512 research outputs found
Improved Pseudofermion Approach for All-Point Propagators
Quark propagators with arbitrary sources and sinks can be obtained more
efficiently using a pseudofermion method with a mode-shifted action.
Mode-shifting solves the problem of critical slowing down (for light quarks)
induced by low eigenmodes of the Dirac operator. The method allows the full
physical content of every gauge configuration to be extracted, and should be
especially helpful for unquenched QCD calculations. The method can be applied
for all the conventional quark actions: Wilson, Sheikoleslami-Wohlert,
Kogut-Susskind, as well as Ginsparg-Wilson compliant overlap actions. The
statistical properties of the method are examined and examples of physical
processes under study are presented.Comment: LateX, 26 pages, 10 eps figure
Dynamical surface structures in multi-particle-correlated surface growths
We investigate the scaling properties of the interface fluctuation width for
the -mer and -particle-correlated deposition-evaporation models. These
models are constrained with a global conservation law that the particle number
at each height is conserved modulo . In equilibrium, the stationary
roughness is anomalous but universal with roughness exponent ,
while the early time evolution shows nonuniversal behavior with growth exponent
varying with models and . Nonequilibrium surfaces display diverse
growing/stationary behavior. The -mer model shows a faceted structure, while
the -particle-correlated model a macroscopically grooved structure.Comment: 16 pages, 10 figures, revte
Quantum Kinetic Theory III: Quantum kinetic master equation for strongly condensed trapped systems
We extend quantum kinetic theory to deal with a strongly Bose-condensed
atomic vapor in a trap. The method assumes that the majority of the vapor is
not condensed, and acts as a bath of heat and atoms for the condensate. The
condensate is described by the particle number conserving Bogoliubov method
developed by one of the authors. We derive equations which describe the
fluctuations of particle number and phase, and the growth of the Bose-Einstein
condensate. The equilibrium state of the condensate is a mixture of states with
different numbers of particles and quasiparticles. It is not a quantum
superposition of states with different numbers of particles---nevertheless, the
stationary state exhibits the property of off-diagonal long range order, to the
extent that this concept makes sense in a tightly trapped condensate.Comment: 3 figures submitted to Physical Review
Experiments of Interfacial Roughening in Hele-Shaw Flows with Weak Quenched Disorder
We have studied the kinetic roughening of an oil--air interface in a forced
imbibition experiment in a horizontal Hele--Shaw cell with quenched disorder.
Different disorder configurations, characterized by their persistence length in
the direction of growth, have been explored by varying the average interface
velocity v and the gap spacing b. Through the analysis of the rms width as a
function of time, we have measured a growth exponent beta ~= 0.5 that is almost
independent of the experimental parameters. The analysis of the roughness
exponent alpha through the power spectrum have shown different behaviors at
short (alpha_1) and long (alpha_2) length scales, separated by a crossover
wavenumber q_c. The values of the measured roughness exponents depend on
experimental parameters, but at large velocities we obtain alpha_1 ~= 1.3
independently of the disorder configuration. The dependence of the crossover
wavenumber with the experimental parameters has also been investigated,
measuring q_c ~ v^{0.47} for the shortest persistence length, in agreement with
theoretical predictions.Comment: 20 pages, 22 figure
Adiabatic Output Coupling of a Bose Gas at Finite Temperatures
We develop a general theory of adiabatic output coupling from trapped atomic
Bose-Einstein Condensates at finite temperatures. For weak coupling, the output
rate from the condensate, and the excited levels in the trap, settles in a time
proportional to the inverse of the spectral width of the coupling to the output
modes. We discuss the properties of the output atoms in the quasi-steady-state
where the population in the trap is not appreciably depleted. We show how the
composition of the output beam, containing condensate and thermal component,
may be controlled by changing the frequency of the output coupler. This
composition determines the first and second order coherence of the output beam.
We discuss the changes in the composition of the bose gas left in the trap and
show how nonresonant output coupling can stimulate either the evaporation of
thermal excitations in the trap or the growth of non-thermal excitations, when
pairs of correlated atoms leave the condensate.Comment: 22 pages, 6 Figs. To appear in Physical Review A All the typos from
the previous submission have been fixe
Solutions of Gross-Pitaevskii equations beyond the hydrodynamic approximation: Application to the vortex problem
We develop the multiscale technique to describe excitations of a
Bose-Einstein condensate (BEC) whose characteristic scales are comparable with
the healing length, thus going beyond the conventional hydrodynamical
approximation. As an application of the theory we derive approximate explicit
vortex and other solutions. The dynamical stability of the vortex is discussed
on the basis of the mathematical framework developed here, the result being
that its stability is granted at least up to times of the order of seconds,
which is the condensate lifetime. Our analytical results are confirmed by the
numerical simulations.Comment: To appear in Phys. Rev.
Effective interaction between helical bio-molecules
The effective interaction between two parallel strands of helical
bio-molecules, such as deoxyribose nucleic acids (DNA), is calculated using
computer simulations of the "primitive" model of electrolytes. In particular we
study a simple model for B-DNA incorporating explicitly its charge pattern as a
double-helix structure. The effective force and the effective torque exerted
onto the molecules depend on the central distance and on the relative
orientation. The contributions of nonlinear screening by monovalent counterions
to these forces and torques are analyzed and calculated for different salt
concentrations. As a result, we find that the sign of the force depends
sensitively on the relative orientation. For intermolecular distances smaller
than it can be both attractive and repulsive. Furthermore we report a
nonmonotonic behaviour of the effective force for increasing salt
concentration. Both features cannot be described within linear screening
theories. For large distances, on the other hand, the results agree with linear
screening theories provided the charge of the bio-molecules is suitably
renormalized.Comment: 18 pages, 18 figures included in text, 100 bibliog
Adsorption of mono- and multivalent cat- and anions on DNA molecules
Adsorption of monovalent and multivalent cat- and anions on a deoxyribose
nucleic acid (DNA) molecule from a salt solution is investigated by computer
simulation. The ions are modelled as charged hard spheres, the DNA molecule as
a point charge pattern following the double-helical phosphate strands. The
geometrical shape of the DNA molecules is modelled on different levels ranging
from a simple cylindrical shape to structured models which include the major
and minor grooves between the phosphate strands. The densities of the ions
adsorbed on the phosphate strands, in the major and in the minor grooves are
calculated. First, we find that the adsorption pattern on the DNA surface
depends strongly on its geometrical shape: counterions adsorb preferentially
along the phosphate strands for a cylindrical model shape, but in the minor
groove for a geometrically structured model. Second, we find that an addition
of monovalent salt ions results in an increase of the charge density in the
minor groove while the total charge density of ions adsorbed in the major
groove stays unchanged. The adsorbed ion densities are highly structured along
the minor groove while they are almost smeared along the major groove.
Furthermore, for a fixed amount of added salt, the major groove cationic charge
is independent on the counterion valency. For increasing salt concentration the
major groove is neutralized while the total charge adsorbed in the minor groove
is constant. DNA overcharging is detected for multivalent salt. Simulations for
a larger ion radii, which mimic the effect of the ion hydration, indicate an
increased adsorbtion of cations in the major groove.Comment: 34 pages with 14 figure
Moments of Nucleon Light Cone Quark Distributions Calculated in Full Lattice QCD
Moments of the quark density, helicity, and transversity distributions are
calculated in unquenched lattice QCD. Calculations of proton matrix elements of
operators corresponding to these moments through the operator product expansion
have been performed on lattices for Wilson fermions at using configurations from the SESAM collaboration and at
using configurations from SCRI. One-loop perturbative renormalization
corrections are included. At quark masses accessible in present calculations,
there is no statistically significant difference between quenched and full QCD
results, indicating that the contributions of quark-antiquark excitations from
the Dirac Sea are small. Close agreement between calculations with cooled
configurations containing essentially only instantons and the full gluon
configurations indicates that quark zero modes associated with instantons play
a dominant role. Naive linear extrapolation of the full QCD calculation to the
physical pion mass yields results inconsistent with experiment. Extrapolation
to the chiral limit including the physics of the pion cloud can resolve this
discrepancy and the requirements for a definitive chiral extrapolation are
described.Comment: 53 Pages Revtex, 26 Figures, 9 Tables. Added additional reference and
updated referenced data in Table I
'My language, my people': language and ethnic identity among Britishâborn South Asians
This study explores how a group of second generation Asians (SGA) understood and defined language, focusing upon the role they perceived language to have played in their identity. Twelve SGA were interviewed and the data were subjected to qualitative thematic analysis. Four superordinate themes are reported, entitled 'Mother tongue and self', #A sense of ownership and affiliation', 'Negotiating linguistic identities in social space' and 'The quest for a positive linguistic identity'. Participants generally expressed a desire to maintain continuity of selfâdefinition as Asian, primarily through the maintenance of the heritage language (HL). An imperfect knowledge of the HL was said to have a negative impact upon psychological wellâbeing. There were ambivalent responses to the perception of language norms, and various strategies were reported for dealing with dilemmatic situations and identity threat arising from bilingualism. Recommendations are offered for interventions that might aid the âmanagementâ of bilingualism among SGA
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