4,938 research outputs found
Classification of zero-energy resonances by dissociation of Feshbach molecules
We study the dissociation of Feshbach molecules by a magnetic field sweep
across a zero-energy resonance. In the limit of an instantaneous magnetic field
change, the distribution of atomic kinetic energy can have a peak indicating
dominance of the molecular closed-channel spin configuration over the entrance
channel. The extent of this dominance influences physical properties such as
stability with respect to collisions, and so the readily measurable presence or
absence of the corresponding peak provides a practical method of classifying
zero-energy resonances. Currently achievable ramp speeds, e.g. those
demonstrated by Duerr et al. [Phys. Rev. A 70, 031601 (2005)], are fast enough
to provide magnetic field changes that may be interpreted as instantaneous. We
study the transition from sudden magnetic field changes to asymptotically wide,
linear ramps. In the latter limit, the predicted form of the atomic kinetic
energy distribution is independent of the specific implementation of the
two-body physics, provided that the near-resonant scattering properties are
properly accounted for.Comment: 10 pages, 5 eps figure
The search for planetary mass companions to field brown dwarfs with HST/NICMOS
We present the results of a high-resolution spectral differential imaging
survey of 12 nearby, relatively young field L dwarfs (<1 Gyr) carried out with
HST/NICMOS to search for planetary mass companions at small physical
separations from their host. The survey resolved two brown dwarf binaries: the
L dwarf system Kelu-1AB and the newly discovered L/T transition system 2MASS
J031059+164815AB. For both systems common proper motion has already been
confirmed in follow-up observations which have been published elsewhere. The
derived separations of the binaries are smaller than 6 AU and consistent with
previous brown dwarf binary statistics. Their mass ratios of q > 0.8 confirm
the preference for equal mass systems similar to a large number of other
surveys. Furthermore, we found tentative evidence for a companion to the L4
dwarf 2MASS W033703-175807, straddling the brown dwarf/planetary mass boundary
and revealing an uncommonly low mass ratio system (q ~ 0.2) compared to the
vast majority of previously found brown dwarf binaries. With a derived minimum
mass of 10 - 15 Mjup, a planetary nature of the secondary cannot be ruled out
yet. However, it seems more likely to be a very low mass brown dwarf secondary
at the border of the spectral T/Y transition regime, primarily due to its
similarities to recently found very cool T dwarfs. This would make it one of
the closest resolved brown dwarf binaries (0.087" 0.015", corresponding
to 2.52 0.44 AU at a distance of 29 pc) with the coolest (Teff ~ 600-630
K) and least massive companion to any L or T dwarf.Comment: 33 pages, 8 figures, 2 tables, accepted for publication by Ap
Dynamics of Natural Killer cell receptor revealed by quantitative analysis of photoswitchable protein
Natural Killer (NK) cell activation is dynamically regulated by numerous
activating and inhibitory surface receptors that accumulate at the immune
synapse. Quantitative analysis of receptor dynamics has been limited by
methodologies which rely on indirect measurements such as fluorescence recovery
after photobleaching. Here, we report a novel approach to study how proteins
traffic to and from the immune synapse using NK cell receptors tagged with the
photoswitchable fluorescent protein tdEosFP, which can be irreversibly
photoswitched from a green to red fluorescent state by ultraviolet light. Thus,
following a localized switching event, the movement of the photoswitched
molecules can be temporally and spatially resolved by monitoring fluorescence
in two regions of interest. By comparing images with mathematical models, we
evaluated the diffusion coefficient of the receptor KIR2DL1 (0.23 +- 0.06
micron^2/s) and assessed how synapse formation affects receptor dynamics. Our
data conclude that the inhibitory NK cell receptor KIR2DL1 is continually
trafficked into the synapse and remains surprisingly stable there. Unexpectedly
however, in NK cells forming synapses with multiple target cells
simultaneously, KIR2DL1 at one synapse can relocate to another synapse. Thus,
our results reveal a previously undetected inter-synaptic exchange of protein.Comment: 25 pages, 5 figure
Three body problem in a dilute Bose-Einstein condensate
We derive the explicit three body contact potential for a dilute condensed
Bose gas from microscopic theory. The three body coupling constant exhibits the
general form predicted by T.T. Wu [Phys. Rev. 113, 1390 (1959)] and is
determined in terms of the amplitudes of two and three body collisions in
vacuum. In the present form the coupling constant becomes accessible to
quantitative studies which should provide the crucial link between few body
collisions and the stability of condensates with attractive two body forces
Random Hierarchical Matrices: Spectral Properties and Relation to Polymers on Disordered Trees
We study the statistical and dynamic properties of the systems characterized
by an ultrametric space of states and translationary non-invariant symmetric
transition matrices of the Parisi type subjected to "locally constant"
randomization. Using the explicit expression for eigenvalues of such matrices,
we compute the spectral density for the Gaussian distribution of matrix
elements. We also compute the averaged "survival probability" (SP) having sense
of the probability to find a system in the initial state by time . Using the
similarity between the averaged SP for locally constant randomized Parisi
matrices and the partition function of directed polymers on disordered trees,
we show that for times (where is some critical
time) a "lacunary" structure of the ultrametric space occurs with the
probability . This means that the escape from some bounded
areas of the ultrametric space of states is locked and the kinetics is confined
in these areas for infinitely long time.Comment: 7 pages, 2 figures (the paper is essentially reworked
Hybrid paramagnon phonon modes at elevated temperatures in EuTiO3
EuTiO3 (ETO) has recently experienced an enormous revival of interest because
of its possible multiferroic properties which are currently in the focus of
research. Unfortunately ETO is an unlikely candidate for enlarged
multifunctionality since the mode softening - typical for ferroelectrics -
remains incomplete, and the antiferromagnetic properties appear at 5.5K only.
However, a strong coupling between lattice and Eu spins exists and leads to the
appearance of a magnon-phonon-hybrid mode at elevated temperatures as evidenced
by electron paramagnetic resonance (EPR), muon spin rotation ({\mu}SR)
experiments and model predictions based on a coupled spin-polarizability
Hamiltonian. This novel finding supports the notion of strong
magneto-dielectric (MD) effects being realized in ETO and opens new strategies
in material design and technological applications.Comment: 9 pages, 4 figure
Nonlocal Kinetic Equation and Simulations of Heavy Ion Reactions
A kinetic equation which combines the quasiparticle drift of Landau's
equation with a dissipation governed by a nonlocal and noninstantaneous
scattering integral in the spirit of Enskog corrections is discussed. Numerical
values of the off-shell contribution to the Wigner distribution, of the
collision duration and of the collision nonlocality are presented for different
realistic potentials. On preliminary results we show that simulations of
quantum molecular dynamics extended by the nonlocal treatment of collisions
leads to a broader proton distribution bringing the theoretical spectra closer
towards the experimental values than the local approach.Comment: Proceedings of the Erice School, published as Vol. 42 of "Progress in
Particle and Nuclear Physics" by ELSEVIE
Production of three-body Efimov molecules in an optical lattice
We study the possibility of associating meta-stable Efimov trimers from three
free Bose atoms in a tight trap realised, for instance, via an optical lattice
site or a microchip. The suggested scheme for the production of these molecules
is based on magnetically tunable Feshbach resonances and takes advantage of the
Efimov effect in three-body energy spectra. Our predictions on the energy
levels and wave functions of three pairwise interacting 85Rb atoms rely upon
exact solutions of the Faddeev equations and include the tightly confining
potential of an isotropic harmonic atom trap. The magnetic field dependence of
these energy levels indicates that it is the lowest energetic Efimov trimer
state that can be associated in an adiabatic sweep of the field strength. We
show that the binding energies and spatial extents of the trimer molecules
produced are comparable, in their magnitudes, to those of the associated
diatomic Feshbach molecule. The three-body molecular state follows Efimov's
scenario when the pairwise attraction of the atoms is strengthened by tuning
the magnetic field strength.Comment: 21 pages, 8 figures (final version
Shortest Path Computation with No Information Leakage
Shortest path computation is one of the most common queries in location-based
services (LBSs). Although particularly useful, such queries raise serious
privacy concerns. Exposing to a (potentially untrusted) LBS the client's
position and her destination may reveal personal information, such as social
habits, health condition, shopping preferences, lifestyle choices, etc. The
only existing method for privacy-preserving shortest path computation follows
the obfuscation paradigm; it prevents the LBS from inferring the source and
destination of the query with a probability higher than a threshold. This
implies, however, that the LBS still deduces some information (albeit not
exact) about the client's location and her destination. In this paper we aim at
strong privacy, where the adversary learns nothing about the shortest path
query. We achieve this via established private information retrieval
techniques, which we treat as black-box building blocks. Experiments on real,
large-scale road networks assess the practicality of our schemes.Comment: VLDB201
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