5,593 research outputs found
Excitation spectrum and instability of a two-species Bose-Einstein condensate
We numerically calculate the density profile and excitation spectrum of a
two-species Bose-Einstein condensate for the parameters of recent experiments.
We find that the ground state density profile of this system becomes unstable
in certain parameter regimes, which leads to a phase transition to a new stable
state. This state displays spontaneously broken cylindrical symmetry. This
behavior is reflected in the excitation spectrum: as we approach the phase
transition point, the lowest excitation frequency goes to zero, indicating the
onset of instability in the density profile. Following the phase transition,
this frequency rises again.Comment: 8 pages, 5 figures, uses REVTe
Half-quantum vortex state in a spin-orbit coupled Bose-Einstein condensate
We investigate theoretically the condensate state and collective excitations
of a two-component Bose gas in two-dimensional harmonic traps subject to
isotropic Rashba spin-orbit coupling. In the weakly interacting regime when the
inter-species interaction is larger than the intra-species interaction
(), we find that the condensate ground state has a
half-quantum-angular-momentum vortex configuration with spatial rotational
symmetry and skyrmion-type spin texture. Upon increasing the interatomic
interaction beyond a threshold , the ground state starts to involve
higher-order angular momentum components and thus breaks the rotational
symmetry. In the case of , the condensate becomes
unstable towards the superposition of two degenerate half-quantum vortex
states. Both instabilities (at and ) can be
determined by solving the Bogoliubov equations for collective density
oscillations of the half-quantum vortex state, and by analyzing the softening
of mode frequencies. We present the phase diagram as functions of the
interatomic interactions and the spin-orbit coupling. In addition, we directly
simulate the time-dependent Gross-Pitaevskii equation to examine the dynamical
properties of the system. Finally, we investigate the stability of the
half-quantum vortex state against both the trap anisotropy and anisotropy in
the spin-orbit coupling term.Comment: 13 pages, 18 figure
Eliminating the mean-field shift in multicomponent Bose-Einstein condensates
We demonstrate that the nonlinear mean-field shift in a multi-component
Bose-Einstein condensate may be eliminated by controlling the two-body
interaction coefficients. This modification is achieved by, e.g., suitably
engineering the environment of the condensate. We consider as an example the
case of a two-component condensate in a tightly confining atom waveguide.
Modification of the atom-atom interactions is then achieved by varying
independently the transverse wave function of the two components. Eliminating
the density dependent phase shift in a high-density atomic beam has important
applications in atom interferometry and precision measurement
Theta oscillations support the interface between language and memory
Recent evidence shows that hippocampal theta oscillations, usually linked to memory and navigation, are also observed during online language processing, suggesting a shared neurophysiological mechanism between language and memory. However, it remains to be established what specific roles hippocampal theta oscillations may play in language, and whether and how theta mediates the communication between the hippocampus and the perisylvian cortical areas, generally thought to support language processing. With whole-head magnetoencephalographic (MEG) recordings, the present study investigated these questions with two experiments. Using a violation paradigm, extensively used for studying neural underpinnings of different aspects of linguistic processing, we found increased theta power (4–8 Hz) in the hippocampal formation, when participants read a semantically incorrect vs. correct sentence ending. Such a pattern of results was replicated using different sentence stimuli in another cohort of participants. Importantly, no significant hippocampal theta power increase was found when participants read a semantically correct but syntactically incorrect sentence ending vs. a correct sentence ending. These findings may suggest that hippocampal theta oscillations are specifically linked to lexical-semantic related processing, and not general information processing in sentence reading. Furthermore, we found significantly transient theta phase coupling between the hippocampus and the left superior temporal gyrus, a hub area of the cortical network for language comprehension. This transient theta phase coupling may provide an important channel that links the memory and language systems for the generation of sentence meaning. Overall, these findings help specify the role of hippocampal theta in language, and provide a novel neurophysiological mechanism at the network level that may support the interface between memory and language
Phase separation of Bose-Einstein condensates
The zero-temperature system of two dilute overlapping Bose-Einstein
condensates is unstable against long wavelength excitations if the interaction
strength between the distinguishable bosons exceeds the geometric mean of the
like-boson interaction strengths. If the condensates attract each other, the
instability is similar to the instability of the negative scattering length
condensates. If the condensates repel, they separate spatially into condensates
of equal pressure. We estimate the boundary size, surface tension and energy of
the phase separated condensate system and we discuss the implications for
double condensates in atomic traps.Comment: 11 pages, 1 figur
Using domain ontologies to help track data provenance.
Motivating example. POESIA ontologies and ontological coverages. Ontological estimation of data provenance. Ontological nets for data integration. Data integration operators. Data reconciling through articulation of ontologies. Semantic workflows. Related work. Conclusions
Filling minimality of Finslerian 2-discs
We prove that every Riemannian metric on the 2-disc such that all its
geodesics are minimal, is a minimal filling of its boundary (within the class
of fillings homeomorphic to the disc). This improves an earlier result of the
author by removing the assumption that the boundary is convex. More generally,
we prove this result for Finsler metrics with area defined as the
two-dimensional Holmes-Thompson volume. This implies a generalization of Pu's
isosystolic inequality to Finsler metrics, both for Holmes-Thompson and
Busemann definitions of Finsler area.Comment: 16 pages, v2: improved introduction and formattin
Phonon spectrum and dynamical stability of a quantum degenerate Bose-Fermi mixture
We calculate the phonon excitation spectrum in a zero-temperature
boson-fermion mixture. We show how the sound velocity changes due to the
boson-fermion interaction and we determine the dynamical stability regime of a
homogeneous mixture. We identify a resonant phonon-exchange interaction between
the fermions as the physical mechanism leading to the instability.Comment: 4 pages, 3 figure
Symbiosis between the TRECVid benchmark and video libraries at the Netherlands Institute for Sound and Vision
Audiovisual archives are investing in large-scale digitisation efforts of their analogue holdings and, in parallel, ingesting an ever-increasing amount of born- digital files in their digital storage facilities. Digitisation opens up new access paradigms and boosted re-use of audiovisual content. Query-log analyses show the shortcomings of manual annotation, therefore archives are complementing these annotations by developing novel search engines that automatically extract information from both audio and the visual tracks. Over the past few years, the TRECVid benchmark has developed a novel relationship with the Netherlands Institute of Sound and Vision (NISV) which goes beyond the NISV just providing data and use cases to TRECVid. Prototype and demonstrator systems developed as part of TRECVid are set to become a key driver in improving the quality of search engines at the NISV and will ultimately help other audiovisual archives to offer more efficient and more fine-grained access to their collections. This paper reports the experiences of NISV in leveraging the activities of the TRECVid benchmark
Dynamics of two interacting Bose-Einstein condensates
We analize the dynamics of two trapped interacting Bose-Einstein condensates
and indentify two regimes for the evolution: the regime of slow periodic
oscillations and the regime of strong non-linear mixing leading to the damping
of the relative motion of the condensates. We compare our predictions with an
experiment recently performed at JILA.Comment: 4 pages RevTeX, 3 eps figure
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