85,295 research outputs found
Ground state and vortex states of bosons in an anisotropic trap: A variational approach
We propose a simple variational form of the wave function to describe the
ground state and vortex states of a system of weakly interacting Bose gas in an
anisotropic trap. The proposed wave function is valid for a wide range of the
particle numbers in the trap. It also works well in the case of attractive
interaction between the atoms. Further, it provides an easy and fast method to
calculate the physical quantities of interest. The results compare very well
with those obtained by purely numerical techniques. Using our wave function we
have been able to verify, for the first time, the predicted behaviour of aspect
ratio.Comment: Revised version, To appear in the special issue of European Physical
Journal
Pairing in the continuum: the quadrupole response of the Borromean nucleus 6He
The ground state and low-lying continuum states of 6He are found within a
shell model scheme, in a basis of two-particle states built out of continuum
p-states of the unbound 5He nucleus, using a simple pairing contact-delta
interaction. This accounts for the Borromean character of the bound ground
state, revealing its composition. We investigate the quadrupole response of the
system and we put our calculations into perspective with the latest
experimental results. The calculated quadrupole strength distribution
reproduces the narrow 2+ resonance, while a second wider peak is found at about
3.9 MeV above the g.s. energy.Comment: 5 pages, 5 figure
Electric multipole response of the halo nucleus He
The role of different continuum components in the weakly-bound nucleus He
is studied by coupling unbound spd-waves of He by means of simple pairing
contact-delta interaction. The results of our previous investigations in a
model space containing only p-waves, showed the collective nature of the ground
state and allowed the calculation of the electric quadrupole transitions. We
extend this simple model by including also sd-continuum neutron states and we
investigate the electric monopole, dipole and octupole response of the system
for transitions to the continuum, discussing the contribution of different
configurations.Comment: 22 pages, 10 figure
The Band Gap in Silicon Nanocrystallites
The gap in semiconductor nanocrystallites has been extensively studied both
theoretically and experimentally over the last two decades. We have compared a
recent ``state-of-the-art'' theoretical calculation with a recent
``state-of-the-art'' experimental observation of the gap in Si nanocrystallite.
We find that the two are in substantial disagreement, with the disagreement
being more pronounced at smaller sizes. Theoretical calculations appear to
over-estimate the gap. Recognizing that the experimental observations are for a
distribution of crystallite sizes, we proffer a phenomenological model to
reconcile the theory with the experiment. We suggest that similar
considerations must dictate comparisons between the theory and experiment
vis-a-vis other properties such as radiative rate, decay constant, absorption
coefficient, etc.Comment: 5 pages, latex, 2 figures. (Submitted Physical Review B
Phase diagram and magnetic collective excitations of the Hubbard model in graphene sheets and layers
We discuss the magnetic phases of the Hubbard model for the honeycomb lattice
both in two and three spatial dimensions. A ground state phase diagram is
obtained depending on the interaction strength
U and electronic density n. We find a first order phase transition between
ferromagnetic regions where the spin is maximally polarized (Nagaoka
ferromagnetism) and regions with smaller magnetization (weak ferromagnetism).
When taking into account the possibility of spiral states, we find that the
lowest critical U is obtained for an ordering momentum different from zero. The
evolution of the ordering momentum with doping is discussed. The magnetic
excitations (spin waves) in the antiferromagnetic insulating phase are
calculated from the random-phase-approximation for the spin susceptibility. We
also compute the spin fluctuation correction to the mean field magnetization by
virtual emission/absorpion of spin waves. In the large limit, the
renormalized magnetization agrees qualitatively with the Holstein-Primakoff
theory of the Heisenberg antiferromagnet, although the latter approach produces
a larger renormalization
High bat (Chiroptera) diversity in the Early Eocene of India
The geographic origin of bats is still unknown, and fossils of earliest bats are rare and poorly diversified, with, maybe, the exception of Europe. The earliest bats are recorded from the Early Eocene of North America, Europe, North Africa and Australia where they seem to appear suddenly and simultaneously. Until now, the oldest record in Asia was from the Middle Eocene. In this paper, we report the discovery of the oldest bat fauna of Asia dating from the Early Eocene of the Cambay Formation at Vastan Lignite Mine in Western India. The fossil taxa are described on the basis of well-preserved fragments of dentaries and lower teeth. The fauna is highly diversified and is represented by seven species belonging to seven genera and at least four families. Two genera and five species are new. Three species exhibit very primitive dental characters, whereas four others indicate more advanced states. Unexpectedly, this fauna presents strong affinities with the European faunas from the French Paris Basin and the German Messel locality. This could result from the limited fossil record of bats in Asia, but could also suggest new palaeobiogeographic scenarios involving the relative position of India during the Early Eocene
The Cole-Cole Law for Critical Dynamics in Glass-Forming Liquids
Within the mode-coupling theory (MCT) for glassy dynamics, the asymptotic
low-frequency expansions for the dynamical susceptibilities at critical points
are compared to the expansions for the dynamic moduli; this shows that the
convergence properties of the two expansions can be quite different. In some
parameter regions, the leading-order expansion formula for the modulus
describes the solutions of the MCT equations of motion outside the transient
regime successfully; at the same time, the leading- and next-to-leading order
expansion formulas for the susceptibility fail. In these cases, one can derive
a Cole-Cole law for the susceptibilities; and this law accounts for the
dynamics for frequencies below the band of microscopic excitations and above
the high-frequency part of the alpha-peak. It is shown that this scenario
explains the optical-Kerr-effect data measured for salol and benzophenone
(BZP). For BZP it is inferred that the depolarized light-scattering spectra
exhibit a wing for the alpha-peak within the Gigahertz band. This wing results
from the crossover of the von Schweidler-law part of the alpha-peak to the
high-frequency part of the Cole-Cole peak; and this crossover can be described
quantitatively by the leading-order formulas of MCT for the modulus.Comment: 15 pages, 9 figure
Geodesic acoustic modes in a fluid model of tokamak plasma : the effects of finite beta and collisionality
Starting from the Braginskii equations, relevant for the tokamak edge region,
a complete set of nonlinear equations for the geodesic acoustic modes (GAM) has
been derived which includes collisionality, plasma beta and external sources of
particle, momentum and heat. Local linear analysis shows that the GAM frequency
increases with collisionality at low radial wave number and decreases
at high . GAM frequency also decreases with plasma beta. Radial profiles
of GAM frequency for two Tore Supra shots, which were part of a collisionality
scan, are compared with these calculations. Discrepency between experiment and
theory is observed, which seems to be explained by a finite for the GAM
when flux surface averaged density and temperature are assumed to vanish. It is shown that this agreement is incidental
and self-consistent inclusion of and
responses enhances the disagreement more with at high . So the
discrepancy between the linear GAM calculation, (which persist also for more
"complete" linear models such as gyrokinetics) can probably not be resolved by
simply adding a finite
Low temperature properties of the triangular-lattice antiferromagnet: a bosonic spinon theory
We study the low temperature properties of the triangular-lattice Heisenberg
antiferromagnet with a mean field Schwinger spin-1/2 boson scheme that
reproduces quantitatively the zero temperature energy spectrum derived
previously using series expansions. By analyzing the spin-spin and the boson
density-density dynamical structure factors, we identify the unphysical spin
excitations that come from the relaxation of the local constraint on bosons.
This allows us to reconstruct a free energy based on the physical excitations
only, whose predictions for entropy and uniform susceptibility seem to be
reliable within the temperature range $0< T <0.3J, which is difficult to access
by other methods. The high values of entropy, also found in high temperature
expansions studies, can be attributed to the roton-like narrowed dispersion at
finite temperatures.Comment: 16 pages, 5 figure
Axial Vector Current Matrix Elements and QCD Sum Rules
The matrix element of the isoscalar axial vector current,
, between nucleon
states is computed using the external field QCD sum rule method. The external
field induced correlator, , is calculated
from the spectrum of the isoscalar axial vector meson states. Since it is
difficult to ascertain, from QCD sum rule for hyperons, the accuracy of
validity of flavour SU(3) symmetry in hyperon decays when strange quark mass is
taken into account, we rely on the empirical validity of Cabbibo theory to
dertermine the matrix element between
nucleon states. Combining with our calculation of and the well known nucleon -decay
constant allows us to determine
occuring in the Bjorken sum rule. The result is in reasonable agreement with
experiment. We also discuss the role of the anomaly in maintaining flavour
symmetry and validity of OZI rule.Comment: 8 pages, 4 figures, revtex
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