We study numerically the four-dimensional f J Ising spin-glass at nonzero external magnetic field, We find numerical evidence of the existence of the Almeida-Thouless critical line. The critical exponents differ from those found at zero external magnetic field.

We confirm recent results obtained in a previous work by studying the Ising spin glass at finite magnetic field in four dimensions. Different

We investigate the nature of the spin-glass phase in the four-dimensional king spin glass. We study he probability dishibutions of overlaps, of energy overlaps and ullrmetricity for several sizes. We discover the existence of finite-size scaling in the fails of the first and second of these. This allows us to exmct mame exponents within the spin-glass phase. We also perform studies on quenched thermalization of large samples. Our results together with previous work favour the mean-field picfure of the spin-glass phase.

We compute, in the ( 1 + 1)-dimensional (ha4! model on the lattice, the soliton mass by means of two very different numerical methods. First, we make use of a "creation operator" formalism, measuring the decay of a certain correlation function. Second, we measure the shift of the vacuum energy between the symmetric and the antiperiodic systems. The obtained results are fully compatible. We compute the continuum limit of the mass from the perturbative renormalization group equations. Special attention is paid to ensure that we are working in the scaling region, where physical quantities remain unchanged along any renormalization group trajectory. We compare the continuum value of the soliton mass with its perturbative value up to one-loop calculation. Both quantities show a quite satisfactory agreement. The first is slightly bigger than the perturbative one; this may be due to the contributions of higher-order corrections.PACS nurnbeds): 05.50. f q , 11.1O.Gh

In this paper we calculate the ground state phase diagram of a Josephson Junction ladder when screening field effects are taken into account. We study the ground state configuration as a function of the external field, the penetration depth and the anisotropy of the ladder, using different approximations to the calculation of the induced fields. A series of tongues, characterized by the vortex density ω, is obtained. The vortex density of the ground state, as a function of the external field, is a Devil's staircase, with a plateau for every rational value of ω. The width of each of these steps depends strongly on the approximation made when calculating the inductance effect: if the self-inductance matrix is considered, the ω = 0 phase tends to occupy all the diagram as the penetration depth decreases. If, instead, the whole inductance matrix is considered, the width of any step tends to a non-zero value in the limit of very low penetration depth. We have also analyzed the stability of some simple metastable phases: screening fields are shown to enlarge their stability range.