8,199 research outputs found
Anderson localization in a correlated fermionic mixture
A mixture of two fermionic species with different masses is studied in an
optical lattice. The heavy fermions are subject only to thermal fluctuations,
the light fermions also to quantum fluctuations. We derive the Ising-like
distribution for the heavy atoms and study the localization properties of the
light fermions numerically by a transfer-matrix method. In a two-dimensional
system one-parameter scaling of the localization length is found with a
transition from delocalized states at low temperatures to localized states at
high temperature. The critical exponent of the localization length is
.Comment: 5 pages, 4 figure
Robust Transport Properties in Graphene
Two-dimensional Dirac fermions are used to discuss quasiparticles in graphene
in the presence of impurity scattering. Transport properties are completely
dominated by diffusion. This may explain why recent experiments did not find
weak localization in graphene. The diffusion coefficient of the quasiparticles
decreases strongly with increasing strength of disorder. Using the Kubo
formalism, however, we find a robust minimal conductivity that is independent
of disorder. This is a consequence of the fact that the change of the diffusion
coefficient is fully compensated by a change of the number of delocalized
quasiparticle states.Comment: 4 pages, 1 figur
Modeling of the saturation current of a fission chamber taking into account the distorsion of electric field due to space charge effects
Fission chambers were first made fifty years ago for neutron detection. At
the moment, the French Atomic Energy Commission \textsf{(CEA-Cadarache)} is
developing a sub-miniature fission chamber technology with a diameter of 1.5 mm
working in the current mode (Bign). To be able to measure intense fluxes, it is
necessary to adjust the chamber geometry and the gas pressure before testing it
under real neutron flux. In the present paper, we describe a theoretical method
to foresee the current-voltage characteristics (sensitivity and saturation
plateau) of a fission chamber whose geometrical features are given, taking into
account the neutron flux to be measured (spectrum and intensity). The proposed
theoretical model describes electric field distortion resulting from charge
collection effect. A computer code has been developed on this model basis. Its
application to 3 kinds of fission chambers indicates excellent agreement
between theoretical model and measured characteristics
Evolution of field spiral galaxies up to redshifts z=1
We have gained VLT/FORS spectra and HST/ACS images of a sample of 220 distant
field spiral galaxies. Spatially resolved rotation curves were extracted and
fitted with synthetic velocity fields that take into account all geometric and
observational effects, like blurring due to the slit width and seeing
influence. The maximum rotation velocity Vmax could be determined for 124
galaxies that cover the redshift range 0.1<z<1.0. The luminosity-rotation
velocity distribution of this sample is offset from the Tully-Fisher relation
(TFR) of local low-mass spirals, whereas the distant high-mass spirals are
compatible with the local TFR. We show that the slope of the local and the
intermediate-z TFR would be in compliance if its scatter decreased by more than
a factor of 3 between z~0.5 and z~0. On the other hand, the distant
low-luminosity disks have much lower stellar M/L ratios than their local
counterparts, while high-luminosity disks barely evolved in M/L over the
covered redshift range. This could be the manifestation of the "downsizing"
effect, i.e. the succesive shift of the peak of star formation from high-mass
to low-mass galaxies towards lower redshifts. This trend might be canceled out
in the TF diagram due to the simultaneous evolution of multiple parameters. We
also estimate the ratios between stellar and total masses, finding that these
remained constant since z=1, as would be expected in the context of
hierarchically growing structure. (Abridged)Comment: 20 pages, 5 figures, ApJ, accepte
SCRIPTKELL : a tool for measuring cognitive effort and time processing in writing and other complex cognitive activities
We present SCRIPTKELL, a computer-assisted experimental tool that makes it possible to measure the time and cognitive effort allocated to the subprocesses of writing and other cognitive activities, SCRIPTKELL was designed to easily use and modulate Kellogg's (1986) triple-task procedure,.which consists of a combination of three tasks: a writing task (or another task), a reaction time task (auditory signal detection), and a directed retrospection task (after each signal detection during writing). We demonstrate how this tool can be used to address several novel empirical and theoretical issues. In sum, SCRIPTKELL should facilitate the flexible realization of experimental designs and the investigation of critical issues concerning the functional characteristics of complex cognitive activities
Short note on the excitonic Mott phase
An exciton gas on a lattice is analyzed in terms of a convergent hopping
expansion. For a given chemical potential our calculation provides a sufficient
condition for the hopping rate to obtain an exponential decay of the exciton
correlation function. This result indicates the existence of a Mott phase in
which strong fluctuations destroy the long range correlations in the exciton
gas at any temperature, either by thermal or by quantum fluctuations.Comment: 5 pages, 1 figur
Quantized Transport in Two-Dimensional Spin-Ordered Structures
We study in detail the transport properties of a model of conducting
electrons in the presence of double-exchange between localized spins arranged
on a 2D Kagome lattice, as introduced by Ohgushi, Murakami, and Nagaosa (2000).
The relationship between the canting angle of the spin texture and the
Berry phase field flux per triangular plaquette is derived explicitly
and we emphasize the similarities between this model and Haldane's honeycomb
lattice version of the quantum Hall effect (Haldane, 1988). The quantization of
the transverse (Hall) conductivity is derived explicitly from the
Kubo formula and a direct calculation of the longitudinal conductivity
shows the existence of a metal-insulator transition as a function
of the canting angle (or flux density ). This transition might
be linked to that observable in the manganite compounds or in the pyrochlore
ones, as the spin ordering changes from ferromagnetic to canted.Comment: 17 pages, 12 figure
Sensitive linear response of an electron-hole superfluid in a periodic potential
We consider excitons in a two-dimensional periodic potential and study the
linear response of the excitonic superfluid to an electromagnetic wave at low
and high densities. It turns out that the static structure factor for small
wavevectors is very sensitive to a change of density and temperature. It is a
consequence of the fact that thermal fluctuations play a crucial role at small
wavevectors, since exchanging the order of the two limits, zero temperature and
vanishing wavevector, leads to different results for the structure factor. This
effect could be used for high accuracy measurements in the superfluid exciton
phase, which might be realized by a gated electron-hole gas. The transition of
the exciton system from the superfluid state to a non-superfluid state and its
manifestation by light scattering are discussed.Comment: 9 pages, 5 figure
Interacting bosons in an optical lattice: Bose-Einstein condensates and Mott insulator
A dense Bose gas with hard-core interaction is considered in an optical
lattice. We study the phase diagram in terms of a special mean-field theory
that describes a Bose-Einstein condensate and a Mott insulator with a single
particle per lattice site for zero as well as for non-zero temperatures. We
calculate the densities, the excitation spectrum and the static structure
factor for each of these phases.Comment: 17 pages, 5 figures; 1 figure added, typos remove
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