129 research outputs found
Frequency and damping of hydrodynamic modes in a trapped Bose-condensed gas
Recently it was shown that the Landau-Khalatnikov two-fluid hydrodynamics
describes the collision-dominated region of a trapped Bose condensate
interacting with a thermal cloud. We use these equations to discuss the low
frequency hydrodynamic collective modes in a trapped Bose gas at finite
temperatures. We derive a variational expressions based on these equations for
both the frequency and damping of collective modes. A new feature is our use of
frequency-dependent transport coefficients, which produce a natural cutoff by
eliminating the collisionless low-density tail of the thermal cloud. Above the
superfluid transition, our expression for the damping in trapped inhomogeneous
gases is analogous to the result first obtained by Landau and Lifshitz for
uniform classical fluids. We also use the moment method to discuss the
crossover from the collisionless to the hydrodynamic region. Recent data for
the monopole-quadrupole mode in the hydrodynamic region of a trapped gas of
metastable He is discussed. We also present calculations for the damping of
the analogous monopole-quadrupole condensate mode in the superfluid
phase.Comment: 22 pages, 10 figures, submitted to Physical Review
Propagation of Second sound in a superfluid Fermi gas in the unitary limit
We study sound propagation in a uniform superfluid gas of Fermi atoms in the
unitary limit. The existence of normal and superfluid components leads to
appearance of two sound modes in the collisional regime, referred to as first
and second sound. The second sound is of particular interest as it is a clear
signal of a superfluid component. Using Landau's two-fluid hydrodynamic theory,
we calculate hydrodynamic sound velocities and these weights in the density
response function. The latter is used to calculate the response to a sudden
modification of the external potential generating pulse propagation. The
amplitude of a pulse which is proportional to the weight in the response
function, is calculated the basis of the approach of Nozieres and Schmitt-Rink
(NSR) for the BCS-BEC crossover. We show that, in a superfluid Fermi gas at
unitarity, the second sound pulse is excited with an appreciate amplitude by
density perturbations.Comment: 12 pages, 9 figures. This version includes an erratum concerning the
temperature dependence of hydrodynamic sound weights in Phys. Rev. A 80,
043613 (2009
Bose-Einstein condensation of magnons in TlCuCl
A quantitative study of the field-induced magnetic ordering in TlCuCl in
terms of a Bose-Einstein condensation (BEC) of magnons is presented. It is
shown that the hitherto proposed simple BEC scenario is in quantitative and
qualitative disagreement with experiment. It is further shown that even very
small Dzyaloshinsky-Moriya interactions or a staggered tensor component of
a certain type can change the BEC picture qualitatively. Such terms lead to a
nonzero condensate density for all temperatures and a gapped quasiparticle
spectrum. Including this type of interaction allows us to obtain good agreement
with experimental data.Comment: 2 pages, 2 figures, submitted to SCES'0
Spin dynamics of a trapped spin-1 Bose Gas above the Bose-Einstein transition temperature
We study collective spin oscillations in a spin-1 Bose gas above the
Bose-Einstein transition temperature. Starting from the Heisenberg equation of
motion, we derive a kinetic equation describing the dynamics of a thermal gas
with the spin-1 degree of freedom. Applying the moment method to the kinetic
equation, we study spin-wave collective modes with dipole symmetry. The dipole
modes in the spin-1 system are found to be classified into the three type of
modes. The frequency and damping rate are obtained as functions of the peak
density. The damping rate is characterized by three relaxation times associated
with collisions.Comment: 19 pages, 5 figur
Atom-molecule equilibration in a degenerate Fermi gas with resonant interactions
We present a nonequilibrium kinetic theory describing atom-molecule
population dynamics in a two-component Fermi gas with a Feshbach resonance. Key
collision integrals emerge that govern the relaxation of the atom-molecule
mixture to chemical and thermal equilibrium. Our focus is on the pseudogap
regime where molecules form above the superfluid transition temperature. In
this regime, we formulate a simple model for the atom-molecule population
dynamics. The model predicts the saturation of molecule formation that has been
observed in recent experiments, and indicates that a dramatic enhancement of
the atom-molecule conversion efficiency occurs at low temperatures.Comment: Updated manuscript on July 5, 2004. Four pages with three embedded
figure
Magnetic structures of RbCuCl_3 in a transverse field
A recent high-field magnetization experiment found a phase transition of
unknown character in the layered, frustrated antiferromagnet RbCuCl_3, in a
transverse field (in the layers). Motivated by these results, we have examined
the magnetic structures predicted by a model of RbCuCl_3, using the classical
approximation. At small fields, we obtain the structure already known to be
optimal, an incommensurate (IC) spiral with wave vector q in the layers. At
higher fields, we find a staircase of long-period commensurate (C) phases
(separated initially by the low-field IC phase), then two narrow IC phases,
then a fourth IC phase (also with intermediate C phases), and finally the
ferromagnetically aligned phase at the saturation field H_S. The
three-sublattice C states familiar from the theory of the triangular
antiferromagnet are never optimal. The C phases and the two intermediate IC
phases were previously unknown in this context. The magnetization is
discontinuous at a field \approx 0.4H_S, in qualitative agreement with
experiment, though we find much fine structure not reported.Comment: 9 pages, 8 figure
Antiferromagnetic Quantum Spins on the Pyrochlore Lattice
The ground state of the S=1/2 Heisenberg antiferromagnet on the pyrochlore
lattice is theoretically investigated. Starting from the limit of isolated
tetrahedra, I include interactions between the tetrahedra and obtain an
effective model for the spin-singlet ground state multiplet by third-order
perturbation. I determine its ground state using the mean-field approximation
and found a dimerized state with a four-sublattice structure, which agrees with
the proposal by Harris et al. I also discuss chirality correlations and spin
correlations for this state.Comment: 4 pages in 2-column format, 5 figures; To appear in J. Phys. Soc.
Jpn. (Mar, 2001
Boundary quantum criticality in models of magnetic impurities coupled to bosonic baths
We investigate quantum impurity problems, where a local magnetic moment is
coupled to the spin density of a bosonic environment, leading to bosonic
versions of the standard Kondo and Anderson impurity models. In a physical
situation, these bosonic environments can correspond either to deconfined
spinons in certain classes of Z_2 frustrated antiferromagnets, or to particles
in a multicomponent Bose gase (in which case the spin degree of freedom is
attributed to hyperfine levels). Using renormalization group techniques, we
establish that our impurity models, which feature an exchange interaction
analogous to Kondo impurities in Fermi liquids, allow the flow towards a stable
strong-coupling state. Since the low-energy bosons live around a single point
in momentum space, and there is no Fermi surface, an impurity quantum phase
transition occurs at intermediate coupling, separating screened and unscreened
phases. This behavior is qualitatively different from previously studied
spin-isotropic variants of the spin-boson model, which display stable
intermediate-coupling fixed points and no screening.Comment: 15 pages, 10 fig
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