378 research outputs found
Spinor BECs in a double-well: population transfer and Josephson oscillations
The dynamics of an F=1 spinor condensate in a two-well potential is studied
within the framework of the Gross-Pitaevskii equation. We derive two-mode
equations relating the population imbalances, the phase differences among the
condensates at each side of the barrier and the time evolution of the different
Zeeman populations for the case of small population imbalances. The case of
zero total magnetization is scrutinized in this limit demonstrating the ability
of a two mode analysis to describe to a large extent the dynamics observed in
the Gross-Pitaevskii equations. It is also demonstrated that the time evolution
of the different total populations fully decouples from the Josephson tunneling
phenomena. All the relevant time scales are clearly identified with microscopic
properties of the atom-atom interactions
Fragmented condensation in Bose-Hubbard trimers with tunable tunnelling
We consider a Bose-Hubbard trimer, i.e. an ultracold Bose gas populating
three quantum states. The latter can be either different sites of a triple-well
potential or three internal states of the atoms. The bosons can tunnel between
different states with variable tunnelling strength between two of them. This
will allow us to study; i) different geometrical configurations, i.e. from a
closed triangle to three aligned wells and ii) a triangular configuration with
a -phase, i.e. by setting one of the tunnellings negative. By solving the
corresponding three-site Bose-Hubbard Hamiltonian we obtain the ground state of
the system as a function of the trap topology. We characterise the different
ground states by means of the coherence and entanglement properties. For small
repulsive interactions, fragmented condensates are found for the -phase
case. These are found to be robust against small variations of the tunnelling
in the small interaction regime. A low-energy effective many-body Hamiltonian
restricted to the degenerate manifold provides a compelling description of the
-phase degeneration and explains the low-energy spectrum as excitations of
discrete semifluxon states
Dynamic generation of spin-squeezed states in bosonic Josephson junctions
We analyze the formation of squeezed states in a condensate of ultracold
bosonic atoms confined by a double-well potential. The emphasis is set on the
dynamical formation of such states from initially coherent many-body quantum
states. Two cases are described: the squeezing formation in the evolution of
the system around the stable point, and in the short time evolution in the
vicinity of an unstable point. The latter is shown to produce highly squeezed
states on very short times. On the basis of a semiclassical approximation to
the Bose-Hubbard Hamiltonian, we are able to predict the amount of squeezing,
its scaling with and the speed of coherent spin formation with simple
analytical formulas which successfully describe the numerical Bose-Hubbard
results. This new method of producing highly squeezed spin states in systems of
ultracold atoms is compared to other standard methods in the literature.Comment: 12 pages, revised discussion + added reference
Sincell: an R/Bioconductor package for statistical assessment of cell-state hierarchies from single-cell RNA-seq
Cell differentiation processes are achieved through a continuum of hierarchical intermediate cell states that might be captured by single-cell RNA seq. Existing computational approaches for the assessment of cell-state hierarchies from single-cell data can be formalized under a general framework composed of (i) a metric to assess cell-to-cell similarities (with or without a dimensionality reduction step) and (ii) a graph-building algorithm (optionally making use of a cell clustering step). The Sincell R package implements a methodological toolbox allowing flexible workflows under such a framework. Furthermore, Sincell contributes new algorithms to provide cell-state hierarchies with statistical support while accounting for stochastic factors in single-cell RNA seq. Graphical representations and functional association tests are provided to interpret hierarchies. The functionalities of Sincell are illustrated in a real case study, which demonstrates its ability to discriminate noisy from stable cell-state hierarchies.
AVAILABILITY AND IMPLEMENTATION: Sincell is an open-source R/Bioconductor package available at http://bioconductor.org/packages/sincell. A detailed manual and a vignette are provided with the package.
CONTACT: [email protected]
SUPPLEMENTARY INFORMATION: Supplementary data are available at Bioinformatics online
Double and single pion photoproduction within a dynamical coupled-channels model
Within a dynamical coupled-channels model which has already been fixed from
analyzing the data of the pi N -> pi N and gamma N -> pi N reactions, we
present the predicted double pion photoproduction cross sections up to the
second resonance region, W< 1.7 GeV. The roles played by the different
mechanisms within our model in determining both the single and double pion
photoproduction reactions are analyzed, focusing on the effects due to the
direct gamma N -> pi pi N mechanism, the interplay between the resonant and
non-resonant amplitudes, and the coupled-channels effects. The model parameters
which can be determined most effectively in the combined studies of both the
single and double pion photoproduction data are identified for future studies.Comment: Version to appear in PRC. 16 pages, 13 figure
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