1,443 research outputs found
Low-lying excitations of a trapped rotating Bose-Einstein condensate
We investigate the low-lying excitations of a weakly-interacting,
harmonically-trapped Bose-Einstein condensed gas under rotation, in the limit
where the angular mometum of the system is much less than the number of the
atoms in the trap. We show that in the asymptotic limit the
excitation energy, measured from the energy of the lowest state, is given by
, where is the number of octupole
excitations and is the unit of the interaction energy.Comment: 3 pages, RevTex, 2 ps figures, submitted to PR
Operator-Algebraic Approach to the Yrast Spectrum of Weakly Interacting Trapped Bosons
We present an operator-algebraic approach to deriving the low-lying
quasi-degenerate spectrum of weakly interacting trapped N bosons with total
angular momentum \hbar L for the case of small L/N, demonstrating that the
lowest-lying excitation spectrum is given by 27 g n_3(n_3-1)/34, where g is the
strength of the repulsive contact interaction and n_3 the number of excited
octupole quanta. Our method provides constraints for these quasi-degenerate
many-body states and gives higher excitation energies that depend linearly on
N.Comment: 7 pages, one figur
Low-Lying Excitations from the Yrast Line of Weakly Interacting Trapped Bosons
Through an extensive numerical study, we find that the low-lying,
quasi-degenerate eigenenergies of weakly-interacting trapped N bosons with
total angular momentum L are given in case of small L/N and sufficiently small
L by E = L hbar omega + g[N(N-L/2-1)+1.59 n(n-1)/2], where omega is the
frequency of the trapping potential and g is the strength of the repulsive
contact interaction; the last term arises from the pairwise repulsive
interaction among n octupole excitations and describes the lowest-lying
excitation spectra from the Yrast line. In this case, the quadrupole modes do
not interact with themselves and, together with the octupole modes, exhaust the
low-lying spectra which are separated from others by N-linear energy gaps.Comment: 5 pages, RevTeX, 2 figures, revised version, submitted to PR
A Comment on "The Far Future of Exoplanet Direct Characterization" - the Case for Interstellar Space Probes
Following on from ideas presented in a recent paper by Schneider et al.
(2010) on "The Far Future of Exoplanet Direct Characterization", I argue that
they have exaggerated the technical obstacles to performing such 'direct
characterization' by means of fast (order 0.1c) interstellar space probes. A
brief summary of rapid interstellar spaceflight concepts that may be found in
the literature is presented. I argue that the presence of interstellar dust
grains, while certainly something which will need to be allowed for in
interstellar vehicle design, is unlikely to be the kind of 'show stopper'
suggested by Schneider et al. Astrobiology as a discipline would be a major
beneficiary of developing an interstellar spaceflight capability, albeit in the
longer term, and I argue that astrobiologists should keep an open mind to the
possibilities.Comment: Accepted for publication in Astrobiolog
Social Dimensions of Urban Flood Experience, Exposure, and Concern
With growing urban populations and climate change, urban flooding is an important global issue, even in dryland regions. Flood risk assessments are usually used to identify vulnerable locations and populations, flooding experience patterns, or levels of concern about flooding, but rarely are all of these approaches combined. Furthermore, the social dynamics of flood concerns, exposure, and experience are underexplored. We combined geographic and survey data on household‐level measures of flood experience, concern, and exposure in Utah\u27s urbanizing Wasatch Front. We asked: (1) Are socially vulnerable groups more likely to be exposed to flood risk? (2) How common are flooding experiences among urban residents, and how are these experiences related to sociodemographic characteristics and exposure? and (3) How concerned are urban residents about flooding, and does concern vary by exposure, flood experience, and sociodemographic characteristics? Although floodplain residents were more likely to be White and have higher incomes, respondents who were of a racial/ethnic minority, were older, had less education, and were living in floodplains were more likely to report flood experiences and concern about flooding. Flood risk management approaches need to address social as well as physical sources of vulnerability to floods and recognize social sources of variation in flood experiences and concern
Nonlinear dynamics for vortex lattice formation in a rotating Bose-Einstein condensate
We study the response of a trapped Bose-Einstein condensate to a sudden
turn-on of a rotating drive by solving the two-dimensional Gross-Pitaevskii
equation. A weakly anisotropic rotating potential excites a quadrupole shape
oscillation and its time evolution is analyzed by the quasiparticle projection
method. A simple recurrence oscillation of surface mode populations is broken
in the quadrupole resonance region that depends on the trap anisotropy, causing
stochastization of the dynamics. In the presence of the phenomenological
dissipation, an initially irrotational condensate is found to undergo damped
elliptic deformation followed by unstable surface ripple excitations, some of
which develop into quantized vortices that eventually form a lattice. Recent
experimental results on the vortex nucleation should be explained not only by
the dynamical instability but also by the Landau instability; the latter is
necessary for the vortices to penetrate into the condensate.Comment: RevTex4, This preprint includes no figures. You can download the
complete article and figures at
http://matter.sci.osaka-cu.ac.jp/bsr/cond-mat.htm
CMAS challenges to CMC-T/EBC systems
Gas turbine technology is undergoing a major transition with the recent implementation of SiC based ceramic composites (CMCs) in aircraft engines. While the potential improvement in temperature capability (≥1500°C) is unprecedented, there are a number of issues that limit the full exploitation of such potential. In addition to the longstanding concern for low temperature oxidative embrittlement and the limited temperature capability of current bond coats and matrices, the susceptibility of the protective SiO2 to volatilization in the combustion environment requires the use of environmental barrier coatings (EBCs) to achieve durability targets. Most EBC concepts, however, are based on silicates and are thus susceptible to degradation by molten silicate deposits generically known as CMAS originating from mineral debris ingested into engines with the intake air. This presentation will discuss the thermodynamic and mechanistic foundation of the degradation of EBCs by CMAS, recent progress in establishing the relevant phase equilibria for these systems, and the role of the CMAS composition on the extent of degradation, as well as perspective on mitigation. (Research supported by ONR, AFOSR and the P&W Center of Excellence in Composites at UCSB.
Measurement of the Total (p,Pi) Cross Sections Through Residual Activity
Supported by the National Science Foundation and Indiana Universit
Solitons, solitonic vortices, and vortex rings in a confined Bose-Einstein condensate
Quasi-one-dimensional solitons that may occur in an elongated Bose-Einstein
condensate become unstable at high particle density. We study two basic modes
of instability and the corresponding bifurcations to genuinely
three-dimensional solitary waves such as axisymmetric vortex rings and
non-axisymmetric solitonic vortices. We calculate the profiles of the above
structures and examine their dependence on the velocity of propagation along a
cylindrical trap. At sufficiently high velocity, both the vortex ring and the
solitonic vortex transform into an axisymmetric soliton. We also calculate the
energy-momentum dispersions and show that a Lieb-type mode appears in the
excitation spectrum for all particle densities.Comment: RevTeX 9 pages, 9 figure
Vortex nucleation in Bose-Einstein condensates in time-dependent traps
Vortex nucleation in a Bose-Einstein condensate subject to a stirring
potential is studied numerically using the zero-temperature, two-dimensional
Gross-Pitaevskii equation. It is found that this theory is able to describe the
creation of vortices, but not the crystallization of a vortex lattice. In the
case of a rotating, slightly anisotropic harmonic potential, the numerical
results reproduce experimental findings, thereby showing that finite
temperatures are not necessary for vortex excitation below the quadrupole
frequency. In the case of a condensate subject to stirring by a narrow rotating
potential, the process of vortex excitation is described by a classical model
that treats the multitude of vortices created by the stirrer as a continuously
distributed vorticity at the center of the cloud, but retains a potential flow
pattern at large distances from the center.Comment: 22 pages, 7 figures. Changes after referee report: one new figure,
new refs. No conclusions altere
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