28,087 research outputs found
The Effect of Spatial Curvature on the Classical and Quantum Strings
We study the effects of the spatial curvature on the classical and quantum
string dynamics. We find the general solution of the circular string motion in
static Robertson-Walker spacetimes with closed or open sections. This is given
closely and completely in terms of elliptic functions. The physical properties,
string length, energy and pressure are computed and analyzed. We find the {\it
back-reaction} effect of these strings on the spacetime: the self-consistent
solution to the Einstein equations is a spatially closed spacetime with
a selected value of the curvature index (the scale f* is normalized to
unity). No self-consistent solutions with exist. We semi-classically
quantize the circular strings and find the mass in each case. For
the very massive strings, oscillating on the full hypersphere, have {\it independent} of and the level spacing {\it
grows} with while the strings oscillating on one hemisphere (without
crossing the equator) have and a {\it finite} number of
states For there are infinitely many string states
with masses that is, the level spacing grows {\it slower} than
The stationary string solutions as well as the generic string fluctuations
around the center of mass are also found and analyzed in closed form.Comment: 30 pages Latex + three tables and five figures (not included
Local moments and symmetry breaking in metallic PrMnSbO
We report a combined experimental and theoretical investigation of the
layered antimonide PrMnSbO which is isostructural to the parent phase of the
iron pnictide superconductors. We find linear resistivity near room temperature
and Fermi liquid-like T^{2} behaviour below 150 K. Neutron powder diffraction
shows that unfrustrated C-type Mn magnetic order develops below \sim 230 K,
followed by a spin-flop coupled to induced Pr order. At T \sim 35 K, we find a
tetragonal to orthorhombic (T-O) transition. First principles calculations show
that the large magnetic moments observed in this metallic compound are of local
origin. Our results are thus inconsistent with either the itinerant or
frustrated models proposed for symmetry breaking in the iron pnictides. We show
that PrMnSbO is instead a rare example of a metal where structural distortions
are driven by f-electron degrees of freedom
Faint star counts in the near-infrared
We discuss near-infrared star counts at the Galactic pole with a view to
guiding the NGST and ground-based NIR cameras. Star counts from deep K-band
images from the CFHT are presented, and compared with results from the 2MASS
survey and some Galaxy models. With appropriate corrections for detector
artifacts and galaxies, the data agree with the models down to K~18, but
indicate a larger population of fainter red stars. There is also a significant
population of compact galaxies that extend to the observational faint limit of
K=20.5. Recent Galaxy models agree well down to K19, but diverge at
fainter magnitudes.Comment: 14 pages and 4 diagrams; to appear in PAS
GRB afterglow light curves from uniform and non-uniform jets
Here we calculate the GRB afterglow light curves from a relativistic jet as
seen by observers at a wide range of viewing angles from the jet axis, and the
jet is uniform or non-uniform. We find that, for uniform jet the afterglow
light curves for different viewing angles are somewhat different: in general,
there are two breaks in the light curve, corresponding to the time and
respectively. However, for non-uniform jet, the things become more complicated.
For the case , we can obtain the analytical results, for
there should be two breaks in the light curve correspond to
and respectively, while for
there should be only one break corresponds to
, and this provides a possible explanation for some
rapidly fading afterglows whose light curves have no breaks since the time at
which is much earlier than our first observation
time. For the case , our numerical results show that, the
afterglow light curves are strongly affected by the values of ,
and . If the values of and are larger,
there will be a prominent flattening in the afterglow light curve, which is
quite different from the uniform jet, and after the flattening a very sharp
break will be occurred at the time $\gamma\sim (\theta_v + \theta_c)^{-1}.Comment: Latex, 5 pages, accepted for publication by A&
Time dependent Ginzburg-Landau equation for an N-component model of self-assembled fluids
We study the time evolution of an N-component model of bicontinuous
microemulsions based on a time dependent Ginzburg-Landau equation quenched from
an high temperature uncorrelated state to the low temperature phases. The
behavior of the dynamical structure factor is obtained, in each
phase, in the framework of the large- limit with both conserved (COP) and
non conserved (NCOP) order parameter dynamics. At zero temperature the system
shows multiscaling in the unstructured region up to the tricritical point for
the COP whereas ordinary scaling is obeyed for NCOP. In the structured phase,
instead, the conservation law is found to be irrelevant and the form , with and , is
obtained in every case. Simple scaling relations are also derived for the
structure factor as a function of the final temperature of the thermal bath.Comment: 9 pages,Apste
Exact Nonperturbative Unitary Amplitudes for 1->N Transitions
I present an extension to arbitrary N of a previously proposed field
theoretic model, in which unitary amplitudes for processes were
obtained. The Born amplitude in this extension has the behavior
expected in a bosonic field theory. Unitarity
is violated when , or when Numerical
solutions of the coupled Schr\"odinger equations shows that for weak coupling
and a large range of N>\ncrit, the exact unitary amplitude is reasonably fit
by a factorized expression |A(1->N)| \sim (0.73 /N) \cdot \exp{(-0.025/\g2)}.
The very small size of the coefficient 1/\g2 , indicative of a very weak
exponential suppression, is not in accord with standard discussions based on
saddle point analysis, which give a coefficient The weak dependence
on could have experimental implications in theories where the exponential
suppression is weak (as in this model). Non-perturbative contributions to
few-point correlation functions in this theory would arise at order $K\ \simeq\
\left((0.05/\g2)+ 2\ ln{N}\right)/ \ ln{(1/\g2)}\g2.$Comment: 11 pages, 3 figures (not included
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