression effectiveness compared to statistical or probability-based approaches. In this work, we apply dictionary-based techniques to the compression of inverted lists, showing that the high degree of regularity that these integer sequences exhibit is a good match for certain types of dictionary methods, and that an important new trade-off balance between compression effectiveness and compression efficiency can be achieved. Our observations are supported by experiments using the document-level inverted index data for two large text collections, and a wide range of other index compression implementations as reference points. Those experiments demonstrate that the gap between efficiency and effectiveness can be substantially narrowed
Angular momentum transport in convectively unstable shear flows
Angular momentum transport owing to hydrodynamic turbulent convection is
studied using local three dimensional numerical simulations employing the
shearing box approximation. We determine the turbulent viscosity from
non-rotating runs over a range of values of the shear parameter and use a
simple analytical model in order to extract the non-diffusive contribution
(Lambda-effect) to the stress in runs where rotation is included. Our results
suggest that the turbulent viscosity is of the order of the mixing length
estimate and weakly affected by rotation. The Lambda-effect is non-zero and a
factor of 2-4 smaller than the turbulent viscosity in the slow rotation regime.
We demonstrate that for Keplerian shear, the angular momentum transport can
change sign and be outward when the rotation period is greater than the
turnover time, i.e. when the Coriolis number is below unity. This result seems
to be relatively independent of the value of the Rayleigh number.Comment: 10 pages, 12 figures, published version. Version with higher
resolution figures can be found at http://www.helsinki.fi/~kapyla/publ.htm
Band Ranking via Extended Coefficient of Variation for Hyperspectral Band Selection
Hundreds of narrow bands over a continuous spectral range make hyperspectral imagery rich in information about objects, while at the same time causing the neighboring bands to be highly correlated. Band selection is a technique that provides clear physical-meaning results for hyperspectral dimensional reduction, alleviating the difficulty for transferring and processing hyperspectral images caused by a property of hyperspectral images: large data volumes. In this study, a simple and efficient band ranking via extended coefficient of variation (BRECV) is proposed for unsupervised hyperspectral band selection. The naive idea of the BRECV algorithm is to select bands with relatively smaller means and lager standard deviations compared to their adjacent bands. To make this simple idea into an algorithm, and inspired by coefficient of variation (CV), we constructed an extended CV matrix for every three adjacent bands to study the changes of means and standard deviations, and accordingly propose a criterion to allocate values to each band for ranking. A derived unsupervised band selection based on the same idea while using entropy is also presented. Though the underlying idea is quite simple, and both cluster and optimization methods are not used, the BRECV method acquires qualitatively the same level of classification accuracy, compared with some state-of-the-art band selection methodsPeer reviewe
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