10,789 research outputs found
Universal dynamics on the way to thermalisation
It is demonstrated how a many-body system far from thermal equilibrium can
exhibit universal dynamics in passing a non-thermal fixed point. As an example,
the process of Bose-Einstein (BE) condensation of a dilute cold gas is
considered. If the particle flux into the low-energy modes, induced, e.g., by a
cooling quench, is sufficiently strong, the Bose gas develops a characteristic
power-law single-particle spectrum , and critical slowing down
in time occurs. The fixed point is shown to be marked by the creation and
dilution of tangled vortex lines. Alternatively, for a weak cooling quench and
particle flux, the condensation process runs quasi adiabatically, passing by
the fixed point in far distance, and signatures of critical scaling remain
absent.Comment: 5+2 pages, 8 figure
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
Supercurrent Stability in a Quasi-1D Weakly Interacting Bose Gas
We discuss a possibility of observing superfluid phenomena in a quasi-1D
weakly interacting Bose gas at finite temperatures. The weakness of interaction
in combination with generic properties of 1D liquids can result in a situation
when relaxational time of supercurrent is essentially larger than the time of
experimental observation, and the behavior of the system is indistinguishable
from that of a genuine superfluid.Comment: Revtex, 4 pages, no figures; Submitted to Phys. Rev. A (Brief
Reports
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
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
Accidental suppression of Landau damping of the transverse breathing mode in elongated Bose-Einstein condensates
We study transverse radial oscillations of an elongated Bose-Einstein
condensate using finite temperature simulations, in the context of a recent
experiment at ENS. We demonstrate the existence of a mode corresponding to an
in-phase collective oscillation of both the condensate and thermal cloud.
Excitation of this mode accounts for the very small damping rate observed
experimentally, and we find excellent quantitative agreement between experiment
and theory. In contrast to other condensate modes, interatomic collisions are
found to be the dominant damping mechanism in this case.Comment: 4 pages, 3 figure
Transformation of a Racemic Mixture by a Chiral Reagent or Catalyst to Give Regioisomeric Products
The transformation of a racemic mixture under the influence of a chiral reagent or catalyst is discussed in the case where regioisomeric products are obtained. The general relationships correlating the ee\u27s and the quantities of the various products are given.
The special case of asymmetric Baeyer-Villiger oxidation of racemic ketones is taken as an example. Sometimes regioisomeric products are derived from opposite enantiomers, which implies that the re- gioselectivity of a reaction on an enantiomerically pure starting material will be controlled by the absolute configuration of the chiral reagent or catalyst
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