36,055 research outputs found

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

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    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)(h,v). In the (h,v)(h,v) plane, the phase boundary is approached along straight lines δv=kδh\delta v=k \delta h, where (δh,δv)(\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 δfconst(δh)3/2\delta f \sim const (\delta h)^{3/2} for any direction other than the tangential one. In the latter case δf\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

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    Here we have obtained the {\it BVRI CCD} photometry down to a limiting magnitude of VV \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 10×10\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_{o}= 12.16 ±\pm 0.10 mag, which corresponds to a distance of 2700 ±\pm125 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

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    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

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    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

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    I present an extension to arbitrary N of a previously proposed field theoretic model, in which unitary amplitudes for 1>81->8 processes were obtained. The Born amplitude in this extension has the behavior A(1>N)tree = gN1 N!A(1->N)^{tree}\ =\ g^{N-1}\ N! expected in a bosonic field theory. Unitarity is violated when A(1>N)>1|A(1->N)|>1, or when N>Ncrite/g.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 1. \sim 1.\ The weak dependence on NN 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 $K\ \simeq\ \left((0.05/\g2)+ 2\ ln{N}\right)/ \ ln{(1/\g2)}inanexpansioninpowersof in an expansion in powers of \g2.$Comment: 11 pages, 3 figures (not included

    Effective degrees of freedom of the Quark-Gluon Plasma

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    The effective degrees of freedom of the Quark-Gluon Plasma are studied in the temperature range 12\sim 1-2 Tc 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 10\sim 10. The chiral mass and the effective number of bosonic degrees of freedom decrease with increasing temperature and at T2T \sim 2 TcT_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