615 research outputs found
Size dependence of the photoinduced magnetism and long-range ordering in Prussian blue analog nanoparticles of rubidium cobalt hexacyanoferrate
Nanoparticles of rubidium cobalt hexacyanoferrate
(RbCo[Fe(CN)]HO) were synthesized using different
concentrations of the polyvinylpyrrolidone (PVP) to produce four different
batches of particles with characteristic diameters ranging from 3 to 13 nm.
Upon illumination with white light at 5 K, the magnetization of these particles
increases. The long-range ferrimagnetic ordering temperatures and the coercive
fields evolve with nanoparticle size. At 2 K, particles with diameters less
than approximately 10 nm provide a Curie-like magnetic signal.Comment: 10 pages, 6 figures in text, expanded text and dat
Stability of de Sitter spacetime under isotropic perturbations in semiclassical gravity
A spatially flat Robertson-Walker spacetime driven by a cosmological constant
is non-conformally coupled to a massless scalar field. The equations of
semiclassical gravity are explicitly solved for this case, and a
self-consistent de Sitter solution associated with the Bunch-Davies vacuum
state is found (the effect of the quantum field is to shift slightly the
effective cosmological constant). Furthermore, it is shown that the corrected
de Sitter spacetime is stable under spatially-isotropic perturbations of the
metric and the quantum state. These results are independent of the free
renormalization parameters.Comment: 19 pages, REVTeX
The T=1 capsid protein of Penicillium chrysogenum virus is formed by a repeated helix-rich core indicative of gene duplication
et al.Penicillium chrysogenum virus (PcV), a member of the Chrysoviridae family, is a double-stranded RNA (dsRNA) fungal virus with a multipartite genome, with each RNA molecule encapsidated in a separate particle. Chrysoviruses lack an extracellular route and are transmitted during sporogenesis and cell fusion. The PcV capsid, based on a T=1 lattice containing 60 subunits of the 982-amino-acid capsid protein, remains structurally undisturbed throughout the viral cycle, participates in genome metabolism, and isolates the virus genome from host defense mechanisms. Using three-dimensional cryoelectron microscopy, we determined the structure of the PcV virion at 8.0 Å resolution. The capsid protein has a high content of rod-like densities characteristic of α-helices, forming a repeated α-helical core indicative of gene duplication. Whereas the PcV capsid protein has two motifs with the same fold, most dsRNA virus capsid subunits consist of dimers of a single protein with similar folds. The spatial arrangement of the α-helical core resembles that found in the capsid protein of the L-A virus, a fungal totivirus with an undivided genome, suggesting a conserved basic fold. The encapsidated genome is organized in concentric shells; whereas the inner dsRNA shells are well defined, the outermost layer is dense due to numerous interactions with the inner capsid surface, specifically, six interacting areas per monomer. The outermost genome layer is arranged in an icosahedral cage, sufficiently well ordered to allow for modeling of an A-form dsRNA. The genome ordering might constitute a framework for dsRNA transcription at the capsid interior and/or have a structural role for capsid stability. Copyright © 2010, American Society for Microbiology. All Rights Reserved.This work was supported by grants from the Spanish Ministry of Science and Innovation (BFU 2008-02328/BMC and S-0505-Mat-0238 to J.L.C. and BIO2008-02361 to J.R.C.) and the NIH Intramural Research Program with support from the Center for Information Technology.Peer Reviewe
Free-Field Realization of D-dimensional Cylindrical Gravitational Waves
We find two-dimensional free-field variables for D-dimensional general
relativity on spacetimes with D-2 commuting spacelike Killing vector fields and
non-compact spatial sections for D>4. We show that there is a canonical
transformation which maps the corresponding two-dimensional dilaton gravity
theory into a two-dimensional diffeomorphism invariant theory of the free-field
variables. We also show that the spacetime metric components can be expressed
as asymptotic series in negative powers of the dilaton, with coefficients which
can be determined in terms of the free fields.Comment: 15 pages, Late
Real time approach to tunneling in open quantum systems: decoherence and anomalous diffusion
Macroscopic quantum tunneling is described using the master equation for the
reduced Wigner function of an open quantum system at zero temperature. Our
model consists of a particle trapped in a cubic potential interacting with an
environment characterized by dissipative and normal and anomalous diffusion
coefficients. A representation based on the energy eigenfunctions of the
isolated system, i.e. the system uncoupled to the environment, is used to write
the reduced Wigner function, and the master equation becomes simpler in that
representation. The energy eigenfunctions computed in a WKB approximation
incorporate the tunneling effect of the isolated system and the effect of the
environment is described by an equation that it is in many ways similar to a
Fokker-Planck equation. Decoherence is easily identified from the master
equation and we find that when the decoherence time is much shorter than the
tunneling time the master equation can be approximated by a Kramers like
equation describing thermal activation due to the zero point fluctuations of
the quantum environment. The effect of anomalous diffusion can be dealt with
perturbatively and its overall effect is to inhibit tunneling.Comment: 25 pages, 1 figur
Molecular and all solid DFT studies of the magnetic and chemical bonding properties within KM[Cr(CN)] (M = V, Ni) complexes
A study at both the molecular and extended solid level in the framework DFT
is carried out for KM[Cr(CN)] (M = V, Ni). From molecular calculations, the
exchange parameters J are obtained, pointing to the expected magnetic ground
states, i.e., antiferromagnetic for M = V with J = -296.5 cm and
ferromagnetic for M = Ni with J = +40.5 cm. From solid state
computations the same ground states and J magnitudes are confirmed from energy
differences. Furthermore an analysis of the site projected density of states
and of the chemical bonding is developed in which the cyanide ion linkage is
analyzed addressing some isomerism aspects.Comment: new results, 5 tables, 7 fig
Mode decomposition and renormalization in semiclassical gravity
We compute the influence action for a system perturbatively coupled to a
linear scalar field acting as the environment. Subtleties related to
divergences that appear when summing over all the modes are made explicit and
clarified. Being closely connected with models used in the literature, we show
how to completely reconcile the results obtained in the context of stochastic
semiclassical gravity when using mode decomposition with those obtained by
other standard functional techniques.Comment: 4 pages, RevTeX, no figure
Backreaction from non-conformal quantum fields in de Sitter spacetime
We study the backreaction on the mean field geometry due to a non-conformal
quantum field in a Robertson-Walker background. In the regime of small mass and
small deviation from conformal coupling, we compute perturbatively the
expectation value of the stress tensor of the field for a variety of vacuum
states, and use it to obtain explicitly the semiclassical gravity solutions for
isotropic perturbations around de Sitter spacetime, which is found to be
stable. Our results show clearly the crucial role of the non-local terms that
appear in the effective action: they cancel the contribution from local terms
proportional to the logarithm of the scale factor which would otherwise become
dominant at late times and prevent the existence of a stable self-consistent de
Sitter solution. Finally, the opposite regime of a strongly non-conformal field
with a large mass is also considered.Comment: 31 page
Dissipation, noise and vacuum decay in quantum field theory
We study the process of vacuum decay in quantum field theory focusing on the
stochastic aspects of the interaction between long and short-wavelength modes.
This interaction results in a diffusive behavior of the reduced Wigner function
describing the state of the long-wavelength modes, and thereby to a finite
activation rate even at zero temperature. This effect can make a substantial
contribution to the total decay rate.Comment: 5 page
Stochastic Gravity: Beyond Semiclassical Gravity
The back-reaction of a classical gravitational field interacting with quantum
matter fields is described by the semiclassical Einstein equation, which has
the expectation value of the quantum matter fields stress tensor as a source.
The semiclassical theory may be obtained from the quantum field theory of
gravity interacting with N matter fields in the large N limit. This theory
breaks down when the fields quantum fluctuations are important. Stochastic
gravity goes beyond the semiclassical limit and allows for a systematic and
self-consistent description of the metric fluctuations induced by these quantum
fluctuations. The correlation functions of the metric fluctuations obtained in
stochastic gravity reproduce the correlation functions in the quantum theory to
leading order in an 1/N expansion. Two main applications of stochastic gravity
are discussed. The first, in cosmology, to obtain the spectrum of primordial
metric perturbations induced by the inflaton fluctuations, even beyond the
linear approximation. The second, in black hole physics, to study the
fluctuations of the horizon of an evaporating black hole.Comment: 12 pages, no figures, proceedings of the XXIX Spanish Relativity
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