408 research outputs found
Field-induced decay dynamics in square-lattice antiferromagnet
Dynamical properties of the square-lattice Heisenberg antiferromagnet in
applied magnetic field are studied for arbitrary value S of the spin. Above the
threshold field for two-particle decays, the standard spin-wave theory yields
singular corrections to the excitation spectrum with logarithmic divergences
for certain momenta. We develop a self-consistent approximation applicable for
S >= 1, which avoids such singularities and provides regularized magnon decay
rates. Results for the dynamical structure factor obtained in this approach are
presented for S = 1 and S = 5/2.Comment: 12 pages, 11 figures, final versio
Collapse and revival of excitations in Bose-Einstein condensates
We study the energies and decay of elementary excitations in weakly
interacting Bose-Einstein condensates within a finite-temperature gapless
second-order theory. The energy shifts for the high-lying collective modes turn
out to be systematically negative compared with the
Hartree-Fock-Bogoliubov-Popov approximation and the decay of the low-lying
modes is found to exhibit collapse and revival effects. In addition,
perturbation theory is used to qualitatively explain the experimentally
observed Beliaev decay process of the scissors mode.Comment: 9 pages, 5 figure
Fermi-Bose mapping for one-dimensional Bose gases
One-dimensional Bose gases are considered, interacting either through the
hard-core potentials or through the contact delta potentials. Interest in these
gases gained momentum because of the recent experimental realization of
quasi-one-dimensional Bose gases in traps with tightly confined radial motion,
achieving the Tonks-Girardeau (TG) regime of strongly interacting atoms. For
such gases the Fermi-Bose mapping of wavefunctions is applicable. The aim of
the present communication is to give a brief survey of the problem and to
demonstrate the generality of this mapping by emphasizing that: (i) It is valid
for nonequilibrium wavefunctions, described by the time-dependent Schr\"odinger
equation, not merely for stationary wavefunctions. (ii) It gives the whole
spectrum of all excited states, not merely the ground state. (iii) It applies
to the Lieb-Liniger gas with the contact interaction, not merely to the TG gas
of impenetrable bosons.Comment: Brief review, Latex file, 15 page
Coherence time of a Bose-Einstein condensate
Temporal coherence is a fundamental property of macroscopic quantum systems,
such as lasers in optics and Bose-Einstein condensates in atomic gases and it
is a crucial issue for interferometry applications with light or matter waves.
Whereas the laser is an "open" quantum system, ultracold atomic gases are
weakly coupled to the environment and may be considered as isolated. The
coherence time of a condensate is then intrinsic to the system and its
derivation is out of the frame of laser theory. Using quantum kinetic theory,
we predict that the interaction with non-condensed modes gradually smears out
the condensate phase, with a variance growing as A t^2+B t+C at long times t,
and we give a quantitative prediction for A, B and C. Whereas the coefficient A
vanishes for vanishing energy fluctuations in the initial state, the
coefficients B and C are remarkably insensitive to these fluctuations. The
coefficient B describes a diffusive motion of the condensate phase that sets
the ultimate limit to the condensate coherence time. We briefly discuss the
possibility to observe the predicted phase spreading, also including the effect
of particle losses.Comment: 17 pages, 8 figures; typos correcte
Optically-Induced Polarons in Bose-Einstein Condensates: Monitoring Composite Quasiparticle Decay
Nonresonant light-scattering off atomic Bose-Einstein condensates (BECs) is
predicted to give rise to hitherto unexplored composite quasiparticles:
unstable polarons, i.e., local ``impurities'' dressed by virtual phonons.
Optical monitoring of their spontaneous decay can display either Zeno or
anti-Zeno deviations from the Golden Rule, and thereby probe the temporal
correlations of elementary excitations in BECs.Comment: 4 pages, 3 figure
Effective field theory and dispersion law of the phonons of a non-relativistic superfluid
We study the recently proposed effective field theory for the phonon of an
arbitrary non-relativistic superfluid. After computing the one-loop phonon
self-energy, we obtain the low temperature T contributions to the phonon
dispersion law at low momentum, and see that the real part of those can be
parametrized as a thermal correction to the phonon velocity. Because the
phonons are the quanta of the sound waves, at low momentum their velocity
should agree with the speed of sound. We find that our results match at order
T^4ln(T) with those predicted by Andreev and Khalatnikov for the speed of
sound, derived from the superfluid hydrodynamical equations and the phonon
kinetic theory. We get also higher order corrections of order T^4, which are
not reproduced pushing naively the kinetic theory computation. Finally, as an
application, we consider the cold Fermi gas in the unitarity limit, and find a
universal expression for the low T relative correction to the speed of sound
for these systems.Comment: 14 pages, 2 figures. References adde
Risk Factors and Predictive Models for Conversion of Laparoscopic Cholecystectomy to Open Surgery, and Surgical Quality Outcome Measures
Background: Laparoscopic cholecystectomy is the preferred surgical operation for symptomatic gallstone disease. Conversion of laparoscopic cholecystectomy to open surgery is used to prevent intra-abdominal organ injury, for open common bile duct exploration and to repair intra-abdominal organ injury
Thermodynamics of a Bose-Einstein Condensate with Weak Disorder
We consider the thermodynamics of a homogeneous superfluid dilute Bose gas in
the presence of weak quenched disorder. Following the zero-temperature approach
of Huang and Meng, we diagonalize the Hamiltonian of a dilute Bose gas in an
external random delta-correlated potential by means of a Bogoliubov
transformation. We extend this approach to finite temperature by combining the
Popov and the many-body T-matrix approximations. This approach permits us to
include the quasi-particle interactions within this temperature range. We
derive the disorder-induced shifts of the Bose-Einstein critical temperature
and of the temperature for the onset of superfluidity by approaching the
transition points from below, i.e., from the superfluid phase. Our results lead
to a phase diagram consistent with that of the finite-temperature theory of
Lopatin and Vinokur which was based on the replica method, and in which the
transition points were approached from above.Comment: 11 pages, 5 figure
Packing dimension of mean porous measures
We prove that the packing dimension of any mean porous Radon measure on
may be estimated from above by a function which depends on mean
porosity. The upper bound tends to as mean porosity tends to its maximum
value. This result was stated in \cite{BS}, and in a weaker form in \cite{JJ1},
but the proofs are not correct. Quite surprisingly, it turns out that mean
porous measures are not necessarily approximable by mean porous sets. We verify
this by constructing an example of a mean porous measure on
such that for all mean porous sets .Comment: Revised versio
Commensurate and incommensurate ground states of Cs_2CuCl_4 in a magnetic field
We present calculations of the magnetic ground state of Cs_2CuCl_4 in an
applied magnetic field, with the aim of understanding the commensurately
ordered state that has been discovered in recent experiments. This layered
material is a realization of a Heisenberg antiferromagnet on an anisotropic
triangular lattice. Its behavior in a magnetic field depends on field
orientation, because of weak Dzyaloshinskii-Moriya interactions.We study the
system by mapping the spin-1/2 Heisenberg Hamiltonian onto a Bose gas with hard
core repulsion. This Bose gas is dilute, and calculations are controlled, close
to the saturation field. We find a zero-temperature transition between
incommensurate and commensurate phases as longitudinal field strength is
varied, but only incommensurate order in a transverse field. Results for both
field orientations are consistent with experiment.Comment: 5 Pages, 3 Figure
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