20,480 research outputs found
Flow effects on multifragmentation in the canonical model
A prescription to incorporate the effects of nuclear flow on the process of
multifragmentation of hot nuclei is proposed in an analytically solvable
canonical model. Flow is simulated by the action of an effective negative
external pressure. It favors sharpening the signatures of liquid-gas phase
transition in finite nuclei with increased multiplicity and a lowered phase
transition temperature.Comment: 13 pages, 5 Post Script figures (accepted for publication in PRC
Exchange-correlation potential for Current Density Functional Theory of frequency dependent linear response
The dynamical, long-wavelength longitudinal and transverse
exchange-correlation potentials for a homogeneous electron gas are evaluated in
a microscopic model based on an approximate decoupling of the equation of
motion for the current-current response function. The transverse spectrum turns
out to be very similar to the longitudinal one. We obtain evidence for a strong
spectral structure near twice the plasma frequency due to a two-plasmon
threshold for two-pair excitations, which may be observable in inelastic
scattering experiments. Our results give the entire input needed to implement
the Time-Dependent Current Density Functional Theory scheme recently developed
by G. Vignale and W. Kohn [Phys. Rev. Lett. 77, 2037 (1996)] and are fitted to
analytic functions to facilitate such applications.Comment: 6 pages, 3 figure
Nuclear Chemical and Mechanical Instability and the Liquid-Gas Phase Transition in Nuclei
The thermodynamic properties of nuclei are studied in a mean field model
using a Skryme interaction. Properties of two component systems are
investigated over the complete range of proton fraction from a system of pure
neutrons to a system of only protons. Besides volume, symmetry, and Coulomb
effects we also include momentum or velocity dependent forces. Applications of
the results developed are then given which include nuclear mechanical and
chemical instability and an associated liquid/gas phase transition in two
component systems. The velocity dependence leads to further changes in the
coexistence curve and nuclear mechanical and chemical instability curves.Comment: 21 pages, 9 figures, Results are changed due to error in progra
Rare isotope production in statistical multifragmentation
Producing rare isotopes through statistical multifragmentation is
investigated using the Mekjian method for exact solutions of the canonical
ensemble. Both the initial fragmentation and the the sequential decay are
modeled in such a way as to avoid Monte Carlo and thus provide yields for
arbitrarily scarce fragments. The importance of sequential decay, exact
particle-number conservation and the sensitivities to parameters such as
density and temperature are explored. Recent measurements of isotope ratios
from the fragmentation of different Sn isotopes are interpreted within this
picture.Comment: 10 eps figure
Excitations in time-dependent density-functional theory
An approximate solution to the time-dependent density functional theory
(TDDFT) response equations for finite systems is developed, yielding
corrections to the single-pole approximation. These explain why allowed
Kohn-Sham transition frequencies and oscillator strengths are usually good
approximations to the true values, and why sometimes they are not. The
approximation yields simple expressions for G\"orling-Levy perturbation theory
results, and a method for estimating expectation values of the unknown
exchange-correlation kernel.Comment: 4 pages, 1 tabl
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
Maximum flow and topological structure of complex networks
The problem of sending the maximum amount of flow between two arbitrary
nodes and of complex networks along links with unit capacity is
studied, which is equivalent to determining the number of link-disjoint paths
between and . The average of over all node pairs with smaller degree
is for large with a constant implying that the statistics of is related to the
degree distribution of the network. The disjoint paths between hub nodes are
found to be distributed among the links belonging to the same edge-biconnected
component, and can be estimated by the number of pairs of edge-biconnected
links incident to the start and terminal node. The relative size of the giant
edge-biconnected component of a network approximates to the coefficient .
The applicability of our results to real world networks is tested for the
Internet at the autonomous system level.Comment: 7 pages, 4 figure
Dilute Multi Alpha Cluster States in Nuclei
Dilute multi cluster condensed states with spherical and axially
deformed shapes are studied with the Gross-Pitaevskii equation and Hill-Wheeler
equation, where the cluster is treated as a structureless boson.
Applications to self-conjugate nuclei show that the dilute
states of C to Ca with appear in the energy region
from threshold up to about 20 MeV, and the critical number of bosons
that the dilute system can sustain as a self-bound nucleus is
estimated roughly to be . We discuss the characteristics of the
dilute states with emphasis on the dependence of their energies
and rms radii.Comment: 44 pages, 8 figure
Cosmological Implications of Lyman-Break Galaxy Clustering
We review our analysis of the clustering properties of ``Lyman-break''
galaxies (LBGs) at redshift z~3, previously discussed in Wechsler et al (1998).
We examine the likelihood of spikes found by Steidel et al (1998) in the
redshift distribution of LBGs, within a suite of models for the evolution of
structure in the Universe. Using high-resolution dissipationless N-body
simulations, we analyze deep pencil-beam surveys from these models in the same
way that they are actually observed, identifying LBGs with the most massive
dark matter halos. We find that all the models (with SCDM as a marginal
exception) have a substantial probability of producing spikes similar to those
observed, because the massive halos are much more clumped than the underlying
matter -- i.e., they are biased. Therefore, the likelihood of such a spike is
not a good discriminator among these models. The LBG correlation functions are
less steep than galaxies today (gamma~1.4), but show similar or slightly longer
correlation lengths. We have extened this analysis and include a preliminary
comparison to the new data presented in Adelberger et al (1998). We also
discuss work in progress, in which we use semi-analytic models to identify
Lyman-break galaxies within dark-matter halos.Comment: 4 pages, 2 figures, Latex, uses aipproc.sty; to appear in the
proceedings of the 9th Annual October Maryland Astrophysics Conference,
"After the Dark Ages: When the Galaxies Were Young (the Universe at 2<z<5)
String Spectrum of 1+1-Dimensional Large N QCD with Adjoint Matter
We propose gauging matrix models of string theory to eliminate unwanted
non-singlet states. To this end we perform a discretised light-cone
quantisation of large N gauge theory in 1+1 dimensions, with scalar or
fermionic matter fields transforming in the adjoint representation of SU(N).
The entire spectrum consists of bosonic and fermionic closed-string
excitations, which are free as N tends to infinity. We analyze the general
features of such bound states as a function of the cut-off and the gauge
coupling, obtaining good convergence for the case of adjoint fermions. We
discuss possible extensions of the model and the search for new non-critical
string theories.Comment: 20 pages (7 figures available from authors as postscipt files),
PUPT-134
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