836 research outputs found
The microlocal spectrum condition and Wick polynomials of free fields on curved spacetimes
Quantum fields propagating on a curved spacetime are investigated in terms of
microlocal analysis. We discuss a condition on the wave front set for the
corresponding n-point distributions, called ``microlocal spectrum condition''
(SC). On Minkowski space, this condition is satisfied as a consequence of
the usual spectrum condition. Based on Radzikowski's determination of the wave
front set of the two-point function of a free scalar field, satisfying the
Hadamard condition in the Kay and Wald sense, we construct in the second part
of this paper all Wick polynomials including the energy-momentum tensor for
this field as operator valued distributions on the manifold and prove that they
satisfy our microlocal spectrum condition.Comment: 21 pages, AMS-LaTeX, 2 figures appended as Postscript file
Exact analytical expression for magnetoresistance using quantum groups
We obtain an exact analytical expression for magnetoresistance using
noncommutative geometry and quantum groups.Then we will show that there is a
deep relationship between magnetoresistance and the quantum group ,
from which we understand the quantum interpretation of the quantum corrections
to the conductivity.Comment: 8 pages, 3 figures, replaced with the version published in Physics
Letters
Electromagnetic nucleon-delta transition in the perturbative chiral quark model
We apply the perturbative chiral quark model to the gamma N -> Delta
transition. The four momentum dependence of the respective transverse helicity
amplitudes A(1/2) and A(3/2) is determined at one loop in the pseudoscalar
Goldstone boson fluctuations. Inclusion of excited states in the quark
propagator is shown to result in a reasonable description of the experimental
values for the helicity amplitudes at the real photon point.Comment: 25 page
Electronic structures of doped anatase : (M=Co, Mn, Fe, Ni)
We have investigated electronic structures of a room temperature diluted
magnetic semiconductor : Co-doped anatase . We have obtained the
half-metallic ground state in the local-spin-density approximation(LSDA) but
the insulating ground state in the LSDA++SO incorporating the spin-orbit
interaction. In the stoichiometric case, the low spin state of Co is realized
with the substantially large orbital moment. However, in the presence of oxygen
vacancies near Co, the spin state of Co becomes intermediate. The
ferromagnetisms in the metallic and insulating phases are accounted for by the
double-exchange-like and the superexchange mechanism, respectively. Further,
the magnetic ground states are obtained for Mn and Fe doped ,
while the paramagnetic ground state for Ni-doped .Comment: 5 pages, 4 figure
A conditional Smg6 mutant mouse model reveals circadian clock regulation through the nonsense-mediated mRNA decay pathway.
Nonsense-mediated messenger RNA (mRNA) decay (NMD) has been intensively studied as a surveillance pathway that degrades erroneous transcripts arising from mutations or RNA processing errors. While additional roles in physiological control of mRNA stability have emerged, possible functions in mammalian physiology in vivo remain unclear. Here, we created a conditional mouse allele that allows converting the NMD effector nuclease SMG6 from wild-type to nuclease domain-mutant protein. We find that NMD down-regulation affects the function of the circadian clock, a system known to require rapid mRNA turnover. Specifically, we uncover strong lengthening of free-running circadian periods for liver and fibroblast clocks and direct NMD regulation of Cry2 mRNA, encoding a key transcriptional repressor within the rhythm-generating feedback loop. Transcriptome-wide changes in daily mRNA accumulation patterns in the entrained liver, as well as an altered response to food entrainment, expand the known scope of NMD regulation in mammalian gene expression and physiology
A novel Smg6 mouse model reveals regulation of circadian period and daily CRY2 accumulation through the nonsense-mediated mRNA decay pathway
Nonsense-mediated mRNA decay (NMD) has been intensively studied as a surveillance pathway that degrades erroneous transcripts arising from mutations or RNA processing errors. While additional roles in controlling regular mRNA stability have emerged, possible functions in mammalian physiology in vivo have remained unclear. Here, we report a novel conditional mouse allele that allows converting the NMD effector nuclease SMG6 from wild-type to nuclease domain-mutant protein. We analyzed how NMD downregulation affects the function of the circadian clock, a system known to require rapid mRNA turnover. We uncover strong lengthening of free-running circadian periods for liver and fibroblast clocks, and direct NMD regulation of Cry2 mRNA, encoding a key transcriptional repressor within the rhythm-generating feedback loop. In the entrained livers of Smg6 mutant animals we reveal transcriptome-wide alterations in daily mRNA accumulation patterns, altogether expanding the known scope of NMD regulation in mammalian gene expression and physiology
Anisotropic transport in the two-dimensional electron gas in the presence of spin-orbit coupling
In a two-dimensional electron gas as realized by a semiconductor quantum
well, the presence of spin-orbit coupling of both the Rashba and Dresselhaus
type leads to anisotropic dispersion relations and Fermi contours. We study the
effect of this anisotropy on the electrical conductivity in the presence of
fixed impurity scatterers. The conductivity also shows in general an anisotropy
which can be tuned by varying the Rashba coefficient. This effect provides a
method of detecting and investigating spin-orbit coupling by measuring
spin-unpolarized electrical currents in the diffusive regime. Our approach is
based on an exact solution of the two-dimensional Boltzmann equation and
provides also a natural framework for investigating other transport effects
including the anomalous Hall effect.Comment: 10 pages, 1 figure included. Discussion of experimental impact
enlarged; error in calculation of conductivity contribution corrected (cf.
Eq. (A14)), no changes in qualitative results and physical consequence
A search for ferromagnetism in transition-metal-doped piezoelectric ZnO
We present the results of a computational study of ZnO in the presence of Co
and Mn substitutional impurities. The goal of our work is to identify potential
ferromagnetic ground states within the (Zn,Co)O or (Zn,Mn)O material systems
that are also good candidates for piezoelectricity. We find that, in contrast
to previous results, robust ferromagnetism is not obtained by substitution of
Co or Mn on the Zn site, unless additional carriers (holes) are also
incorporated. We propose a practical scheme for achieving such -type doping
in ZnO
Self-optimization, community stability, and fluctuations in two individual-based models of biological coevolution
We compare and contrast the long-time dynamical properties of two
individual-based models of biological coevolution. Selection occurs via
multispecies, stochastic population dynamics with reproduction probabilities
that depend nonlinearly on the population densities of all species resident in
the community. New species are introduced through mutation. Both models are
amenable to exact linear stability analysis, and we compare the analytic
results with large-scale kinetic Monte Carlo simulations, obtaining the
population size as a function of an average interspecies interaction strength.
Over time, the models self-optimize through mutation and selection to
approximately maximize a community fitness function, subject only to
constraints internal to the particular model. If the interspecies interactions
are randomly distributed on an interval including positive values, the system
evolves toward self-sustaining, mutualistic communities. In contrast, for the
predator-prey case the matrix of interactions is antisymmetric, and a nonzero
population size must be sustained by an external resource. Time series of the
diversity and population size for both models show approximate 1/f noise and
power-law distributions for the lifetimes of communities and species. For the
mutualistic model, these two lifetime distributions have the same exponent,
while their exponents are different for the predator-prey model. The difference
is probably due to greater resilience toward mass extinctions in the food-web
like communities produced by the predator-prey model.Comment: 26 pages, 12 figures. Discussion of early-time dynamics added. J.
Math. Biol., in pres
Investigation of quasi-periodic varaiations in hard X-rays of solar flares
The aim of the present paper is to use quasi-periodic oscillations in hard
X-rays (HXRs) of solar flares as a diagnostic tool for investigation of
impulsive electron acceleration. We have selected a number of flares which
showed quasi-periodic oscillations in hard X-rays and their loop-top sources
could be easily recognized in HXR images. We have considered MHD standing waves
to explain the observed HXR oscillations. We interpret these HXR oscillations
as being due to oscillations of magnetic traps within cusp-like magnetic
structures. This is confirmed by a good correlation between periods of the
oscillations and the sizes of the loop-top sources. We argue that a model of
oscillating magnetic traps is adequate to explain the observations. During the
compressions of a trap particles are accelerated, but during its expansions
plasma, coming from chromospheric evaporation, fills the trap, which explains
the large number of electrons being accelerated during a sequence of strong
impulses. The advantage of our model of oscillating magnetic traps is that it
can explain both the impulses of electron acceleration and quasi-periodicity of
their distribution in time.Comment: 21 pages, 11 figures, 3 tables, submitted to Solar Physic
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