392 research outputs found
Rapid and safe ASAP acquisition with EXACT NMR
EXACT acquisition allows the fast and safer acquisition of ASAP 2D NMR experiments in just a few seconds.</p
Activity driven modeling of time varying networks
Network modeling plays a critical role in identifying statistical
regularities and structural principles common to many systems. The large
majority of recent modeling approaches are connectivity driven. The structural
patterns of the network are at the basis of the mechanisms ruling the network
formation. Connectivity driven models necessarily provide a time-aggregated
representation that may fail to describe the instantaneous and fluctuating
dynamics of many networks. We address this challenge by defining the activity
potential, a time invariant function characterizing the agents' interactions
and constructing an activity driven model capable of encoding the instantaneous
time description of the network dynamics. The model provides an explanation of
structural features such as the presence of hubs, which simply originate from
the heterogeneous activity of agents. Within this framework, highly dynamical
networks can be described analytically, allowing a quantitative discussion of
the biases induced by the time-aggregated representations in the analysis of
dynamical processes.Comment: 10 pages, 4 figure
Nucleation of vortex arrays in rotating anisotropic Bose-Einstein condensates
The nucleation of vortices and the resulting structures of vortex arrays in
dilute, trapped, zero-temperature Bose-Einstein condensates are investigated
numerically. Vortices are generated by rotating a three-dimensional,
anisotropic harmonic atom trap. The condensate ground state is obtained by
propagating the Gross-Pitaevskii equation in imaginary time. Vortices first
appear at a rotation frequency significantly larger than the critical frequency
for vortex stabilization. This is consistent with a critical velocity mechanism
for vortex nucleation. At higher frequencies, the structures of the vortex
arrays are strongly influenced by trap geometry.Comment: 5 pages, two embedded figures. To appear in Phys. Rev. A (RC
Prospects for p-wave paired BCS states of fermionic atoms
We present theoretical prospects for creating p-wave paired BCS states of
magnetic trapped fermionic atoms. Based on our earlier proposal of using dc
electric fields to control both the strength and anisotropic characteristic of
atom-atom interaction and our recently completed multi-channel atomic collision
calculations we discover that p-wave pairing with K and Rb
in the low field seeking maximum spin polarized state represent excellent
choices for achieving superfluid BCS states; and may be realizable with current
technology in laser cooling, magnetic trapping, and evaporative/sympathetic
cooling, provided the required strong electric field can be applied. We also
comment on the prospects of similar p-wave paired BCS states in Li, and
more generally on creating other types exotic BCS states. Our study will open a
new area in the vigorous pursuit to create a quantum degenerate fermionic atom
vapor.Comment: to be publishe
Ballistic matter waves with angular momentum: Exact solutions and applications
An alternative description of quantum scattering processes rests on
inhomogeneous terms amended to the Schroedinger equation. We detail the
structure of sources that give rise to multipole scattering waves of definite
angular momentum, and introduce pointlike multipole sources as their limiting
case. Partial wave theory is recovered for freely propagating particles. We
obtain novel results for ballistic scattering in an external uniform force
field, where we provide analytical solutions for both the scattering waves and
the integrated particle flux. Our theory directly applies to p-wave
photodetachment in an electric field. Furthermore, illustrating the effects of
extended sources, we predict some properties of vortex-bearing atom laser beams
outcoupled from a rotating Bose-Einstein condensate under the influence of
gravity.Comment: 42 pages, 8 figures, extended version including photodetachment and
semiclassical theor
Collective oscillations of an interacting trapped Fermi gas
We calculate the effects of two-body interactions on the low frequency
oscillations of a normal Fermi gas confined in a harmonic trap. The mean field
contribution to the collective frequencies is evaluated in the collisionless
regime using a sum rule approach. We also discuss the transition between the
collisionless and hydrodynamic regime with special emphasis to the spin dipole
mode in which two atomic clouds occupying different spin states oscillate in
opposite phase. The spin dipole mode is predicted to be overdamped in the
hydrodynamic regime. The relaxation time is calculated as a function of
temperature and the effects of Fermi statistics are explicitly pointed out.Comment: 4 pages, 1 figure include
Effect of quantum group invariance on trapped Fermi gases
We study the properties of a thermodynamic system having the symmetry of a
quantum group and interacting with a harmonic potential. We calculate the
dependence of the chemical potential, heat capacity and spatial distribution of
the gas on the quantum group parameter and the number of spatial dimensions
. In addition, we consider a fourth-order interaction in the quantum group
fields , and calculate the ground state energy up to first order.Comment: LaTeX file, 20 pages, four figures, uses epsf.sty, packaged as a
single tar.gz uuencoded fil
Automated sperm head morphology analyzer for open-source software
Abstract Sperm head morphology has been identified as a characteristic that can be used to predict a male's semen quality. In the present study, we have developed an automated sperm head morphology analysis (ASMA) plug-in for open-source ImageJ software (http://rsbweb.nih.gov/ij/). We describe the plug-in's functionality, and confirm its validity for sperm head morphology analysis using fish sperm. Sperm head morphological measurements (length and width) made with the ASMA plug-in did not differ from manual measurements. Using the plug-in to measure sperm head-shaped objects of known size, the associated plug-in error rate was Ϝ 0.5%. Brightness and contrast ratios influenced sperm head measurements, suggesting the need for standardized protocols. This plug-in was effective at measuring elliptical (i.e., Atlantic cod) as well as slightly irregular (i.e., Chinook salmon) shaped sperm heads. In conclusion, our ASMA plug-in represents a versatile alternative to costly sperm morphology software
Vortices in Bose-Einstein Condensates: Some Recent Developments
In this brief review we summarize a number of recent developments in the
study of vortices in Bose-Einstein condensates, a topic of considerable
theoretical and experimental interest in the past few years. We examine the
generation of vortices by means of phase imprinting, as well as via dynamical
instabilities. Their stability is subsequently examined in the presence of
purely magnetic trapping, and in the combined presence of magnetic and optical
trapping. We then study pairs of vortices and their interactions, illustrating
a reduced description in terms of ordinary differential equations for the
vortex centers. In the realm of two vortices we also consider the existence of
stable dipole clusters for two-component condensates. Last but not least, we
discuss mesoscopic patterns formed by vortices, the so-called vortex lattices
and analyze some of their intriguing dynamical features. A number of
interesting future directions are highlighted.Comment: 24 pages, 8 figs, ws-mplb.cls, to appear in Modern Physics Letters B
(2005
Stability of vortices in rotating taps: a 3d analysis
We study the stability of vortex-lines in trapped dilute gases subject to
rotation. We solve numerically both the Gross-Pitaevskii and the Bogoliubov
equations for a 3d condensate in spherically and cilyndrically symmetric
stationary traps, from small to very large nonlinearities. In the stationary
case it is found that the vortex states with unit and charge are
energetically unstable. In the rotating trap it is found that this energetic
instability may only be suppressed for the vortex-line, and that the
multicharged vortices are never a local minimum of the energy functional, which
implies that the absolute minimum of the energy is not an eigenstate of the
operator, when the angular speed is above a certain value, .Comment: 10 pages, 7 figures in EPS forma
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