No abstract

We improve the theoretical estimates of the critical exponents for the threedimensional Heisenberg universality class. We find γ = 1.3960(9), ν = 0.7112(5), η = 0.0375(5), α = −0.1336(15), β = 0.3689(3), and δ = 4.783(3). We consider an improved lattice φ 4 Hamiltonian with suppressed leading scaling corrections. Our results are obtained by combining Monte Carlo simulations based on finite-size scaling methods and high-temperature expansions. The critical exponents are computed from high-temperature expansions specialized to the φ 4 improved model. By the same technique we determine the coefficients of the small-magnetization expansion of the equation of state. This expansion is extended analytically by means of approximate parametric representations, obtaining the equation of state in the whole critical region. We also determine a number of universal amplitude ratios.

We improve the theoretical estimates of the critical exponents for the threedimensional XY universality class. We find α = −0.0146(8), γ = 1.3177(5), ν = 0.67155(27), η = 0.0380(4), β = 0.3485(2), and δ = 4.780(2). We observe a discrepancy with the most recent experimental estimate of α; this discrepancy calls for further theoretical and experimental investigations. Our results are obtained by combining Monte Carlo simulations based on finite-size scaling methods, and high-temperature expansions. Two improved models (with suppressed leading scaling corrections) are selected by Monte Carlo computation. The critical exponents are computed from high-temperature expansions specialized to these improved models. By the same technique we determine the coefficients of the small-magnetization expansion of the equation of state. This expansion is extended analytically by means of approximate parametric representations, obtaining the equation of state in the whole critical region. We also determine the specific-heat amplitude ratio.

We compute a number of universal amplitude ratios in the three-dimensional Ising universality class. To this end, we perform Monte Carlo simulations of the improved Blume-Capel model on the simple cubic lattice. For example, we obtain A + /A − = 0.536(2) and C + /C − = 4.713 (7), where A ± and C ± are the amplitudes of the specific heat and the magnetic susceptibility, respectively. The subscripts + and − indicate the high and the low temperature phase, respectively. We compare our results with those obtained from previous Monte Carlo simulations, high and low temperature series expansions, field theoretic methods and experiments.