10,611 research outputs found
Fragmentation Phase Transition in Atomic Clusters II - Coulomb Explosion of Metal Clusters -
We discuss the role and the treatment of polarization effects in many-body
systems of charged conducting clusters and apply this to the statistical
fragmentation of Na-clusters. We see a first order microcanonical phase
transition in the fragmentation of for Z=0 to 8. We can
distinguish two fragmentation phases, namely evaporation of large particles
from a large residue and a complete decay into small fragments only. Charging
the cluster shifts the transition to lower excitation energies and forces the
transition to disappear for charges higher than Z=8. At very high charges the
fragmentation phase transition no longer occurs because the cluster
Coulomb-explodes into small fragments even at excitation energy .Comment: 19 text pages +18 *.eps figures, my e-mail adress: [email protected]
submitted to Z. Phys.
On the inequivalence of statistical ensembles
We investigate the relation between various statistical ensembles of finite
systems. If ensembles differ at the level of fluctuations of the order
parameter, we show that the equations of states can present major differences.
A sufficient condition for this inequivalence to survive at the thermodynamical
limit is worked out. If energy consists in a kinetic and a potential part, the
microcanonical ensemble does not converge towards the canonical ensemble when
the partial heat capacities per particle fulfill the relation
.Comment: 4 pages, 4 figure
Critical dynamics of an isothermal compressible non-ideal fluid
A pure fluid at its critical point shows a dramatic slow-down in its
dynamics, due to a divergence of the order-parameter susceptibility and the
coefficient of heat transport. Under isothermal conditions, however, sound
waves provide the only possible relaxation mechanism for order-parameter
fluctuations. Here we study the critical dynamics of an isothermal,
compressible non-ideal fluid via scaling arguments and computer simulations of
the corresponding fluctuating hydrodynamics equations. We show that, below a
critical dimension of 4, the order-parameter dynamics of an isothermal fluid
effectively reduces to "model A," characterized by overdamped sound waves and a
divergent bulk viscosity. In contrast, the shear viscosity remains finite above
two dimensions. Possible applications of the model are discussed.Comment: 19 pages, 7 figures; v3: minor corrections and clarifications; as
published in Phys. Rev.
Vortices in Bose-Einstein-Condensed Atomic Clouds
The properties of vortex states in a Bose-Einstein condensed cloud of atoms
are considered at zero temperature. Using both analytical and numerical methods
we solve the time-dependent Gross-Pitaevskii equation for the case when a cloud
of atoms containing a vortex is released from a trap. In two dimensions we find
the simple result that the time dependence of the cloud radius is given by
, where is the trap frequency. We calculate and
compare the expansion of the vortex core and the cloud radius for different
numbers of particles and interaction strengths, in both two and three
dimensions, and discuss the circumstances under which vortex states may be
observed experimentally.Comment: Revtex, 11 pages including 5 eps figures, submitted to Phys. Rev. A;
new reference added, remark added in Sec. IIIB, axis label added in Fig.
On the criterion for Bose-Einstein condensation for particles in traps
We consider the criterion for Bose condensation for particles in a harmonic
trap. For a fixed angular momentum, the lowest energy state for a cloud of
bosons with attractive interactions is the ground state of the cloud with all
the angular momentum in the center-of-mass motion, and the one-particle reduced
density matrix generally does not have a single large eigenvalue, but a number
of them, suggesting that the state is an example of a fragmented condensate
(Wilkin, Gunn, and Smith, Phys. Rev. Lett. 80, 2265 (1998)). We show that a
convenient way to describe correlations in the system is by defining an
internal one-particle reduced density matrix, in which the center-of-mass
motion is eliminated, and that this has a single eigenvalue equal to the number
of particles for the problem considered here. Our considerations indicate that
care is necessary in formulating a criterion for Bose-Einstein condensation.Comment: 2 pages, RevTex, Submitted to Phys. Rev.
Time-dependent quantum transport: A practical scheme using density functional theory
We present a computationally tractable scheme of time-dependent transport
phenomena within open-boundary time-dependent density-functional-theory. Within
this approach all the response properties of a system are determined from the
time-propagation of the set of ``occupied'' Kohn-Sham orbitals under the
influence of the external bias. This central idea is combined with an
open-boundary description of the geometry of the system that is divided into
three regions: left/right leads and the device region (``real simulation
region''). We have derived a general scheme to extract the set of initial
states in the device region that will be propagated in time with proper
transparent boundary-condition at the device/lead interface. This is possible
due to a new modified Crank-Nicholson algorithm that allows an efficient
time-propagation of open quantum systems. We illustrate the method in
one-dimensional model systems as a first step towards a full first-principles
implementation. In particular we show how a stationary current develops in the
system independent of the transient-current history upon application of the
bias. The present work is ideally suited to study ac transport and
photon-induced charge-injection. Although the implementation has been done
assuming clamped ions, we discuss how it can be extended to include dissipation
due to electron-phonon coupling through the combined simulation of the
electron-ion dynamics as well as electron-electron correlations.Comment: 14 pages, 9 figures, one of which consist of two separate file
Binary Bose-Einstein Condensate Mixtures in Weakly and Strongly Segregated Phases
We perform a mean-field study of the binary Bose-Einstein condensate mixtures
as a function of the mutual repulsive interaction strength. In the phase
segregated regime, we find that there are two distinct phases: the weakly
segregated phase characterized by a `penetration depth' and the strongly
segregated phase characterized by a healing length. In the weakly segregated
phase the symmetry of the shape of each condensate will not take that of the
trap because of the finite surface tension, but its total density profile still
does. In the strongly segregated phase even the total density profile takes a
different symmetry from that of the trap because of the mutual exclusion of the
condensates. The lower critical condensate-atom number to observe the complete
phase segregation is discussed. A comparison to recent experimental data
suggests that the weakly segregated phase has been observed.Comment: minor change
Non-perturbative Debye mass in finite T QCD
Employing a non-perturbative gauge invariant definition of the Debye
screening mass m_D in the effective field theory approach to finite T QCD, we
use 3d lattice simulations to determine the leading O(g^2) and to estimate the
next-to-leading O(g^3) corrections to m_D in the high temperature region. The
O(g^2) correction is large and modifies qualitatively the standard
power-counting hierarchy picture of correlation lengths in high temperature
QCD.Comment: 4 pages, Late
Chiral bosons and improper constraints
We argue that a consistent quantization of the Floreanini-Jackiw model, as a
constrained system, should start by recognizing the improper nature of the
constraints. Then each boundary conditon defines a problem which must be
treated sparately. The model is settled on a compact domain which allows for a
discrete formulation of the dynamics; thus, avoiding the mixing of local with
collective coordinates. For periodic boundary conditions the model turns out to
be a gauge theory whose gauge invariant sector contains only chiral
excitations. For antiperiodoc boundary conditions, the mode is a second-class
theory where the excitations are also chiral. In both cases, the equal-time
algebra of the quantum energy-momentum densities is a Virasoro algebra. The
Poincar\'e symmetry holds for the finite as well as for the infinite domain.Comment: 13 pages, Revtex file, IF.UFRGS Preprin
Using the local density approximation and the LYP, BLYP, and B3LYP functionals within Reference--State One--Particle Density--Matrix Theory
For closed-shell systems, the local density approximation (LDA) and the LYP,
BLYP, and B3LYP functionals are shown to be compatible with reference-state
one-particle density-matrix theory, where this recently introduced formalism is
based on Brueckner-orbital theory and an energy functional that includes exact
exchange and a non-universal correlation-energy functional. The method is
demonstrated to reduce to a density functional theory when the
exchange-correlation energy-functional has a simplified form, i.e., its
integrand contains only the coordinates of two electron, say r1 and r2, and it
has a Dirac delta function -- delta(r1 - r2) -- as a factor. Since Brueckner
and Hartree--Fock orbitals are often very similar, any local exchange
functional that works well with Hartree--Fock theory is a reasonable
approximation with reference-state one-particle density-matrix theory. The LDA
approximation is also a reasonable approximation. However, the Colle--Salvetti
correlation-energy functional, and the LYP variant, are not ideal for the
method, since these are universal functionals. Nevertheless, they appear to
provide reasonable approximations. The B3LYP functional is derived using a
linear combination of two functionals: One is the BLYP functional; the other
uses exact exchange and a correlation-energy functional from the LDA.Comment: 26 Pages, 0 figures, RevTeX 4, Submitted to Mol. Phy
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