36,055 research outputs found

### Direction dependent free energy singularity of the asymmetric six-vertex model

The transition from the ordered commensurate phase to the incommensurate
gaussian phase of the antiferroelectric asymmetric six-vertex model is
investigated by keeping the temperature constant below the roughening point and
varying the external fields $(h,v)$. In the $(h,v)$ plane, the phase boundary
is approached along straight lines $\delta v=k \delta h$, where $(\delta
h,\delta v)$ measures the displacement from the phase boundary. It is found
that the free energy singularity displays the exponent 3/2 typical of the
Pokrovski-Talapov transition $\delta f \sim const (\delta h)^{3/2}$ for any
direction other than the tangential one. In the latter case $\delta f$ shows a
discontinuity in the third derivative.Comment: 18 pages, Latex, 1 figure, minor corrections and two references
change

### CCD BVRI and 2MASS Photometry of the Poorly Studied Open Cluster NGC 6631

Here we have obtained the {\it BVRI CCD} photometry down to a limiting
magnitude of $V \sim$ 20 for the southern poorly studied open cluster NGC 6631.
It is observed from the {\it 1.88 m} Telescope of Kottamia Observatory in
Egypt. About 3300 stars have been observed in an area of $\sim 10^{\prime}
\times 10^{\prime}$ around the cluster center. The main photometric parameters
have been estimated and compared with the results that determined for the
cluster using {\it JHKs 2MASS} photometric database. The cluster's diameter is
estimated to be 10 arcmin; the reddening E(B-V)= 0.68 $\pm$ 0.10 mag, E(J-H)=
0.21 $\pm$ 0.10 mag, the true modulus (m-M)$_{o}$= 12.16 $\pm$ 0.10 mag, which
corresponds to a distance of 2700 $\pm$125 pc and age of 500 $\pm$ 50 Myr.Comment: 13 pages, 6 figure

### Quasi-long-range order in the random anisotropy Heisenberg model: functional renormalization group in 4-\epsilon dimensions

The large distance behaviors of the random field and random anisotropy O(N)
models are studied with the functional renormalization group in 4-\epsilon
dimensions. The random anisotropy Heisenberg (N=3) model is found to have a
phase with the infinite correlation radius at low temperatures and weak
disorder. The correlation function of the magnetization obeys a power law <
m(x) m(y) >\sim |x-y|^{-0.62\epsilon}. The magnetic susceptibility diverges at
low fields as \chi \sim H^{-1+0.15\epsilon}. In the random field O(N) model the
correlation radius is found to be finite at the arbitrarily weak disorder for
any N>3. The random field case is studied with a new simple method, based on a
rigorous inequality. This approach allows one to avoid the integration of the
functional renormalization group equations.Comment: 12 pages, RevTeX; a minor change in the list of reference

### WSRT Faraday tomography of the Galactic ISM at \lambda \sim 0.86 m

We investigate the distribution and properties of Faraday rotating and
synchrotron emitting regions in the Galactic ISM in the direction of the
Galactic anti-centre. We apply Faraday tomography to a radio polarization
dataset that we obtained with the WSRT. We developed a new method to calculate
a linear fit to periodic data, which we use to determine rotation measures from
our polarization angle data. From simulations of a Faraday screen + noise we
could determine how compatible the data are with Faraday screens. An
unexpectedly large fraction of 14% of the lines-of-sight in our dataset show an
unresolved main component in the Faraday depth spectrum. For lines-of-sight
with a single unresolved component we demonstrate that a Faraday screen in
front of a synchrotron emitting region that contains a turbulent magnetic field
component can explain the data.Comment: 5 pages, 5 figures. Accepted for publication as a Letter to the
Editor in A&

### 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 $1->8$ processes were
obtained. The Born amplitude in this extension has the behavior
$A(1->N)^{tree}\ =\ g^{N-1}\ N!$ expected in a bosonic field theory. Unitarity
is violated when $|A(1->N)|>1$, or when $N>\N_crit\simeq e/g.$ 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 $\sim 1.\$ The weak dependence
on $N$ 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 $K\ \simeq\
\left((0.05/\g2)+ 2\ ln{N}\right)/ \ ln{(1/\g2)}$in an expansion in powers of$\g2.$Comment: 11 pages, 3 figures (not included

### Effective degrees of freedom of the Quark-Gluon Plasma

The effective degrees of freedom of the Quark-Gluon Plasma are studied in the
temperature range $\sim 1-2$ $T_c$. Employing lattice results for the pressure
and the energy density, we constrain the quasiparticle chiral invariant mass to
be of order 200 MeV and the effective number of bosonic resonant states to be
at most of order $\sim 10$. The chiral mass and the effective number of bosonic
degrees of freedom decrease with increasing temperature and at $T \sim 2$ $T_c$
only quark and gluon quasiparticles survive. Some remarks regarding the role of
the gluon condensation and the baryon number-strangeness correlation are also
presented.Comment: 4 pages, 1 figur

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