10,035 research outputs found
Calculation of the microcanonical temperature for the classical Bose field
The ergodic hypothesis asserts that a classical mechanical system will in
time visit every available configuration in phase space. Thus, for an ergodic
system, an ensemble average of a thermodynamic quantity can equally well be
calculated by a time average over a sufficiently long period of dynamical
evolution. In this paper we describe in detail how to calculate the temperature
and chemical potential from the dynamics of a microcanonical classical field,
using the particular example of the classical modes of a Bose-condensed gas.
The accurate determination of these thermodynamics quantities is essential in
measuring the shift of the critical temperature of a Bose gas due to
non-perturbative many-body effects.Comment: revtex4, 10 pages, 1 figure. v2: updated to published version. Fuller
discussion of numerical results, correction of some minor error
Studies of Radiative Penguin B Decays at BaBar
We summarize results on a number of observations of penguin dominated
radiative decays of the B meson. Such decays are forbidden at tree level and
proceed via electroweak loops. As such they may be sensitive to physics beyond
the standard model. The observations have been made at the BaBar experiment at
PEP-II, the asymmetric B factory at SLAC.Comment: 3 pages, 5 figure
Motion of a condensate in a shaken and vibrating harmonic trap
The dynamics of a Bose-Einstein condensate (BEC) in a time-dependent harmonic
trapping potential is determined for arbitrary variations of the position of
the center of the trap and its frequencies. The dynamics of the BEC wavepacket
is soliton-like. The motion of the center of the wavepacket, and the spatially
and temporally dependent phase (which affects the coherence properties of the
BEC) multiplying the soliton-like part of the wavepacket, are analytically
determined.Comment: Accepted for publication in J. Phys. B: At Mol Opt Phy
Quantum Glassiness
Describing matter at near absolute zero temperature requires understanding a
system's quantum ground state and the low energy excitations around it, the
quasiparticles, which are thermally populated by the system's contact to a heat
bath. However, this paradigm breaks down if thermal equilibration is
obstructed. This paper presents solvable examples of quantum many-body
Hamiltonians of systems that are unable to reach their ground states as the
environment temperature is lowered to absolute zero. These examples, three
dimensional generalizations of quantum Hamiltonians proposed for topological
quantum computing, 1) have no quenched disorder, 2) have solely local
interactions, 3) have an exactly solvable spectrum, 4) have topologically
ordered ground states, and 5) have slow dynamical relaxation rates akin to
those of strong structural glasses.Comment: 4 page
Dynamics of long-range order in an exciton-polariton condensate
We report on time resolved measurements of the first order spatial coherence
in an exciton polariton Bose-Einstein condensate. Long range spatial coherence
is found to set in right at the onset of stimulated scattering, on a picosecond
time scale. The coherence reaches its maximum value after the population and
decays slower, staying up to a few hundreds of picoseconds. This behavior can
be qualitatively reproduced, using a stochastic classical field model
describing interaction between the polariton condensate and the exciton
reservoir within a disordered potential.Comment: 7 pages, 4 figure
Critical Dynamics of a Two-dimensional Superfluid near a Non-Thermal Fixed Point
Critical dynamics of an ultracold Bose gas far from equilibrium is studied in
two spatial dimensions. Superfluid turbulence is created by quenching the
equilibrium state close to zero temperature. Instead of immediately
re-thermalizing, the system approaches a meta-stable transient state,
characterized as a non-thermal fixed point. A focus is set on the vortex
density and vortex-antivortex correlations which characterize the evolution
towards the non-thermal fixed point and the departure to final
(quasi-)condensation. Two distinct power-law regimes in the vortex-density
decay are found and discussed in terms of a vortex binding-unbinding transition
and a kinetic description of vortex scattering. A possible relation to decaying
turbulence in classical fluids is pointed out. By comparing the results to
equilibrium studies of a two-dimensional Bose gas, an intuitive understanding
of the location of the non-thermal fixed point in a reduced phase space is
developed.Comment: 11 pages, 13 figures; PRA versio
Condensation and vortex formation in Bose-gas upon cooling
The mechanism for the transition of a Bose gas to the superfluid state via
thermal fluctuations is considered. It is shown that in the process of external
cooling some critical fluctuations (instantons) are formed above the critical
temperature. The probability of the instanton formation is calculated in the
three and two-dimensional cases. It is found that this probability increases as
the system approaches the transition temperature. It is shown that the
evolution of an individual instanton is impossible without the formation of
vortices in its superfluid part
Exact Results for Three-Body Correlations in a Degenerate One-Dimensional Bose Gas
Motivated by recent experiments we derive an exact expression for the
correlation function entering the three-body recombination rate for a
one-dimensional gas of interacting bosons. The answer, given in terms of two
thermodynamic parameters of the Lieb-Liniger model, is valid for all values of
the dimensionless coupling and contains the previously known results
for the Bogoliubov and Tonks-Girardeau regimes as limiting cases. We also
investigate finite-size effects by calculating the correlation function for
small systems of 3, 4, 5 and 6 particles.Comment: 4 pages, 2 figure
Superfluidity in a gas of strongly-interacting bosons
We consider small systems of bosonic atoms rotating in a toroidal trap. Using
the method of exact numerical diagonalization of the many-body Hamiltonian, we
examine the transition from the Bose-Einstein condensed state to the
Tonks-Girardeau state. The system supports persistent currents in a wide range
between the two limits, even in the absence of Bose-Einstein condensation.Comment: 7 pages, 3 figures, revised version, to appear in Europh. Let
Existence of two-channel Kondo regime for tunneling impurities with resonant scattering
Dynamical tunneling systems have been proposed earlier to display a
two-channel Kondo effect, the orbital index of the particle playing the role of
a pseudospin in the equivalent Kondo problem, and the spin being a silent
channel index. However, as shown recently by Aleiner et al. [Phys. Rev. Lett.
86, 2629 (2001)], the predicted two-channel Kondo behavior can never be
observed in the weak coupling regime, where the tunneling induced splitting of
the levels of the tunneling system always dominates the physics. Here we show
that the above scenario changes completely if the conduction electrons are
scattered by resonant scattering off the tunneling impurity; Then - as a
non-perturbative analysis reveals - the two-channel Kondo regime can easily be
reached.Comment: 10 PRB page
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