1,995 research outputs found
Collision of Polymers in a Vacuum
In a number of experimental situations, single polymer molecules can be
suspended in a vacuum. Here collisions between such molecules are considered.
The limit of high collision velocity is investigated numerically for a variety
of conditions. The distribution of contact times, scattering angles, and final
velocities are analyzed. In this limit, self avoiding chains are found to
become highly stretched as they collide with each other, and have a
distribution of scattering times that depends on the scattering angle. The
velocity of the molecules after the collisions is similar to predictions of a
model assuming thermal equilibration of molecules during the collision. The
most important difference is a significant subset of molecules that
inelastically scatter but do not substantially change direction.Comment: 7 pages, 6 figure
Phase diagram for the asymmetric nuclear matter in the multifragmentation model
We assume that, in equilibrium, nuclear matter at reduced density and
moderate finite temperature, breaks up into many fragments. A strong support to
this assumption is provided by date accumulated from intermediate energy heavy
ion collisions. The break-up of hot and expanded nuclear matter according to
rules of equilibrium statistical mechanics is the multifragmentation model. The
model gives a first order phase transition. This is studied in detail here.
Phase-equilibrium lines for different degrees of asymmetry are computed.Comment: 22 pages, 10 figure
A Model for Phase Transition based on Statistical Disassembly of Nuclei at Intermediate Energies
Consider a model of particles (nucleons) which has a two-body interaction
which leads to bound composites with saturation properties. These properties
are : all composites have the same density and the ground state energies of
composites with k nucleons are given by -kW+\sigma k^{2/3} where W and \sigma
are positive constants. W represents a volume term and \sigma a surface tension
term. These values are taken from nuclear physics. We show that in the large N
limit where N is the number of particles such an assembly in a large enclosure
at finite temperature shows properties of liquid-gas phase transition. We do
not use the two-body interaction but the gross properties of the composites
only. We show that (a) the p-\rho isotherms show a region where pressure does
not change as changes just as in Maxwell construction of a Van der Waals
gas, (b) in this region the chemical potential does not change and (c) the
model obeys the celebrated Clausius-Clapeyron relations. A scaling law for the
yields of composites emerges. For a finite number of particles N (upto some
thousands) the problem can be easily solved on a computer. This allows us to
study finite particle number effects which modify phase transition effects. The
model is calculationally simple. Monte-Carlo simulations are not needed.Comment: RevTex file, 21 pages, 5 figure
Quasi-Particle Degrees of Freedom versus the Perfect Fluid as Descriptors of the Quark-Gluon Plasma
The hot nuclear matter created at the Relativistic Heavy Ion Collider (RHIC)
has been characterized by near-perfect fluid behavior. We demonstrate that this
stands in contradiction to the identification of QCD quasi-particles with the
thermodynamic degrees of freedom in the early (fluid) stage of heavy ion
collisions. The empirical observation of constituent quark ``'' scaling of
elliptic flow is juxtaposed with the lack of such scaling behavior in
hydrodynamic fluid calculations followed by Cooper-Frye freeze-out to hadrons.
A ``quasi-particle transport'' time stage after viscous effects break down the
hydrodynamic fluid stage, but prior to hadronization, is proposed to reconcile
these apparent contradictions. However, without a detailed understanding of the
transitions between these stages, the ``'' scaling is not a necessary
consequence of this prescription. Also, if the duration of this stage is too
short, it may not support well defined quasi-particles. By comparing and
contrasting the coalescence of quarks into hadrons with the similar process of
producing light nuclei from nucleons, it is shown that the observation of
``'' scaling in the final state does not necessarily imply that the
constituent degrees of freedom were the relevant ones in the initial state.Comment: 9 pages, 7 figures, Updated text and figure
A Paradox of State-Dependent Diffusion and How to Resolve It
Consider a particle diffusing in a confined volume which is divided into two
equal regions. In one region the diffusion coefficient is twice the value of
the diffusion coefficient in the other region. Will the particle spend equal
proportions of time in the two regions in the long term? Statistical mechanics
would suggest yes, since the number of accessible states in each region is
presumably the same. However, another line of reasoning suggests that the
particle should spend less time in the region with faster diffusion, since it
will exit that region more quickly. We demonstrate with a simple microscopic
model system that both predictions are consistent with the information given.
Thus, specifying the diffusion rate as a function of position is not enough to
characterize the behaviour of a system, even assuming the absence of external
forces. We propose an alternative framework for modelling diffusive dynamics in
which both the diffusion rate and equilibrium probability density for the
position of the particle are specified by the modeller. We introduce a
numerical method for simulating dynamics in our framework that samples from the
equilibrium probability density exactly and is suitable for discontinuous
diffusion coefficients.Comment: 21 pages, 6 figures. Second round of revisions. This is the version
that will appear in Proc Roy So
Sub-milliKelvin spatial thermometry of a single Doppler cooled ion in a Paul trap
We report on observations of thermal motion of a single, Doppler-cooled ion
along the axis of a linear radio-frequency quadrupole trap. We show that for a
harmonic potential the thermal occupation of energy levels leads to Gaussian
distribution of the ion's axial position. The dependence of the spatial thermal
spread on the trap potential is used for precise calibration of our imaging
system's point spread function and sub-milliKelvin thermometry. We employ this
technique to investigate the laser detuning dependence of the Doppler
temperature.Comment: 5 pages, 4 figure
Dynamic roughening and fluctuations of dipolar chains
Nonmagnetic particles in a carrier ferrofluid acquire an effective dipolar
moment when placed in an external magnetic field. This fact leads them to form
chains that will roughen due to Brownian motion when the magnetic field is
decreased. We study this process through experiments, theory and simulations,
three methods that agree on the scaling behavior over 5 orders of magnitude.
The RMS width goes initially as , then as before it
saturates. We show how these results complement existing results on polymer
chains, and how the chain dynamics may be described by a recent non-Markovian
formulation of anomalous diffusion.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let
Relaxation paths for single modes of vibrations in isolated molecules
A numerical simulation of vibrational excitation of molecules was devised,
and used to excite computational models of common molecules into a prescribed,
pure, normal vibration mode in the ground electronic state, with varying,
controlable energy content. The redistribution of this energy (either
non-chaotic or irreversible IVR) within the isolated, free molecule is then
followed in time with a view to determining the coupling strength between
modes. This work was triggered by the need to predict the general characters of
the infrared spectra to be expected from molecules in interstellar space, after
being excited by photon absorption or reaction with a radical. It is found that
IVR from a pure normal mode is very "restricted" indeed at energy contents of
one mode quantum or so. However, as this is increased, or when the excitation
is localized, our approach allows us to isolate, describe and quantify a number
of interesting phenomena, known to chemists and in non-linear mechanics, but
difficult to demonstrate experimentally: frequency dragging, mode locking or
quenching or, still, instability near a potential surface crossing, the first
step to generalized chaos as the energy content per mode is increased.Comment: 25 pages, 15 figures; accepted by J. Atom. Phys.
Fcc-bcc transition for Yukawa interactions determined by applied strain deformation
Calculations of the work required to transform between bcc and fcc phases
yield a high-precision bcc-fcc transition line for monodisperse point Yukawa
(screened-Couloumb) systems. Our results agree qualitatively but not
quantitatively with previously published simulations and phenomenological
criteria for the bcc-fcc transition. In particular, the bcc-fcc-fluid triple
point lies at a higher inverse screening length than previously reported.Comment: RevTex4, 9 pages, 6 figures. Discussion of phase coexistence
extended, a few other minor clarifications added, referencing improved.
Accepted for publication by Physical Review
Nonzero orbital angular momentum superfluidity in ultracold Fermi gases
We analyze the evolution of superfluidity for nonzero orbital angular
momentum channels from the Bardeen-Cooper-Schrieffer (BCS) to the Bose-Einstein
condensation (BEC) limit in three dimensions. First, we analyze the low energy
scattering properties of finite range interactions for all possible angular
momentum channels. Second, we discuss ground state () superfluid
properties including the order parameter, chemical potential, quasiparticle
excitation spectrum, momentum distribution, atomic compressibility, ground
state energy and low energy collective excitations. We show that a quantum
phase transition occurs for nonzero angular momentum pairing, unlike the s-wave
case where the BCS to BEC evolution is just a crossover. Third, we present a
gaussian fluctuation theory near the critical temperature (),
and we analyze the number of bound, scattering and unbound fermions as well as
the chemical potential. Finally, we derive the time-dependent Ginzburg-Landau
functional near , and compare the Ginzburg-Landau coherence length
with the zero temperature average Cooper pair size.Comment: 28 pages and 24 figure
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