470 research outputs found
Ground-state properties of trapped Bose-Fermi mixtures: role of exchange-correlation
We introduce Density Functional Theory for inhomogeneous Bose-Fermi mixtures,
derive the associated Kohn-Sham equations, and determine the
exchange-correlation energy in local density approximation. We solve
numerically the Kohn-Sham system and determine the boson and fermion density
distributions and the ground-state energy of a trapped, dilute mixture beyond
mean-field approximation. The importance of the corrections due to
exchange--correlation is discussed by comparison with current experiments; in
particular, we investigate the effect of of the repulsive potential energy
contribution due to exchange--correlation on the stability of the mixture
against collapse.Comment: 6 pages, 4 figures (final version as published in Physical Review
Thermodynamics of a Trapped Bose-Fermi Mixture
By using the Hartree-Fock-Bogoliubov equations within the Popov
approximation, we investigate the thermodynamic properties of a dilute binary
Bose-Fermi mixture confined in an isotropic harmonic trap. For mixtures with an
attractive Bose-Fermi interaction we find a sizable enhancement of the
condensate fraction and of the critical temperature of Bose-Einstein
condensation with respect to the predictions for a pure interacting Bose gas.
Conversely, the influence of the repulsive Bose-Fermi interaction is less
pronounced. The possible relevance of our results in current experiments on
trapped {\rm K} mixtures is discussed.Comment: 5 pages + 4 figures; minor changes, final version to appear in Phys.
Rev. A; the extension work on the finite-temperature low-lying excitations
can be found in cond-mat/030763
Demixing in mesoscopic boson-fermion clouds inside cylindrical harmonic traps: quantum phase diagram and role of temperature
We use a semiclassical three-fluid thermodynamic model to evaluate the
phenomena of spatial demixing in mesoscopic clouds of fermionic and bosonic
atoms at high dilution under harmonic confinement, assuming repulsive
boson-boson and boson-fermion interactions and including account of a bosonic
thermal cloud at finite temperature T. The finite system size allows three
different regimes for the equilibrium density profiles at T=0: a fully mixed
state, a partially mixed state in which the overlap between the boson and
fermion clouds is decreasing, and a fully demixed state where the two clouds
have zero overlap. We propose simple analytical rules for the two cross-overs
between the three regimes as functions of the physical system parameters and
support these rules by extensive numerical calculations. A universal ``phase
diagram'' expressed in terms of simple scaling parameters is shown to be valid
for the transition to the regime of full demixing, inside which we identify
several exotic configurations for the two phase-separated clouds in addition to
simple ones consisting of a core of bosons enveloped by fermions and "vice
versa". With increasing temperature the main role of the growing thermal cloud
of bosons is to transform some exotic configurations into more symmetric ones,
until demixing is ultimately lost. For very high values of boson-fermion
repulsive coupling we also report demixing between the fermions and the
thermally excited bosons.Comment: 11 pages, 8 figure
Collective excitations in trapped boson-fermion mixtures: from demixing to collapse
We calculate the spectrum of low-lying collective excitations in a gaseous
cloud formed by a Bose-Einstein condensate and a spin-polarized Fermi gas over
a range of the boson-fermion coupling strength extending from strongly
repulsive to strongly attractive. Increasing boson-fermion repulsions drive the
system towards spatial separation of its components (``demixing''), whereas
boson-fermion attractions drive it towards implosion (``collapse''). The
dynamics of the system is treated in the experimentally relevant collisionless
regime by means of a Random-Phase approximation and the behavior of a
mesoscopic cloud under isotropic harmonic confinement is contrasted with that
of a macroscopic mixture at given average particle densities. In the latter
case the locations of both the demixing and the collapse phase transitions are
sharply defined by the same stability condition, which is determined by the
softening of an eigenmode of either fermionic or bosonic origin. In contrast,
the transitions to either demixing or collapse in a mesoscopic cloud at fixed
confinement and particle numbers are spread out over a range of boson-fermion
coupling strength, and some initial decrease of the frequencies of a set of
collective modes is followed by hardening as evidenced by blue shifts of most
eigenmodes. The spectral hardening can serve as a signal of the impending
transition and is most evident when the number of bosons in the cloud is
relatively large. We propose physical interpretations for these dynamical
behaviors with the help of suitably defined partial compressibilities for the
gaseous cloud under confinement.Comment: 16 pages, 7 figures, revtex
Inter-areal coordination of columnar architectures during visual cortical development
The occurrence of a critical period of plasticity in the visual cortex has
long been established, yet its function in normal development is not fully
understood. Here we show that as the late phase of the critical period unfolds,
different areas of cat visual cortex develop in a coordinated manner.
Orientation columns in areas V1 and V2 become matched in size in regions that
are mutually connected. The same age trend is found for such regions in the
left and right brain hemisphere. Our results indicate that a function of
critical period plasticity is to progressively coordinate the functional
architectures of different cortical areas - even across hemispheres.Comment: 30 pages, 1 table, 6 figure
Finite temperature effects on the collapse of trapped Bose-Fermi mixtures
By using the self-consistent Hartree-Fock-Bogoliubov-Popov theory, we present
a detailed study of the mean-field stability of spherically trapped Bose-Fermi
mixtures at finite temperature. We find that, by increasing the temperature,
the critical particle number of bosons (or fermions) and the critical
attractive Bose-Fermi scattering length increase, leading to a significant
stabilization of the mixture.Comment: 5 pages, 4 figures; minor changes, proof version, to appear in Phys.
Rev. A (Nov. 1, 2003
Quantum phase transitions of light
Recently, condensed matter and atomic experiments have reached a length-scale
and temperature regime where new quantum collective phenomena emerge. Finding
such physics in systems of photons, however, is problematic, as photons
typically do not interact with each other and can be created or destroyed at
will. Here, we introduce a physical system of photons that exhibits strongly
correlated dynamics on a meso-scale. By adding photons to a two-dimensional
array of coupled optical cavities each containing a single two-level atom in
the photon-blockade regime, we form dressed states, or polaritons, that are
both long-lived and strongly interacting. Our zero temperature results predict
that this photonic system will undergo a characteristic Mott insulator
(excitations localised on each site) to superfluid (excitations delocalised
across the lattice) quantum phase transition. Each cavity's impressive photon
out-coupling potential may lead to actual devices based on these quantum
many-body effects, as well as observable, tunable quantum simulators. We
explicitly show that such phenomena may be observable in micro-machined diamond
containing nitrogen-vacancy colour centres and superconducting microwave
strip-line resonators.Comment: 11 pages, 5 figures (2 in colour
Voluntary exercise can strengthen the circadian system in aged mice
Consistent daily rhythms are important to healthy aging according to studies linking disrupted circadian rhythms with negative health impacts. We studied the effects of age and exercise on baseline circadian rhythms and on the circadian system's ability to respond to the perturbation induced by an 8 h advance of the light:dark (LD) cycle as a test of the system's robustness. Mice (male, mPer2luc/C57BL/6) were studied at one of two ages: 3.5 months (n = 39) and >18 months (n = 72). We examined activity records of these mice under entrained and shifted conditions as well as mPER2::LUC measures ex vivo to assess circadian function in the suprachiasmatic nuclei (SCN) and important target organs. Age was associated with reduced running wheel use, fragmentation of activity, and slowed resetting in both behavioral and molecular measures. Furthermore, we observed that for aged mice, the presence of a running wheel altered the amplitude of the spontaneous firing rate rhythm in the SCN in vitro. Following a shift of the LD cycle, both young and aged mice showed a change in rhythmicity properties of the mPER2::LUC oscillation of the SCN in vitro, and aged mice exhibited longer lasting internal desynchrony. Access to a running wheel alleviated some age-related changes in the circadian system. In an additional experiment, we replicated the effect of the running wheel, comparing behavioral and in vitro results from aged mice housed with or without a running wheel (>21 months, n = 8 per group, all examined 4 days after the shift). The impact of voluntary exercise on circadian rhythm properties in an aged animal is a novel finding and has implications for the health of older people living with environmentally induced circadian disruption
Coordinated optimization of visual cortical maps (II) Numerical studies
It is an attractive hypothesis that the spatial structure of visual cortical
architecture can be explained by the coordinated optimization of multiple
visual cortical maps representing orientation preference (OP), ocular dominance
(OD), spatial frequency, or direction preference. In part (I) of this study we
defined a class of analytically tractable coordinated optimization models and
solved representative examples in which a spatially complex organization of the
orientation preference map is induced by inter-map interactions. We found that
attractor solutions near symmetry breaking threshold predict a highly ordered
map layout and require a substantial OD bias for OP pinwheel stabilization.
Here we examine in numerical simulations whether such models exhibit
biologically more realistic spatially irregular solutions at a finite distance
from threshold and when transients towards attractor states are considered. We
also examine whether model behavior qualitatively changes when the spatial
periodicities of the two maps are detuned and when considering more than 2
feature dimensions. Our numerical results support the view that neither minimal
energy states nor intermediate transient states of our coordinated optimization
models successfully explain the spatially irregular architecture of the visual
cortex. We discuss several alternative scenarios and additional factors that
may improve the agreement between model solutions and biological observations.Comment: 55 pages, 11 figures. arXiv admin note: substantial text overlap with
arXiv:1102.335
- …