27,641 research outputs found

    Critical behavior of the Random-Field Ising model at and beyond the Upper Critical Dimension

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    The disorder-driven phase transition of the RFIM is observed using exact ground-state computer simulations for hyper cubic lattices in d=5,6,7 dimensions. Finite-size scaling analyses are used to calculate the critical point and the critical exponents of the specific heat, magnetization, susceptibility and of the correlation length. For dimensions d=6,7 which are larger or equal to the assumed upper critical dimension, d_u=6, mean-field behaviour is found, i.e. alpha=0, beta=1/2, gamma=1, nu=1/2. For the analysis of the numerical data, it appears to be necessary to include recently proposed corrections to scaling at and beyond the upper critical dimension.Comment: 8 pages and 13 figures; A consise summary of this work can be found in the papercore database at http://www.papercore.org/Ahrens201

    Application of the discrete Wentzel-Kramers-Brillouin method to spin tunneling

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    A discrete version of the WKB method is developed and applied to calculate the tunnel splittings between classically degenerate states of spin Hamiltonians. The results for particular model problems are in complete accord with those previously found using instanton methods. The discrete WKB method is more elementary and also yields wavefunctions.Comment: RevTex, 2ps figures, incorporates errata (typos and omitted reference) in JMP pape

    The Evolution of Intermediary Institutions in Europe: From Corporatism to Governance

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    This book investigates the consecutive shifts between three types of intermediary institutions in the European context: Corporatist, Neo-corporatist and Governance institutions. It develops a new conceptual framework for understanding the function and position of intermediary institutions in society, as well as a vocabulary capable of explaining the causes and consequences of these shifts for politics, economy and society at large. The book is designed to fill a gap in three rather distinct, yet also overlapping bodies of literature: European Political Economy, European Integration and governance studies, and socio-legal studies in the European context. Reviews: - Anne Guisset: Transfer: European Review of Labour and Research, 22, 3, 427-429, 2016. - Ian Bruff, Capital & Class, 40, 3, 555 – 57, 2016

    Direct sampling of complex landscapes at low temperatures: the three-dimensional +/-J Ising spin glass

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    A method is presented, which allows to sample directly low-temperature configurations of glassy systems, like spin glasses. The basic idea is to generate ground states and low lying excited configurations using a heuristic algorithm. Then, with the help of microcanonical Monte Carlo simulations, more configurations are found, clusters of configurations are determined and entropies evaluated. Finally equilibrium configuration are randomly sampled with proper Gibbs-Boltzmann weights. The method is applied to three-dimensional Ising spin glasses with +- J interactions and temperatures T<=0.5. The low-temperature behavior of this model is characterized by evaluating different overlap quantities, exhibiting a complex low-energy landscape for T>0, while the T=0 behavior appears to be less complex.Comment: 9 pages, 7 figures, revtex (one sentence changed compared to v2

    Scaling up: A path to effective development

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    Poverty reduction, Hunger, MDGs, Interventions, Political dynamics, Capacity, Development assistance, Scaling up,

    Few-Particle Effects in Semiconductor Quantum Dots: Observation of Multi-Charged-Excitons

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    We investigate experimentally and theoretically few-particle effects in the optical spectra of single quantum dots (QDs). Photo-depletion of the QD together with the slow hopping transport of impurity-bound electrons back to the QD are employed to efficiently control the number of electrons present in the QD. By investigating structurally identical QDs, we show that the spectral evolutions observed can be attributed to intrinsic, multi-particle-related effects, as opposed to extrinsic QD-impurity environment-related interactions. From our theoretical calculations we identify the distinct transitions related to excitons and excitons charged with up to five additional electrons, as well as neutral and charged biexcitons.Comment: 4 pages, 4 figures, revtex. Accepted for publication in Physical Review Letter

    Molecular Dynamics in grafted layers of poly(dimethylsiloxane) (PDMS)

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    Dielectric relaxation spectroscopy 10^-1 Hz to 10^6 Hz) is employed to study the molecular dynamics of poly(dimethylsiloxane) (PDMS, Mw=1.7 10^5 g/mol and Mw=9.6 10^4 g/mol as grafted films with thicknesses d below and above the radius of gyration Rg. For d smaller than Rg the molecular dynamics becomes faster by up to three orders of magnitude with respect to the bulk resulting in a pronounced decrease of the Vogel temperature T0 and hence the calorimetric glass transition temperature Tg. For d larger than Rg the molecular dynamics is comparable to that of the bulk melt. The results are interpreted in terms of a chain confinement effect and compared with the findings for low molecular eight glass forming liquids contained in nanoporous glasses and zeolites. Crystallization effects - well known for PDMS - are observed for films of thicknesses above and below Rg.Comment: 20 pages, 4 figure

    The ground state energy of the Edwards-Anderson spin glass model with a parallel tempering Monte Carlo algorithm

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    We study the efficiency of parallel tempering Monte Carlo technique for calculating true ground states of the Edwards-Anderson spin glass model. Bimodal and Gaussian bond distributions were considered in two and three-dimensional lattices. By a systematic analysis we find a simple formula to estimate the values of the parameters needed in the algorithm to find the GS with a fixed average probability. We also study the performance of the algorithm for single samples, quantifying the difference between samples where the GS is hard, or easy, to find. The GS energies we obtain are in good agreement with the values found in the literature. Our results show that the performance of the parallel tempering technique is comparable to more powerful heuristics developed to find the ground state of Ising spin glass systems.Comment: 30 pages, 17 figures. A new section added. Accepted for publication in Physica

    No spin-glass transition in the "mobile-bond" model

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    The recently introduced ``mobile-bond'' model for two-dimensional spin glasses is studied. The model is characterized by an annealing temperature T_q. On the basis of Monte Carlo simulations of small systems it has been claimed that this model exhibits a non-trivial spin-glass transition at finite temperature for small values of T_q. Here the model is studied by means of exact ground-state calculations of large systems up to N=256^2. The scaling of domain-wall energies is investigated as a function of the system size. For small values T_q<0.95 the system behaves like a (gauge-transformed) ferromagnet having a small fraction of frustrated plaquettes. For T_q>=0.95 the system behaves like the standard two-dimensional +-J spin-glass, i.e. it does NOT exhibit a phase transition at T>0.Comment: 4 pages, 5 figures, RevTe
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