76,988 research outputs found
Phase transitions for a collective coordinate coupled to Luttinger liquids
We study various realizations of collective coordinates, e.g. the position of
a particle, the charge of a Coulomb box or the phase of a Bose or a
superconducting condensate, coupled to Luttinger liquids (LL) with N flavors.
We find that for Luttinger parameter 1/2<K<1 there is a phase transition from a
delocalized phase into a phase with a periodic potential at strong coupling. In
the delocalized phase the dynamics is dominated by an effective mass, i.e.
diffusive in imaginary time, while on the transition line it becomes
dissipative. At K=1/2 there is an additional transition into a localized phase
with no diffusion at zero temperature.Comment: 5 pages, 2 figures, 1 table, Phys. Rev. Lett. (in press
Bose-enhanced chemistry: Amplification of selectivity in the dissociation of molecular Bose-Einstein condensates
We study the photodissociation chemistry of a quantum degenerate gas of
bosonic triatomic molecules, assuming two open rearrangement channels
( or ). The equations of motion are equivalent to those of a
parametric multimode laser, resulting in an exponential buildup of macroscopic
mode populations. By exponentially amplifying a small differential in the
single-particle rate-coefficients, Bose stimulation leads to a nearly complete
selectivity of the collective -body process, indicating a novel type of
ultra-selective quantum degenerate chemistry.Comment: 5 pages, 3 figure
Metastable and transient states of chemical ordering in Fe-V nanocrystalline alloys
Chemical ordering of the disordered alloys Fe0.78V0.22, Fe0.53V0.47, Fe0.39V0.61, and Fe0.37V0.63 was performed by annealing at temperatures from 723 to 973 K. The initial state of chemical disorder was produced by high-energy ball milling, and the evolution of order was measured by neutron diffractometry and by 57Fe Mössbauer spectrometry. The hyperfine magnetic field distributions obtained from the Mössbauer spectra provided quantitative measurements of the number of antisite Fe atoms in the partially ordered alloys. The long-range order parameters in steady state after long annealing times were used as states of metastable equilibrium for a generally successful comparison with the metastable Fe-V phase diagram calculated by Sanchez et al. [Phys. Rev. B 54, 8958 (1996)]. For the metastable equilibrium state of order in Fe0.53V0.47 at low temperatures, the order parameters were smaller than expected. This corresponded to an abundance of antisite atoms, which were not removed effectively by annealing at the lower temperatures
Quantifying the Effect of Non-Larmor Motion of Electrons on the Pressure Tensor
In space plasma, various effects of magnetic reconnection and turbulence
cause the electron motion to significantly deviate from their Larmor orbits.
Collectively these orbits affect the electron velocity distribution function
and lead to the appearance of the "non-gyrotropic" elements in the pressure
tensor. Quantification of this effect has important applications in space and
laboratory plasma, one of which is tracing the electron diffusion region (EDR)
of magnetic reconnection in space observations. Three different measures of
agyrotropy of pressure tensor have previously been proposed, namely,
, and . The multitude of contradictory measures has
caused confusion within the community. We revisit the problem by considering
the basic properties an agyrotropy measure should have. We show that
, and are all defined based on the sum of the
principle minors (i.e. the rotation invariant ) of the pressure tensor. We
discuss in detail the problems of -based measures and explain why they may
produce ambiguous and biased results. We introduce a new measure
constructed based on the determinant of the pressure tensor (i.e. the rotation
invariant ) which does not suffer from the problems of -based
measures. We compare with other measures in 2 and 3-dimension
particle-in-cell magnetic reconnection simulations, and show that can
effectively trace the EDR of reconnection in both Harris and force-free current
sheets. On the other hand, does not show prominent peaks in
the EDR and part of the separatrix in the force-free reconnection simulations,
demonstrating that does not measure all the non-gyrotropic
effects in this case, and is not suitable for studying magnetic reconnection in
more general situations other than Harris sheet reconnection.Comment: accepted by Phys. of Plasm
Optical study of the anisotropic erbium spin flip-flop dynamics
We investigate the erbium flip-flop dynamics as a limiting factor of the
electron spin lifetime and more generally as an indirect source of decoherence
in rare-earth doped insulators. Despite the random isotropic arrangement of
dopants in the host crystal, the dipolar interaction strongly depends on the
magnetic field orientation following the strong anisotropy of the -factor.
In Er:YSiO, we observe by transient optical spectroscopy a three
orders of magnitude variation of the erbium flip-flop rate (10ppm dopant
concentration). The measurements in two different samples, with 10ppm and 50ppm
concentrations, are well-supported by our analytic modeling of the dipolar
coupling between identical spins with an anisotropic -tensor. The model can
be applied to other rare-earth doped materials. We extrapolate the calculation
to Er:CaWO, Er:LiNbO and Nd:YSiO at
different concentrations
The Wide-field High-resolution Infrared TElescope (WHITE)
The Wide-field High-resolution Infrared TElescope (WHITE) will be dedicated
in the first years of its life to carrying out a few (well focused in terms of
science objectives and time) legacy surveys.
WHITE would have an angular resolution of ~0.3'' homogeneous over ~0.7 sq.
deg. in the wavelength range 1 - 5 um, which means that we will very
efficiently use all the available observational time during night time and day
time. Moreover, the deepest observations will be performed by summing up
shorter individual frames. We will have a temporal information that can be used
to study variable objects.
The three key science objectives of WHITE are : 1) A complete survey of the
Magellanic Clouds to make a complete census of young stellar objects in the
clouds and in the bridge and to study their star formation history and the link
with the Milky Way. The interaction of the two clouds with our Galaxy might the
closest example of a minor merging event that could be the main driver of
galaxy evolution in the last 5 Gyrs. 2) The building of the first sample of
dusty supernovae at z<1.2 in the near infrared range (1-5 um) to constrain the
equation of state from these obscured objects, study the formation of dust in
galaxies and build the first high resolution sample of high redshift galaxies
observed in their optical frame 3) A very wide weak lensing survey over that
would allow to estimate the equation of state in a way that would favourably
compete with space projects.Comment: Invited talk to the 2nd ARENA Conference : "The Astrophysical Science
Cases at Dome C" Potsdam 17-21 September, 200
Words are Malleable: Computing Semantic Shifts in Political and Media Discourse
Recently, researchers started to pay attention to the detection of temporal
shifts in the meaning of words. However, most (if not all) of these approaches
restricted their efforts to uncovering change over time, thus neglecting other
valuable dimensions such as social or political variability. We propose an
approach for detecting semantic shifts between different viewpoints--broadly
defined as a set of texts that share a specific metadata feature, which can be
a time-period, but also a social entity such as a political party. For each
viewpoint, we learn a semantic space in which each word is represented as a low
dimensional neural embedded vector. The challenge is to compare the meaning of
a word in one space to its meaning in another space and measure the size of the
semantic shifts. We compare the effectiveness of a measure based on optimal
transformations between the two spaces with a measure based on the similarity
of the neighbors of the word in the respective spaces. Our experiments
demonstrate that the combination of these two performs best. We show that the
semantic shifts not only occur over time, but also along different viewpoints
in a short period of time. For evaluation, we demonstrate how this approach
captures meaningful semantic shifts and can help improve other tasks such as
the contrastive viewpoint summarization and ideology detection (measured as
classification accuracy) in political texts. We also show that the two laws of
semantic change which were empirically shown to hold for temporal shifts also
hold for shifts across viewpoints. These laws state that frequent words are
less likely to shift meaning while words with many senses are more likely to do
so.Comment: In Proceedings of the 26th ACM International on Conference on
Information and Knowledge Management (CIKM2017
Synaptic nanomodules underlie the organization and plasticity of spine synapses.
Experience results in long-lasting changes in dendritic spine size, yet how the molecular architecture of the synapse responds to plasticity remains poorly understood. Here a combined approach of multicolor stimulated emission depletion microscopy (STED) and confocal imaging in rat and mouse demonstrates that structural plasticity is linked to the addition of unitary synaptic nanomodules to spines. Spine synapses in vivo and in vitro contain discrete and aligned subdiffraction modules of pre- and postsynaptic proteins whose number scales linearly with spine size. Live-cell time-lapse super-resolution imaging reveals that NMDA receptor-dependent increases in spine size are accompanied both by enhanced mobility of pre- and postsynaptic modules that remain aligned with each other and by a coordinated increase in the number of nanomodules. These findings suggest a simplified model for experience-dependent structural plasticity relying on an unexpectedly modular nanomolecular architecture of synaptic proteins
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