882 research outputs found
Reheating in a Brane Monodromy Inflation Model
We study reheating in a recently proposed brane "monodromy inflation" model
in which the inflaton is the position of a D4 brane on a "twisted torus".
Specifically, we study the repeated collisions between the D4 brane and a D6
brane (on which the Standard Model fields are assumed to be localized) at a
fixed position along the monodromy direction as the D4 brane rolls down its
potential. We find that there is no trapping of the rolling D4 brane until it
reaches the bottom of its potential, and that reheating is entirely described
by the last brane encounter. Previous collisions have negligible effect on the
brane velocity and hence on the reheat temperature. In the context of our
setup, reheating is efficient and the reheat temperature is therefore high.Comment: 13 pages, reference adde
Symbolic dynamics for the -centre problem at negative energies
We consider the planar -centre problem, with homogeneous potentials of
degree -\a<0, \a \in [1,2). We prove the existence of infinitely many
collisions-free periodic solutions with negative and small energy, for any
distribution of the centres inside a compact set. The proof is based upon
topological, variational and geometric arguments. The existence result allows
to characterize the associated dynamical system with a symbolic dynamics, where
the symbols are the partitions of the centres in two non-empty sets
Ramanujan sums for signal processing of low frequency noise
An aperiodic (low frequency) spectrum may originate from the error term in
the mean value of an arithmetical function such as M\"obius function or
Mangoldt function, which are coding sequences for prime numbers. In the
discrete Fourier transform the analyzing wave is periodic and not well suited
to represent the low frequency regime. In place we introduce a new signal
processing tool based on the Ramanujan sums c_q(n), well adapted to the
analysis of arithmetical sequences with many resonances p/q. The sums are
quasi-periodic versus the time n of the resonance and aperiodic versus the
order q of the resonance. New results arise from the use of this
Ramanujan-Fourier transform (RFT) in the context of arithmetical and
experimental signalsComment: 11 pages in IOP style, 14 figures, 2 tables, 16 reference
Hamiltonian dynamics of the two-dimensional lattice phi^4 model
The Hamiltonian dynamics of the classical model on a two-dimensional
square lattice is investigated by means of numerical simulations. The
macroscopic observables are computed as time averages. The results clearly
reveal the presence of the continuous phase transition at a finite energy
density and are consistent both qualitatively and quantitatively with the
predictions of equilibrium statistical mechanics. The Hamiltonian microscopic
dynamics also exhibits critical slowing down close to the transition. Moreover,
the relationship between chaos and the phase transition is considered, and
interpreted in the light of a geometrization of dynamics.Comment: REVTeX, 24 pages with 20 PostScript figure
Exactly solvable model of quantum diffusion
We study the transport property of diffusion in a finite translationally
invariant quantum subsystem described by a tight-binding Hamiltonian with a
single energy band and interacting with its environment by a coupling in terms
of correlation functions which are delta-correlated in space and time. For weak
coupling, the time evolution of the subsystem density matrix is ruled by a
quantum master equation of Lindblad type. Thanks to the invariance under
spatial translations, we can apply the Bloch theorem to the subsystem density
matrix and exactly diagonalize the time evolution superoperator to obtain the
complete spectrum of its eigenvalues, which fully describe the relaxation to
equilibrium. Above a critical coupling which is inversely proportional to the
size of the subsystem, the spectrum at given wavenumber contains an isolated
eigenvalue describing diffusion. The other eigenvalues rule the decay of the
populations and quantum coherences with decay rates which are proportional to
the intensity of the environmental noise. On the other hand, an analytical
expression is obtained for the dispersion relation of diffusion. The diffusion
coefficient is proportional to the square of the width of the energy band and
inversely proportional to the intensity of the environmental noise because
diffusion results from the perturbation of quantum tunneling by the
environmental fluctuations in this model. Diffusion disappears below the
critical coupling.Comment: Submitted to J. Stat. Phy
YBCO-buffered NdBCO film with higher thermal stability in seeding REBCO Growth
In this work, we report a strengthened superheating effect caused by a
buffering YBa2Cu3Oy (Y123 or YBCO) layer in the Nd1+xBa2-xCu3O7-y (Nd123 or
NdBCO) thin film with MgO substrate (i.e., NdBCO/YBCO/MgO thin film). In the
cold-seeding melt-textured (MT) growth, the NdBCO/YBCO/MgO film presented an
even higher superheating level, about 20 {\deg}C higher than that of
non-buffered NdBCO film (i.e., NdBCO/MgO film). Using this NdBCO/YBCO/MgO film
as seeds and undergoing a maximum processing temperature (Tmax) up to 1120
{\deg}C, we succeeded in growing various RE1+xBa2-xCu3O7-y (REBCO, RE=rare
elements) bulk superconductors, including Gd1+xBa2-xCu3O7-y (GdBCO),
Sm1+xBa2-xCu3O7-y (SmBCO) and NdBCO that have high peritectic temperatures
(Tp). The pole figure (X-Ray \phi-scan) measurement reveals that the
NdBCO/YBCO/MgO film has better in-plane alignment than the NdBCO/MgO film,
indicating that the induced intermediate layer improves the crystallinity of
the NdBCO film, which could be the main origin of the enhanced thermal
stability. In short, possessing higher thermal stability and enduring a higher
Tmax in the MT process, the NdBCO/YBCO/MgO film is beneficial to the growth of
bulk superconductors in two aspects: (1) broad application for high-Tp REBCO
materials; (2) effective suppression against heterogeneous nucleation, which is
of great assistance in growing large and high-performance REBCO crystals.Comment: 9 pages, 4 figure
Development of high amylose wheat through TILLING
BACKGROUND: Wheat (Triticum spp.) is an important source of food worldwide and the focus of considerable efforts to identify new combinations of genetic diversity for crop improvement. In particular, wheat starch composition is a major target for changes that could benefit human health. Starches with increased levels of amylose are of interest because of the correlation between higher amylose content and elevated levels of resistant starch, which has been shown to have beneficial effects on health for combating obesity and diabetes. TILLING (Targeting Induced Local Lesions in Genomes) is a means to identify novel genetic variation without the need for direct selection of phenotypes. RESULTS: Using TILLING to identify novel genetic variation in each of the A and B genomes in tetraploid durum wheat and the A, B and D genomes in hexaploid bread wheat, we have identified mutations in the form of single nucleotide polymorphisms (SNPs) in starch branching enzyme IIa genes (SBEIIa). Combining these new alleles of SBEIIa through breeding resulted in the development of high amylose durum and bread wheat varieties containing 47-55% amylose and having elevated resistant starch levels compared to wild-type wheat. High amylose lines also had reduced expression of SBEIIa RNA, changes in starch granule morphology and altered starch granule protein profiles as evaluated by mass spectrometry. CONCLUSIONS: We report the use of TILLING to develop new traits in crops with complex genomes without the use of transgenic modifications. Combined mutations in SBEIIa in durum and bread wheat varieties resulted in lines with significantly increased amylose and resistant starch contents
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