8,344 research outputs found
Phytochrome modulation of calcium fluxes in wheat (Triticum aestivum L.) protoplasts
Employing the metallochromic dye murexide and by monitoring the uptake of radiolabelled calcium, photoreversible calcium fluxes were measured in wheat leaf protoplast suspensions. Results obtained by both methods were identical - red light promoted and subsequent far-red irradiation reversed an influx of Ca++ ions into the protoplasts. These findings imply phytochrome regulation of Ca++ fluxes across the plasma membrane. The influx of Ca++ stimulated by 2 min red irradiation could be maintained in total darkness for the initial 16-18 min after illumination, after which a 6-8 min efflux process was triggered and the basal Ca++ level restored. Verapamil, a calcium channel blocker, inhibited the red-promoted influx, whereas the far-red mediated efflux could be checked by the use of the ATPase inhibitor vanadate, and also by the calmodulin antagonist chlorpromazine, thus suggesting a role of ion channels and pumps in phytochrome-controlled Ca++ fluxes. The possible involvement of phosphoinositides in phytochrome-modulated calcium fluxes was also investigated
Inelastic X-ray scattering in correlated (Mott) insulators
We calculate the inelastic light scattering from X-rays, which allows the
photon to transfer both energy and momentum to the strongly correlated charge
excitations. We find that the charge transfer peak and the low energy peak both
broaden and disperse through the Brillouin zone similar to what is seen in
experiments in materials like Ca_2 Cu O_2 Cl_2.Comment: 5 pages Revtex4, 6 figure
Behaviour of three charged particles on a plane under perpendicular magnetic field
We consider the problem of three identical charged particles on a plane under
a perpendicular magnetic field and interacting through Coulomb repulsion. This
problem is treated within Taut's framework, in the limit of vanishing center of
mass vector , which corresponds to the strong magnetic
field limit, occuring for example in the Fractional Quantum Hall Effect. Using
the solutions of the biconfluent Heun equation, we compute the eigenstates and
show that there is two sets of solutions. The first one corresponds to a system
of three independent anyons which have their angular momenta fixed by the value
of the magnetic field and specified by a dimensionless parameter , the ratio of , the magnetic length, over , the Bohr
radius. This anyonic character, consistent with quantum mechanics of identical
particles in two dimensions, is induced by competing physical forces. The
second one corresponds to the case of the Landau problem when .
Finally we compare these states with the quantum Hall states and find that the
Laughlin wave functions are special cases of our solutions under certains
conditions.Comment: 15 pages, 3 figures, Accepeted in JP
Optical Conductivity in Mott-Hubbard Systems
We study the transfer of spectral weight in the optical spectra of a strongly
correlated electron system as a function of temperature and interaction
strength. Within a dynamical mean field theory of the Hubbard model that
becomes exact in the limit of large lattice coordination, we predict an
anomalous enhancement of spectral weight as a function of temperature in the
correlated metallic state and report on experimental measurements which agree
with this prediction in . We argue that the optical conductivity
anomalies in the metal are connected to the proximity to a crossover region in
the phase diagram of the model.Comment: 12 pages and 4 figures, to appear in Phys. Rev. Lett., v 75, p 105
(1995
Bridging the gap between evidence and policy for infectious diseases: How models can aid public health decision-making.
The dominant approach to decision-making in public health policy for infectious diseases relies heavily on expert opinion, which often applies empirical evidence to policy questions in a manner that is neither systematic nor transparent. Although systematic reviews are frequently commissioned to inform specific components of policy (such as efficacy), the same process is rarely applied to the full decision-making process. Mathematical models provide a mechanism through which empirical evidence can be methodically and transparently integrated to address such questions. However, such models are often considered difficult to interpret. In addition, models provide estimates that need to be iteratively re-evaluated as new data or considerations arise. Using the case study of a novel diagnostic for tuberculosis, a framework for improved collaboration between public health decision-makers and mathematical modellers that could lead to more transparent and evidence-driven policy decisions for infectious diseases in the future is proposed. The framework proposes that policymakers should establish long-term collaborations with modellers to address key questions, and that modellers should strive to provide clear explanations of the uncertainty of model structure and outputs. Doing so will improve the applicability of models and clarify their limitations when used to inform real-world public health policy decisions
Interchangeable punishments during aversive conditioning in Drosophila
Using Drosophila melanogaster larvae we asked whether distinct aversive stimuli have a common neuralrepresentation during associative learning. We tested the interchangeability of heat shock and electroshock punishments when used within a single olfactory associative conditioning experiment. We find that compared to animals trained with the repetitive use of a single punishment, the use of two alternating punishments results in similar associative learning. Additionally, the two punishments are shown to have different sensory origins. Therefore, while punishments are processed differently by the larvae of Drosophila melanogaster, the value of the stimulus is preserved
The role of plasma slowdown in the generation of Rhea's Alfven wings
AlfvĂ©n wings are known to form when a conducting or mass-loading object slows down a flowing plasma in its vicinity. AlfvĂ©n wings are not expected to be generated when an inert moon such as Rhea interacts with Saturn's magnetosphere, where the plasma impacting the moon is absorbed and the magnetic flux passes unimpeded through the moon. However, in two close polar passes of Rhea, Cassini clearly observed magnetic field signatures consistent with AlfvĂ©n wings. In addition, observations from a high-inclination flyby (Distanceâ>â100 RRh) of Rhea on 3 June 2010 showed that the AlfvĂ©n wings continue to propagate away from Rhea even at this large distance. We have performed three-dimensional hybrid simulations of Rhea's interaction with Saturn's magnetosphere which show that the wake refilling process generates a plasma density gradient directed in the direction of corotating plasma. The resulting plasma pressure gradient exerts a force directed toward Rhea and slows down the plasma streaming into the wake along field lines. As on the same field lines, outside of the wake, the plasma continues to move close to its full speed, this differential motion of plasma bends the magnetic flux tubes, generating AlfvĂ©n wings in the wake. The current system excited by the AlfvĂ©n wings transfers momentum to the wake plasma extracting it from plasma outside the wake. Our work demonstrates that AlfvĂ©n wings can be excited even when a moon does not possess a conducting exosphere
Heavy Quasi-Particle in the Two-Orbital Hubbard Model
The two-orbital Hubbard model with the Hund coupling is investigated in a
metallic phase close to the Mott insulator. We calculate the one-particle
spectral function and the optical conductivity within dynamical mean field
theory, for which the effective impurity problem is solved by using the
non-crossing approximation. For a metallic system close to quarter filling, a
heavy quasi-particle band is formed by the Hubbard interaction, the effective
mass of which is not so sensitive to the orbital splitting and the Hund
coupling. In contrast, a heavy quasi-particle band near half filling disappears
in the presence of the orbital splitting, but is induced again by the
introduction of the Hund coupling, resulting in a different type of heavy
quasi-particles.Comment: 6page, 7eps figures, to appear in J. Phys. Soc. Jp
Drude weight and dc-conductivity of correlated electrons
The Drude weight and the dc-conductivity of strongly
correlated electrons are investigated theoretically. Analytic results are
derived for the homogeneous phase of the Hubbard model in
dimensions, and for spinless fermions in this limit with -corrections
systematically included to lowest order. It is found that is
finite for all , displaying Fermi liquid behavior, , at low temperatures. The validity of this result for finite dimensions
is examined by investigating the importance of Umklapp scattering processes and
vertex corrections. A finite dc-conductivity for is argued to be a
generic feature of correlated lattice electrons in not too low dimensions.Comment: 15 pages, uuencoded compressed PS-fil
Transfer of Spectral Weight in Spectroscopies of Correlated Electron Systems
We study the transfer of spectral weight in the photoemission and optical
spectra of strongly correlated electron systems. Within the LISA, that becomes
exact in the limit of large lattice coordination, we consider and compare two
models of correlated electrons, the Hubbard model and the periodic Anderson
model. The results are discussed in regard of recent experiments. In the
Hubbard model, we predict an anomalous enhancement optical spectral weight as a
function of temperature in the correlated metallic state which is in
qualitative agreement with optical measurements in . We argue that
anomalies observed in the spectroscopy of the metal are connected to the
proximity to a crossover region in the phase diagram of the model. In the
insulating phase, we obtain an excellent agreement with the experimental data
and present a detailed discussion on the role of magnetic frustration by
studying the resolved single particle spectra. The results for the periodic
Anderson model are discussed in connection to recent experimental data of the
Kondo insulators and . The model can successfully explain
the different energy scales that are associated to the thermal filling of the
optical gap, which we also relate to corresponding changes in the density of
states. The temperature dependence of the optical sum rule is obtained and its
relevance for the interpretation of the experimental data discussed. Finally,
we argue that the large scattering rate measured in Kondo insulators cannot be
described by the periodic Anderson model.Comment: 19 pages + 29 figures. Submitted to PR
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