31,059 research outputs found

    Calculation of the incremental stress-strain relation of a polygonal packing

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    The constitutive relation of the quasi-static deformation on two dimensional packed samples of polygons is calculated using molecular dynamic simulations. The stress values at which the system remains stable are bounded by a failure surface, that shows a power law dependence on the pressure. Below the failure surface, non linear elasticity and plastic deformation are obtained, which are evaluated in the framework of the incremental linear theory. The results shows that the stiffness tensor can be directly related to the micro-contact rearrangements. The plasticity obeys a non-associated flow rule, with a plastic limit surface that does not agree with the failure surface.Comment: 11 pages, 20 figur

    Equilibrium spin-glass transition of magnetic dipoles with random anisotropy axes on a site diluted lattice

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    We study partially occupied lattice systems of classical magnetic dipoles which point along randomly oriented axes. Only dipolar interactions are taken into account. The aim of the model is to mimic collective effects in disordered assemblies of magnetic nanoparticles. From tempered Monte Carlo simulations, we obtain the following equilibrium results. The zero temperature entropy approximately vanishes. Below a temperature T_c, given by k_B T_c= (0.95 +- 0.1)x e_d, where e_d is a nearest neighbor dipole-dipole interaction energy and x is the site occupancy rate, we find a spin glass phase. In it, (1) the mean value , where q is the spin overlap, decreases algebraically with system size N as N increases, and (2) D|q| = 0.5 (T/x)^1/2, independently of N, where D|q| is the root mean square deviation of |q|.Comment: 7 LaTeX pages, 7 eps figures. Submitted to PRB on 30 December 200

    Multi-ion sensing of dipolar noise sources in ion traps

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    Trapped-ion quantum platforms are subject to `anomalous' heating due to interactions with electric-field noise sources of nature not yet completely known. There is ample experimental evidence that this noise originates at the surfaces of the trap electrodes, and models assuming fluctuating point-like dipoles are consistent with observations, but the exact microscopic mechanisms behind anomalous heating remain undetermined. Here we show how a two-ion probe displays a transition in its dissipation properties, enabling experimental access to the mean orientation of the dipoles and the spatial extent of dipole-dipole correlations. This information can be used to test the validity of candidate microscopic models, which predict correlation lengths spanning several orders of mag- nitude. Furthermore, we propose an experiment to measure these effects with currently-available traps and techniques

    Monte Carlo study of the spin-glass phase of the site-diluted dipolar Ising model

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    By tempered Monte Carlo simulations, we study site-diluted Ising systems of magnetic dipoles. All dipoles are randomly placed on a fraction x of all L^3 sites of a simple cubic lattice, and point along a given crystalline axis. For x_c< x<=1, where x_c = 0.65, we find an antiferromagnetic phase below a temperature which vanishes as x tends to x_c from above. At lower values of x, we find an equilibrium spin-glass (SG) phase below a temperature given by k_B T_{sg} = x e_d, where e_d is a nearest neighbor dipole-dipole interaction energy. We study (a) the relative mean square deviation D_q^2 of |q|, where q is the SG overlap parameter, and (b) xi_L/L, where xi_L is a correlation length. From their variation with temperature and system size, we determine T_{sg}. In the SG phase, we find (i) the mean values and decrease algebraically with L as L increases, (ii) double peaked, but wide, distributions of q/ appear to be independent of L, and (iii) xi_L/L rises with L at constant T, but extrapolations to 1/L -> 0 give finite values. All of this is consistent with quasi-long-range order in the SG phase.Comment: 15 LaTeX pages, 15 figures, 3 tables. (typos fixed in Appendix A
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