135 research outputs found
The Kohn mode for trapped Bose gases within the dielectric formalism
The presence of undamped harmonic center of mass oscillations of a weakly
interacting Bose gas in a harmonic trap is demonstrated within the dielectric
formalism for a previously introduced finite temperature approximation
including exchange. The consistency of the approximation with the Kohn theorem
is thereby demonstrated. The Kohn modes are found explicitly, generalizing an
earlier zero-temperature result found in the literature. It is shown how the
Kohn mode disappears from the single-particle spectrum, while remaining in the
density oscillation spectrum, when the temperature increases from below to
above the condensation temperature.Comment: 6 pages revte
Quasi-equilibria in one-dimensional self-gravitating many body systems
The microscopic dynamics of one-dimensional self-gravitating many-body
systems is studied. We examine two courses of the evolution which has the
isothermal and stationary water-bag distribution as initial conditions. We
investigate the evolution of the systems toward thermal equilibrium. It is
found that when the number of degrees of freedom of the system is increased,
the water-bag distribution becomes a quasi-equilibrium, and also the
stochasticity of the system reduces. This results suggest that the phase space
of the system is effectively not ergodic and the system with large degreees of
freedom approaches to the near-integrable one.Comment: 21pages + 7 figures (available upon request), revtex, submitted to
Physical Review
âI kind of had an avatar switchâ : the role of the self in engagement with an interactive TV drama
This paper reports results from a study which examined
viewersâ cognitive and affective responses to an interactive
TV drama. Ten participants were videoed interacting with
âOur World Warâ [1], and then interviewed about their
experience using the video playback as a retrospective
prompt. An interpretative framework was designed to guide
analysis by probing themes of narrative engagement
identified in previous literature. We report findings relating to five themes of engagement: cognitive, affective,
perspective taking, competence and autonomy, and
transportation. Our data adds to the existing literature on
interactive stories by highlighting the pivotal role of the self in engaging with interactive drama, with self-reflection emerging within each theme. We conclude that two experiential states drive engagement: a transported
experience; and one in which self-reflection limits transportation
Structural Evidence of a Major Conformational Change Triggered by Substrate Binding in DapE Enzymes: Impact on the Catalytic Mechanism
The X-ray crystal structure of the dapE-encoded N-succinyl-l,l-diaminopimelic acid desuccinylase from Haemophilus influenzae (HiDapE) bound by the products of hydrolysis, succinic acid and l,l-DAP, was determined at 1.95 Ă
. Surprisingly, the structure bound to the products revealed that HiDapE undergoes a significant conformational change in which the catalytic domain rotates âŒ50° and shifts âŒ10.1 Ă
(as measured at the position of the Zn atoms) relative to the dimerization domain. This heretofore unobserved closed conformation revealed significant movements within the catalytic domain compared to that of wild-type HiDapE, which results in effectively closing off access to the dinuclear Zn(II) active site with the succinate carboxylate moiety bridging the dinculear Zn(II) cluster in a ÎŒ-1,3 fashion forming a bis(ÎŒ-carboxylato)dizinc(II) core with a ZnâZn distance of 3.8 Ă
. Surprisingly, His194.B, which is located on the dimerization domain of the opposing chain âŒ10.1 Ă
from the dinuclear Zn(II) active site, forms a hydrogen bond (2.9 Ă
) with the oxygen atom of succinic acid bound to Zn2, forming an oxyanion hole. As the closed structure forms upon substrate binding, the movement of His194.B by more than âŒ10 Ă
is critical, based on site-directed mutagenesis data, for activation of the scissile carbonyl carbon of the substrate for nucleophilic attack by a hydroxide nucleophile. Employing the HiDapE product-bound structure as the starting point, a reverse engineering approach called product-based transition-state modeling provided structural models for each major catalytic step. These data provide insight into the catalytic reaction mechanism and also the future design of new, potent inhibitors of DapE enzymes
Relaxation processes in one-dimensional self-gravitating many-body systems
Though one dimensional self-gravitating -body systems have been studied
for three decade, the nature of relaxation was still unclear. There were
inconsistent results about relaxation time; some initial state relaxed in the
time scale , but another state did not relax even after , where is the crossing time. The water-bag distribution was
believed not to relax after . In our previous paper, however,
we found there are two different relaxation times in the water-bag
distribution;in the faster relaxation ( microscopic relaxation ) the
equipartition of energy distribution is attains but the macroscopic
distribution turns into the isothermal distribution in the later relaxation
(macroscopic relaxation). In this paper, we investigated the properties of the
two relaxation. We found that the microscopic relaxation time is , and the macroscopic relaxation time is proportional to , thus
the water-bag does relax. We can see the inconsistency about the relaxation
times is resolved as that we see the two different aspect of relaxations.
Further, the physical mechanisms of the relaxations are presented.Comment: 11 pages, uuencoded, compressed Postscript, no figure, figures
available at ftp://ferio.mtk.nao.ac.jp/pub/tsuchiya/Tsuchiya95.tar.g
Shifts and widths of collective excitations in trapped Bose gases by the dielectric formalism
We present predictions for the temperature dependent shifts and damping
rates. They are obtained by applying the dielectric formalism to a simple model
of a trapped Bose gas. Within the framework of the model we use lowest order
perturbation theory to determine the first order correction to the results of
Hartree-Fock-Bogoliubov-Popov theory for the complex collective excitation
frequencies, and present numerical results for the temperature dependence of
the damping rates and the frequency shifts. Good agreement with the
experimental values measured at JILA are found for the m=2 mode, while we find
disagreements in the shifts for m=0. The latter point to the necessity of a
non-perturbative treatment for an explanation of the temperature-dependence of
the m=0 shifts.Comment: 10 pages revtex, 3 figures in postscrip
Zig-Zag Numberlink is NP-Complete
When can terminal pairs in an grid be connected by
vertex-disjoint paths that cover all vertices of the grid? We prove that this
problem is NP-complete. Our hardness result can be compared to two previous
NP-hardness proofs: Lynch's 1975 proof without the ``cover all vertices''
constraint, and Kotsuma and Takenaga's 2010 proof when the paths are restricted
to have the fewest possible corners within their homotopy class. The latter
restriction is a common form of the famous Nikoli puzzle \emph{Numberlink}; our
problem is another common form of Numberlink, sometimes called \emph{Zig-Zag
Numberlink} and popularized by the smartphone app \emph{Flow Free}
Limitations of squeezing due to collisional decoherence in Bose-Einstein condensates
We study the limitations for entanglement due to collisional decoherence in a
Bose-Einstein condensate. Specifically we consider relative number squeezing
between photons and atoms coupled out from a homogeneous condensate. We study
the decay of excited quasiparticle modes due to collisions, in condensates of
atoms with one or two internal degrees of freedom. The time evolution of these
modes is determined in the linear response approximation to the deviation from
equilibrium. We use Heisenberg-Langevin equations to derive equations of motion
for the densities and higher correlation functions which determine the
squeezing. In this way we can show that decoherence due to quasiparticle
interactions imposes an important limit on the degree of number squeezing which
may be achieved. Our results are also relevant for the determination of
decoherence times in other experiments based on entanglement, e.g. the slowing
and stopping of light in condensed atomic gases using dark states.Comment: 16 pages RevTeX, 3 figure
Energies and damping rates of elementary excitations in spin-1 Bose-Einstein condensed gases
Finite temperature Green's function technique is used to calculate the
energies and damping rates of elementary excitations of the homogeneous,
dilute, spin-1 Bose gases below the Bose-Einstein condensation temperature both
in the density and spin channels. For this purpose the self-consistent
dynamical Hartree-Fock model is formulated, which takes into account the direct
and exchange processes on equal footing by summing up certain classes of
Feynman diagrams. The model is shown to fulfil the Goldstone theorem and to
exhibit the hybridization of one-particle and collective excitations correctly.
The results are applied to the gases of ^{23}Na and ^{87}Rb atoms.Comment: 26 pages, 21 figures. Added 2 new figures, detailed discussio
Thermodynamics of a Trapped Bose-Fermi Mixture
By using the Hartree-Fock-Bogoliubov equations within the Popov
approximation, we investigate the thermodynamic properties of a dilute binary
Bose-Fermi mixture confined in an isotropic harmonic trap. For mixtures with an
attractive Bose-Fermi interaction we find a sizable enhancement of the
condensate fraction and of the critical temperature of Bose-Einstein
condensation with respect to the predictions for a pure interacting Bose gas.
Conversely, the influence of the repulsive Bose-Fermi interaction is less
pronounced. The possible relevance of our results in current experiments on
trapped {\rm K} mixtures is discussed.Comment: 5 pages + 4 figures; minor changes, final version to appear in Phys.
Rev. A; the extension work on the finite-temperature low-lying excitations
can be found in cond-mat/030763
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