18,949 research outputs found
A note on the analogy between superfluids and cosmology
A new analogy between superfluid systems and cosmology is here presented,
which relies strongly on the following ingredient: the back-reaction of the
vacuum to the quanta of sound waves. We show how the presence of thermal
phonons, the excitations above the quantum vacuum for , enable us to
deduce an hydrodynamical equation formally similar to the one obtained for a
perfect fluid in a Universe obeying the Friedmann-Robertson-Walker metric.Comment: Accepted for publication in Modern Physics Letters
Unified approach to structure factors and neutrino processes in nucleon matter
We present a unified approach to neutrino processes in nucleon matter based
on Landau's theory of Fermi liquids that includes one- and
two-quasiparticle-quasihole pair states as well as mean-field effects. We show
how rates of neutrino processes involving two nucleons may be calculated in
terms of the collision integral in the Landau transport equation for
quasiparticles. Using a relaxation time approximation, we solve the transport
equation for density and spin-density fluctuations and derive a general form
for the response functions. We apply our approach to neutral-current processes
in neutron matter, where the spin response function is crucial for calculations
of neutrino elastic and inelastic scattering, neutrino-pair bremsstrahlung and
absorption from strongly-interacting nucleons. We calculate the relaxation
rates using modern nuclear interactions and including many-body contributions,
and find that rates of neutrino processes are reduced compared with estimates
based on the one-pion exchange interaction, which is used in current
simulations of core-collapse supernovae.Comment: 16 pages, 4 figures; NORDITA-2008-30; published versio
Decay of polarons and molecules in a strongly polarized Fermi gas
The ground state of an impurity immersed in a Fermi sea changes from a
polaron to a molecule as the interaction strength is increased.
We show here that the coupling between these two states is strongly
suppressed due to a combination of phase space effects and Fermi statistics,
and that it vanishes much faster than the energy difference between the two
states, thereby confirming the first order nature of the polaron-molecule
transition. In the regime where each state is metastable, we find quasiparticle
lifetimes which are much longer than what is expected for a usual Fermi liquid.
Our analysis indicates that the decay rates are sufficiently slow to be
experimentally observable.Comment: Version accepted in PRL. Added discussion of three-body losses to
deeply bound molecular state
Symmetry protected fractional Chern insulators and fractional topological insulators
In this paper we construct fully symmetric wavefunctions for the
spin-polarized fractional Chern insulators (FCI) and time-reversal-invariant
fractional topological insulators (FTI) in two dimensions using the parton
approach. We show that the lattice symmetry gives rise to many different FCI
and FTI phases even with the same filling fraction (and the same
quantized Hall conductance in FCI case). They have different
symmetry-protected topological orders, which are characterized by different
projective symmetry groups. We mainly focus on FCI phases which are realized in
a partially filled band with Chern number one. The low-energy gauge groups of a
generic FCI wavefunctions can be either or
the discrete group , and in the latter case the associated low-energy
physics are described by Chern-Simons-Higgs theories. We use our construction
to compute the ground state degeneracy. Examples of FCI/FTI wavefunctions on
honeycomb lattice and checkerboard lattice are explicitly given. Possible
non-Abelian FCI phases which may be realized in a partially filled band with
Chern number two are discussed. Generic FTI wavefunctions in the absence of
spin conservation are also presented whose low-energy gauge groups can be
either or . The constructed wavefunctions
also set up the framework for future variational Monte Carlo simulations.Comment: 24 pages, 13 figures, published versio
Landau Damping in a Turbulent Setting
To address the problem of Landau damping in kinetic turbulence, the forcing
of the linearized Vlasov equation by a stationary random source is considered.
It is found that the time-asymptotic density response is dominated by resonant
particle interactions that are synchronized with the source. The energy
consumption of this response is calculated, implying an effective damping rate,
which is the main result of this paper. Evaluating several cases, it is found
that the effective damping rate can differ from the Landau damping rate in
magnitude and also, remarkably, in sign. A limit is demonstrated in which the
density and current become phase-locked, which causes the effective damping to
be negligible; this potentially resolves an energy paradox that arises in the
application of critical balance to a kinetic turbulence cascade.Comment: Introduction significantly expanded to help contextualize results.
Calculations unchange
Bose-Einstein Condensation Temperature of a Homogeneous Weakly Interacting Bose Gas : PIMC study
Using a finite-temperature Path Integral Monte Carlo simulation (PIMC) method
and finite-size scaling, we have investigated the interaction-induced shift of
the phase transition temperature for Bose-Einstein condensation of homogeneous
weakly interacting Bose gases in three dimensions, which is given by a proposed
analytical expression , where
is the critical temperature for an ideal gas, is the s-wave
scattering length, and is the number density. We have used smaller number
densities and more time slices than in the previous PIMC simulations [Gruter
{\it et al.}, Phys. Rev. Lett. {\bf 79}, 3549 (1997)] in order to understand
the difference in the value of the coefficient between their results
and the (apparently) other reliable results in the literature. Our results show
that depends strongly on the
interaction strength while the previous PIMC results are
considerably flatter and smaller than our results. We obtain = 1.32
0.14, in agreement with results from recent Monte Carlo methods of
three-dimensional O(2) scalar field theory and variational
perturbation theory
Quantum ether: photons and electrons from a rotor model
We give an example of a purely bosonic model -- a rotor model on the 3D cubic
lattice -- whose low energy excitations behave like massless U(1) gauge bosons
and massless Dirac fermions. This model can be viewed as a ``quantum ether'': a
medium that gives rise to both photons and electrons. It illustrates a general
mechanism for the emergence of gauge bosons and fermions known as ``string-net
condensation.'' Other, more complex, string-net condensed models can have
excitations that behave like gluons, quarks and other particles in the standard
model. This suggests that photons, electrons and other elementary particles may
have a unified origin: string-net condensation in our vacuum.Comment: 10 pages, 6 figures, RevTeX4. Home page http://dao.mit.edu/~we
High-energy gluon bremsstrahlung in a finite medium: harmonic oscillator versus single scattering approximation
A particle produced in a hard collision can lose energy through
bremsstrahlung. It has long been of interest to calculate the effect on
bremsstrahlung if the particle is produced inside a finite-size QCD medium such
as a quark-gluon plasma. For the case of very high-energy particles traveling
through the background of a weakly-coupled quark-gluon plasma, it is known how
to reduce this problem to an equivalent problem in non-relativistic
two-dimensional quantum mechanics. Analytic solutions, however, have always
resorted to further approximations. One is a harmonic oscillator approximation
to the corresponding quantum mechanics problem, which is appropriate for
sufficiently thick media. Another is to formally treat the particle as having
only a single significant scattering from the plasma (known as the N=1 term of
the opacity expansion), which is appropriate for sufficiently thin media. In a
broad range of intermediate cases, these two very different approximations give
surprisingly similar but slightly differing results if one works to leading
logarithmic order in the particle energy, and there has been confusion about
the range of validity of each approximation. In this paper, I sort out in
detail the parametric range of validity of these two approximations at leading
logarithmic order. For simplicity, I study the problem for small alpha_s and
large logarithms but alpha_s log << 1.Comment: 40 pages, 23 figures [Primary change since v1: addition of new
appendix reviewing transverse momentum distribution from multiple scattering
On an exact hydrodynamic solution for the elliptic flow
Looking for the underlying hydrodynamic mechanisms determining the elliptic
flow we show that for an expanding relativistic perfect fluid the transverse
flow may derive from a solvable hydrodynamic potential, if the entropy is
transversally conserved and the corresponding expansion "quasi-stationary",
that is mainly governed by the temperature cooling. Exact solutions for the
velocity flow coefficients and the temperature dependence of the spatial
and momentum anisotropy are obtained and shown to be in agreement with the
elliptic flow features of heavy-ion collisions.Comment: 10 pages, 4 figure
A new modelling framework for statistical cumulus dynamics
We propose a new modelling framework suitable for the description of atmospheric convective systems as a collection of distinct plumes. The literature contains many examples of models for collections of plumes in which strong simplifying assumptions are made, a diagnostic dependence of convection on the large-scale environment and the limit of many plumes often being imposed from the outset. Some recent studies have sought to remove one or the other of those assumptions. The proposed framework removes both, and is explicitly time-dependent and stochastic in its basic character. The statistical dynamics of the plume collection are defined through simple probabilistic rules applied at the level of individual plumes, and van Kampen's system size expansion is then used to construct the macroscopic limit of the microscopic model. Through suitable choices of the microscopic rules, the model is shown to encompass previous studies in the appropriate limits, and to allow their natural extensions beyond those limits
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