15,093 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
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
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
Renormalization group approach to Fermi Liquid Theory
We show that the renormalization group (RG) approach to interacting fermions
at one-loop order recovers Fermi liquid theory results when the forward
scattering zero sound (ZS) and exchange (ZS) channels are both taken into
account. The Landau parameters are related to the fixed point value of the
``unphysical'' limit of the forward scattering vertex. We specify the
conditions under which the results obtained at one-loop order hold at all order
in a loop expansion. We also emphasize the similarities between our RG approach
and the diagrammatic derivation of Fermi liquid theory.Comment: 4 pages (RevTex) + 1 postcript file, everything in a uuencoded file,
uses epsf (problem with the figure in the first version
Temperature dependence of the spin susceptibility of a clean Fermi gas with repulsion
Spin susceptibility of a clean Fermi gas with repulsion in any dimension is
considered using a supersymmetric low energy theory of interacting spin
excitations and renormalization scheme recently proposed by Aleiner and Efetov
[cond-mat/0602309]. We generalize this method to include the coupling to the
magnetic field. As a result, we obtain for the correction to the
Pauli susceptibility a non-analytic temperature dependence of the form in dimensions where is an
effective -dependent logarithmically renormalized backscattering amplitude.
In one dimension, is proportional to , and we
reproduce a well known result obtained long ago by a direct calculation.Comment: 25 pages, 10 figure
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