4,713 research outputs found
Polaron action for multimode dispersive phonon systems
Path-integral approach to the tight-binding polaron is extended to multiple
optical phonon modes of arbitrary dispersion and polarization. The non-linear
lattice effects are neglected. Only one electron band is considered. The
electron-phonon interaction is of the density-displacement type, but can be of
arbitrary spatial range and shape. Feynman's analytical integration of ion
trajectories is performed by transforming the electron-ion forces to the basis
in which the phonon dynamical matrix is diagonal. The resulting polaron action
is derived for the periodic and shifted boundary conditions in imaginary time.
The former can be used for calculating polaron thermodynamics while the latter
for the polaron mass and spectrum. The developed formalism is the analytical
basis for numerical analysis of such models by path-integral Monte Carlo
methods.Comment: 9 page
Magnetism and the Weiss Exchange Field - A Theoretical Analysis Inspired by Recent Experiments
The huge spin precession frequency observed in recent experiments with
spin-polarized beams of hot electrons shot through magnetized films is
interpreted as being caused by Zeeman coupling of the electron spins to the
so-called Weiss exchange field in the film. A "Stern-Gerlach experiment" for
electrons moving through an inhomogeneous exchange field is proposed. The
microscopic origin of exchange interactions and of large mean exchange fields,
leading to different types of magnetic order, is elucidated. A microscopic
derivation of the equations of motion of the Weiss exchange field is presented.
Novel proofs of the existence of phase transitions in quantum XY-models and
antiferromagnets, based on an analysis of the statistical distribution of the
exchange field, are outlined.Comment: 36 pages, 3 figure
Long-range magnetic fields in the ground state of the Standard Model plasma
In thermal equilibrium the ground state of the plasma of Standard Model
particles is determined by temperature and exactly conserved combinations of
baryon and lepton numbers. We show that at non-zero values of the global
charges a translation invariant and homogeneous state of the plasma becomes
unstable and the system transits into a new state, containing a large-scale
magnetic field. The origin of this effect is the parity-breaking character of
weak interactions and chiral anomaly. This situation can occur in the early
Universe and may play an important role in its subsequent evolution.Comment: 6 pages. Comments are welcom
Magnetic activity, differential rotation and dynamo action in the pulsating F9IV star KIC 5955122
We present photometric spot modeling of the nearly four-year long light-curve
of the Kepler target KIC 5955122 in terms of persisting dark circular surface
features. With a Bayesian technique, we produced a plausible surface map that
shows dozens of small spots. After some artifacts are removed, the residuals
are at \,mmag. The shortest rotational period found is days. The equator-to-pole extrapolated differential rotation is rad/d. The spots are roughly half as bright as the unperturbed stellar
photosphere. Spot latitudes are restricted to the zone latitude.
There is no indication for any near-pole spots. In addition, the p-mode
pulsations enabled us to determine the evolutionary status of the star, the
extension of the convective zone, and its radius and mass. We discuss the
possibility that the clear signature of active regions in the light curve of
the F9IV star KIC 5955122 is produced by a flux-transport dynamo action at the
base of the convection zone. In particular, we argue that this star has evolved
from an active to a quiet status during the Q0--Q16 period of observation, and
we predict, according to our dynamo model, that the characteristic activity
cycle is of the order of the solar one.Comment: 9 pages, 12 figures, to be published on A&
Age-related impairment of human T lymphocytes' activation: specific differences between CD4+ and CD8+ subsets
The relevance of physiological immune aging is of great interest with respect to determining disorders with pathologic immune function in aging individuals. In recent years, the relevance of changes in peripheral lymphocytes in age-associated neurologic diseases has become more evident. Due to the lack of immunological studies, covering more than one event after mitogenic activation, we envisaged a new concept in the present study, aiming to investigate several events, starting from T cell receptor (TCR) ligation up to T cell proliferation. In addition, we addressed the question whether changes are present in the subsets (CD4, CD8) with aging. Phosphorylation of tyrosine residues declines with increasing age in CD4+ cells. Fewer levels of CD69 positive cells after 4 h mitogenic activation, altered expression of cytokines (IL2, IFN-gamma and TNF-alpha; 22 h) and lower proliferation (72 h) were determined in aging. Moreover, it could be shown that CD8+ lymphocytes react more effectively to mitogenic stimulation with reference to CD69 expression and proliferation in both age groups (60 years old). These data indicate that T cell activation, mediated by TCR engagement, is significantly impaired in aging and both subsets are affected. However, bypassing the TCR does not fully restore T cell function, indicating that there are more mechanisms involved than impaired signal transduction through TCR only. The results will be discussed in relation to their relevance in neurodegenerative and psychiatric disorders
On the flux phase conjecture at half-filling: an improved proof
We present a simplification of Lieb's proof of the flux phase conjecture for
interacting fermion systems -- such as the Hubbard model --, at half filling on
a general class of graphs. The main ingredient is a procedure which transforms
a class of fermionic Hamiltonians into reflection positive form. The method can
also be applied to other problems, which we briefly illustrate with two
examples concerning the model and an extended Falicov-Kimball model.Comment: 23 pages, Latex, uses epsf.sty to include 3 eps figures, to appear in
J. Stat. Phys., Dec. 199
The solar differential rotation in the 18th century
The sunspot drawings of Johann Staudacher of 1749--1799 were used to
determine the solar differential rotation in that period. These drawings of the
full disk lack any indication of their orientation. We used a Bayesian
estimator to obtain the position angles of the drawings, the corresponding
heliographic spot positions, a time offset between the drawings and the
differential rotation parameter \delta\Omega, assuming the equatorial rotation
period is the same as today. The drawings are grouped in pairs, and the
resulting marginal distributions for \delta\Omega were multiplied. We obtain
\delta\Omega=-0.048 \pm 0.025 d^-1 (-2.75^o/d) for the entire period. There is
no significant difference to the value of the present Sun. We find an
(insignificant) indication for a change of \delta\Omega throughout the
observing period from strong differential rotation, \delta\Omega\approx -0.07
d^-1, to weaker differential rotation, \delta\Omega\approx-0.04 d^-1.Comment: 6 pages, 6 figures, accepted for Astronomy and Astrophysic
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Changing European storm loss potentials under modified climate conditions according to ensemble simulations of the ECHAM5/MPI-OM1 GMC
A simple storm loss model is applied to an ensemble of ECHAM5/MPI-OM1 GCM simulations in order to estimate changes of insured loss potentials over Europe in the 21st century. Losses are computed based on the daily maximum wind speed for each grid point. The calibration of the loss model is performed using wind data from the ERA40-Reanalysis and German loss data. The obtained annual losses for the present climate conditions (20C, three realisations) reproduce the statistical features of the historical insurance loss data for Germany.
The climate change experiments correspond to the SRES-Scenarios A1B and A2, and for each of them three realisations are considered. On average, insured loss potentials increase for all analysed European regions at the end of the 21st century. Changes are largest for Germany and France, and lowest for Portugal/Spain. Additionally, the spread between the single realisations is large, ranging e.g. for Germany from −4% to +43% in terms of mean annual loss. Moreover, almost all simulations show an increasing interannual variability of storm damage. This assessment is even more pronounced if no adaptation of building structure to climate change is considered. The increased loss potentials are linked with enhanced values for the high percentiles of surface wind maxima over Western and Central Europe, which in turn are associated with an enhanced number and increased intensity of extreme cyclones over the British Isles and the North Sea
Structure and dielectric properties of polar fluids with extended dipoles: results from numerical simulations
The strengths and short-comings of the point-dipole model for polar fluids of
spherical molecules are illustrated by considering the physically more relevant
case of extended dipoles formed by two opposite charges separated by a
distance (dipole moment ). Extensive Molecular Dynamics
simulations on a high density dipolar fluid are used to analyse the dependence
of the pair structure, dielectric constant \eps and dynamics as a function of
the ratio (\sig is the molecular diameter), for a fixed dipole
moment . The point dipole model is found to agree well with the extended
dipole model up to d/\sig \simeq 0.3. Beyond that ratio, \eps shows a
non-trivial variation with d/\sig. When d/\sig>0.6, a transition is
observed towards a hexagonal columnar phase; the corresponding value of the
dipole moment, \mu^2/\sig^3 k T=3, is found to be substantially lower than
the value of the point dipole required to drive a similar transition.Comment: 10 pages, 11 figures; Paper submitted to Molecular Physic
Spin - or, actually: Spin and Quantum Statistics
The history of the discovery of electron spin and the Pauli principle and the
mathematics of spin and quantum statistics are reviewed. Pauli's theory of the
spinning electron and some of its many applications in mathematics and physics
are considered in more detail. The role of the fact that the tree-level
gyromagnetic factor of the electron has the value g = 2 in an analysis of
stability (and instability) of matter in arbitrary external magnetic fields is
highlighted. Radiative corrections and precision measurements of g are
reviewed. The general connection between spin and statistics, the CPT theorem
and the theory of braid statistics are described.Comment: 50 pages, no figures, seminar on "spin
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