962 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
On the Emancipation of PLS-SEM: A Commentary on Rigdon (2012)
Rigdon's (2012) thoughtful article argues that PLS-SEM should free itself from CB-SEM. It should renounce all mechanisms, frameworks, and jargon associated with factor models entirely. In this comment, we shed further light on two subject areas on which Rigdon (2012) touches in his discussion of CB-SEM and PLS-SEM. Rigdon (2012) highlights ways to make better use of PLS-SEM's predictive capabilities, for example, by reverting to set correlations. We discuss this issue in more detail, highlighting the need to examine the predictive capabilities of models when developing and testing theories, and broach the issue of confirmatory versus exploratory modeling. As a result of our discussion, we call for the continuous improvement of the PLS-SEM method to uncover its capabilities for theory testing while retaining its predictive characte
Self-consistent study of Anderson localization in the Anderson-Hubbard model in two and three dimensions
We consider the change in electron localization due to the presence of
electron-electron repulsion in the \HA model. Taking into account local
Mott-Hubbard physics and static screening of the disorder potential, the system
is mapped onto an effective single-particle Anderson model, which is studied
within the self-consistent theory of electron localization. We find rich
nonmonotonic behavior of the localization length in two-dimensional
systems, including an interaction-induced exponential enhancement of for
small and intermediate disorders although remains finite. In three
dimensions we identify for half filling a Mott-Hubbard-assisted Anderson
localized phase existing between the metallic and the Mott-Hubbard-gapped
phases. For small there is re-entrant behavior from the Anderson localized
phase to the metallic phase.Comment: 7 pages, 4 figures, accepted for publication in Phys. Rev. B, journal
versio
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
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
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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
Floquet Formalism of Quantum Pumps
We review Floquet formalism of quantum electron pumps. In the Floquet
formalism the quantum pump is regarded as a time dependent scattering system,
which allows us to go beyond the adiabatic limit. It can be shown that the
well-known adiabatic formula given by Brouwer can be derived from the adiabatic
limit of Floquet formalism. We compare various physical properties of the
quantum pump both in the adiabatic and in the non-adiabatic regime using the
Floquet theory.Comment: Latex2e 16 pages, 6 figures. A review paper to appear in Int. J. Mod.
Phys.
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