927 research outputs found
Lane reduction in driven 2d-colloidal systems through microchannels
The transport behavior of a system of gravitationally driven colloidal
particles is investigated. The particle interactions are determined by the
superparamagnetic behavior of the particles. They can thus be arranged in a
crystalline order by application of an external magnetic field. Therefore the
motion of the particles through a narrow channel occurs in well-defined lanes.
The arrangement of the particles is perturbed by diffusion and the motion
induced by gravity. Due to these combined influences a density gradient forms
along the direction of motion of the particles. A reconfiguration of the
crystal is observed leading to a reduction of the number of lanes. In the
course of the lane reduction transition a local melting of the
quasi-crystalline phase to a disordered phase and a subsequent crystallization
along the motion of the particles is observed. This transition is characterized
experimentally and using Brownian dynamics (BD) simulations.Comment: 4 pages, 4 figure
Two-dimensional Anderson-Hubbard model in DMFT+Sigma approximation
Density of states, dynamic (optical) conductivity and phase diagram of
paramagnetic two-dimensional Anderson-Hubbard model with strong correlations
and disorder are analyzed within the generalized dynamical mean-field theory
(DMFT+Sigma approximation). Strong correlations are accounted by DMFT, while
disorder is taken into account via the appropriate generalization of the
self-consistent theory of localization. We consider the two-dimensional system
with the rectangular "bare" density of states (DOS). The DMFT effective single
impurity problem is solved by numerical renormalization group (NRG). Phases of
"correlated metal", Mott insulator and correlated Anderson insulator are
identified from the evolution of density of states, optical conductivity and
localization length, demonstrating both Mott-Hubbard and Anderson
metal-insulator transitions in two-dimensional systems of the finite size,
allowing us to construct the complete zero-temperature phase diagram of
paramagnetic Anderson-Hubbard model. Localization length in our approximation
is practically independent of the strength of Hubbard correlations. However,
the divergence of localization length in finite size two-dimensional system at
small disorder signifies the existence of an effective Anderson transition.Comment: 10 pages, 10 figures, improve phase diagra
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Small Disks and Semiclassical Resonances
We study the effect on quantum spectra of the existence of small circular
disks in a billiard system. In the limit where the disk radii vanish there is
no effect, however this limit is approached very slowly so that even very small
radii have comparatively large effects. We include diffractive orbits which
scatter off the small disks in the periodic orbit expansion. This situation is
formally similar to edge diffraction except that the disk radii introduce a
length scale in the problem such that for wave lengths smaller than the order
of the disk radius we recover the usual semi-classical approximation; however,
for wave lengths larger than the order of the disk radius there is a
qualitatively different behaviour. We test the theory by successfully
estimating the positions of scattering resonances in geometries consisting of
three and four small disks.Comment: Final published version - some changes in the discussion and the
labels on one figure are correcte
Superiority of semiclassical over quantum mechanical calculations for a three-dimensional system
In systems with few degrees of freedom modern quantum calculations are, in
general, numerically more efficient than semiclassical methods. However, this
situation can be reversed with increasing dimension of the problem. For a
three-dimensional system, viz. the hyperbolic four-sphere scattering system, we
demonstrate the superiority of semiclassical versus quantum calculations.
Semiclassical resonances can easily be obtained even in energy regions which
are unattainable with the currently available quantum techniques.Comment: 10 pages, 1 figure, submitted to Phys. Lett.
Nonadiabatic pumping in classical and quantum chaotic scatterers
We study directed transport in periodically forced scattering systems in the
regime of fast and strong driving where the dynamics is mixed to chaotic and
adiabatic approximations do not apply. The model employed is a square potential
well undergoing lateral oscillations, alternatively as two- or single-parameter
driving. Mechanisms of directed transport are analyzed in terms of asymmetric
irregular scattering processes. Quantizing the system in the framework of
Floquet scattering theory, we calculate directed currents on basis of
transmission and reflection probabilities obtained by numerical wavepacket
scattering. We observe classical as well as quantum transport beyond linear
response, manifest in particular in a non-zero current for single-parameter
driving where according to adiabatic theory, it should vanish identically.Comment: 13 pages, 8 figure
Floquet scattering in parametric electron pumps
A Floquet scattering approach to parametric electron pumps is presented and
compared with Brouwer's adiabatic scattering approach [Phys. Rev. B 58, R10135
(1998)] for a simple scattering model with two harmonically oscillating
delta-function barriers. For small strength of oscillating potentials these two
approaches give exactly equivalent results while for large strength, these
clearly deviate from each other. The validity of the adiabatic theory is also
discussed by using the Wigner delay time obtained from the Floquet scattering
matrix.Comment: 10 pages, 7 figure
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