2,849 research outputs found

    Formation of laser plasma channels in a stationary gas

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    The formation of plasma channels with nonuniformity of about +- 3.5% has been demonstrated. The channels had a density of 1.2x10^19 cm-3 with a radius of 15 um and with length >= 2.5 mm. The channels were formed by 0.3 J, 100 ps laser pulses in a nonflowing gas, contained in a cylindrical chamber. The laser beam passed through the chamber along its axis via pinholes in the chamber walls. A plasma channel with an electron density on the order of 10^18 - 10^19 cm-3 was formed in pure He, N2, Ar, and Xe. A uniform channel forms at proper time delays and in optimal pressure ranges, which depend on the sort of gas. The influence of the interaction of the laser beam with the gas leaking out of the chamber through the pinholes was found insignificant. However, the formation of an ablative plasma on the walls of the pinholes by the wings of the radial profile of the laser beam plays an important role in the plasma channel formation and its uniformity. A low current glow discharge initiated in the chamber slightly improves the uniformity of the plasma channel, while a high current arc discharge leads to the formation of overdense plasma near the front pinhole and further refraction of the laser beam. The obtained results show the feasibility of creating uniform plasma channels in non-flowing gas targets.Comment: 15 pages, 7 figures, submitted to Physics of Plasma

    Random Field and Random Anisotropy Effects in Defect-Free Three-Dimensional XY Models

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    Monte Carlo simulations have been used to study a vortex-free XY ferromagnet with a random field or a random anisotropy on simple cubic lattices. In the random field case, which can be related to a charge-density wave pinned by random point defects, it is found that long-range order is destroyed even for weak randomness. In the random anisotropy case, which can be related to a randomly pinned spin-density wave, the long-range order is not destroyed and the correlation length is finite. In both cases there are many local minima of the free energy separated by high entropy barriers. Our results for the random field case are consistent with the existence of a Bragg glass phase of the type discussed by Emig, Bogner and Nattermann.Comment: 10 pages, including 2 figures, extensively revise

    Ground-State and Domain-Wall Energies in the Spin-Glass Region of the 2D ±J\pm J Random-Bond Ising Model

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    The statistics of the ground-state and domain-wall energies for the two-dimensional random-bond Ising model on square lattices with independent, identically distributed bonds of probability pp of Jij=1J_{ij}= -1 and (1p)(1-p) of Jij=+1J_{ij}= +1 are studied. We are able to consider large samples of up to 3202320^2 spins by using sophisticated matching algorithms. We study L×LL \times L systems, but we also consider L×ML \times M samples, for different aspect ratios R=L/MR = L / M. We find that the scaling behavior of the ground-state energy and its sample-to-sample fluctuations inside the spin-glass region (pcp1pcp_c \le p \le 1 - p_c) are characterized by simple scaling functions. In particular, the fluctuations exhibit a cusp-like singularity at pcp_c. Inside the spin-glass region the average domain-wall energy converges to a finite nonzero value as the sample size becomes infinite, holding RR fixed. Here, large finite-size effects are visible, which can be explained for all pp by a single exponent ω2/3\omega\approx 2/3, provided higher-order corrections to scaling are included. Finally, we confirm the validity of aspect-ratio scaling for R0R \to 0: the distribution of the domain-wall energies converges to a Gaussian for R0R \to 0, although the domain walls of neighboring subsystems of size L×LL \times L are not independent.Comment: 11 pages with 15 figures, extensively revise

    Finite-Size Scaling Critical Behavior of Randomly Pinned Spin-Density Waves

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    We have performed Monte Carlo studies of the 3D XYXY model with random uniaxial anisotropy, which is a model for randomly pinned spin-density waves. We study L×L×LL \times L \times L simple cubic lattices, using LL values in the range 16 to 64, and with random anisotropy strengths of D/2JD / 2 J = 1, 2, 3, 6 and \infty. There is a well-defined finite temperature critical point, TcT_c, for each these values of D/2JD / 2 J. We present results for the angle-averaged magnetic structure factor, S(k)S (k) at TcT_c for L=64L = 64. We also use finite-size scaling analysis to study scaling functions for the critical behavior of the specific heat, the magnetization and the longitudinal magnetic susceptibility. Good data collapse of the scaling functions over a wide range of TT is seen for D/2JD / 2 J = 6 and \infty. For our finite values of D/2JD / 2 J the scaled magnetization function increases with LL below TcT_c, and appears to approach an LL-independent limit for large LL. This suggests that the system is ferromagnetic below TcT_c.Comment: 21 pages in single column format, 20 .eps files, revised and expanded, errors corrected, submitted to PR

    Background gauge invariance in the antifield formalism for theories with open gauge algebras

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    We show that any BRST invariant quantum action with open or closed gauge algebra has a corresponding local background gauge invariance. If the BRST symmetry is anomalous, but the anomaly can be removed in the antifield formalism, then the effective action possesses a local background gauge invariance. The presence of antifields (BRST sources) is necessary. As an example we analyze chiral W3W_3 gravity.Comment: 17pp., Latex, mispelling in my name! corrected, no other change

    Long Range Order in Random Anisotropy Magnets

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    High temperature series for the magnetic susceptibility, χ, of random anisotropy axis models in the limit of infinite anisotropy are presented, for two choices of the number of spin components, m. For m=2, we find T c =1.78 J on the simple cubic lattice, and on the face‐centered cubic lattice we find T c =4.29 J. There is no divergence of χ at finite temperature for m=3 on either lattice. For the four‐dimensional hypercubic lattice, we find finite temperature divergences of χ for both m=2 and m=3

    Series Study of a Spin-Glass Model in Continuous Dimensionality

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    A high-temperature series expansion for the Edwards and Anderson spin-glass order-parameter susceptibility is computed for Ising spins on hypercubic lattices with nearest-neighbor interactions. The series is analyzed by Padé approximants with Rudnick-Nelson-type corrections to scaling. The results agree with the first-order ε expansion of Harris, Lubensky, and Chen. The critical exponent γQ increases monotonically with decreasing dimension, d, for d\u3c6, and apparently tends to infinity at d=4; however, the critical temperature does not appear to go to zero at d=4

    Randomly Dilute Two Dimensional Ising Models

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    Calculations of the specific heat and magnetization of quenched, site‐diluted, N×N square and triangular Ising lattices have been carried out by a Monte Carlo method. For spin concentrations x of 0.8 and 0.9, lattices of size N=64 did not give sharp transitions. For a triangular lattice with N=128 and x=0.904, we found a well‐defined peak in the specific heat and an abrupt change in the magnetization at T=0.865 Tc(1). Linear interpolation gives s≡d/dx[Tc(x)/Tc(1)]x=1=1.40±0.05, in excellent agreement with the high temperature series calculations of Rushbrooke et al. For the square lattice we calculate s=1.5±0.1. We also determined site magnetization as a function of the number of ’’live’’ nearest neighbors
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