1,488 research outputs found

    Metastable States in High Order Short-Range Spin Glasses

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    The mean number of metastable states in higher order short-range spin glasses is estimated analytically using a variational method introduced by Tanaka and Edwards for very large coordination numbers. For lattices with small connectivities, numerical simulations do not show any significant dependence on the relative positions of the interacting spins on the lattice, indicating thus that these systems can be described by a few macroscopic parameters. As an extremely anisotropic model we consider the low autocorrelated binary spin model and we show through numerical simulations that its landscape has an exceptionally large number of local optima

    Fractal geometry of spin-glass models

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    Stability and diversity are two key properties that living entities share with spin glasses, where they are manifested through the breaking of the phase space into many valleys or local minima connected by saddle points. The topology of the phase space can be conveniently condensed into a tree structure, akin to the biological phylogenetic trees, whose tips are the local minima and internal nodes are the lowest-energy saddles connecting those minima. For the infinite-range Ising spin glass with p-spin interactions, we show that the average size-frequency distribution of saddles obeys a power law ∌w−D \sim w^{-D}, where w=w(s) is the number of minima that can be connected through saddle s, and D is the fractal dimension of the phase space

    Model calculations of doubly closed shell nuclei in CBF theory III. j-j coupling and isospin dependence

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    Correlated Basis Function theory and Fermi Hypernetted Chain technique are extended to study medium-heavy, doubly closed shell nuclei in j-j coupling scheme, with different single particle wave functions for protons and neutrons and isospin dependent two-body correlations. Central semirealistic interactions are used. Ground state energies, one-body densities, distribution functions and momentum distributions are calculated for 12C, 16O, 40Ca, 48Ca and 208Pb nuclei. The values of the ground state energies provided by isospin dependent correlations are lower than those obtained with isospin independent correlations. In finite nuclear systems, the two--body Euler equations provide correlation functions variationally more effective than those obtained with the same technique in infinite nuclear matter.Comment: 29 Latex pages plus 6 Postscript figure

    Opportunities for public private partnerships in IRPTN infrastructure

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    The Integrated Rapid Public Transport Network interventions in 13 cities in South Africa came about as a result of a forward thinking public transport strategy being presented by Government in 2007. Since then, costs of both implementation and operation have been shown to be higher than expected. The future budgeted capital funding required to roll out the proposed public transport investments are in excess of the level of grant funding likely to be available from the national fiscus. Third party funding options need to be actively investigated and developed to augment the capital investment allocations provided through the national fiscus. One such third party funding option is Public Private Partnerships in the development and operation of selected public transport infrastructure. This paper aims to reduce the mystique attached to Private Public Partnership (PPP) initiatives and seeks to link the use of PPP principles to the funding difficulties in IRPTNs. The paper concludes that PPP?s may well be a viable source of funding for selected elements or components of the IRPTNs. This will however require interventions at a National level in order to reduce the administrative burden on each of the 13 involved cities.Paper presented at the 34th Annual Southern African Transport Conference 6-9 July 2015 "Working Together to Deliver - Sakha Sonke", CSIR International Convention Centre, Pretoria, South Africa.The Minister of Transport, South AfricaTransportation Research Board of the US

    Landscape statistics of the p-spin Ising model

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    The statistical properties of the local optima (metastable states) of the infinite range Ising spin glass with p-spin interactions in the presence of an external magnetic field h are investigated analytically. The average number of optima as well as the typical overlap between pairs of identical optima are calculated for general p. Similarly to the thermodynamic order parameter, for p>2 and small h the typical overlap q_t is a discontinuous function of the energy. The size of the jump in q_t increases with p and decreases with h, vanishing at finite values of the magnetic field.Comment: 12 pages,te

    Templated growth of metal-organic coordination chains at surfaces

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    Line them up: Metal\u2013organic chains (see scanning tunneling microscopy image) have been created in situ by self\u2010organized growth at a metal surface under ultrahigh vacuum. These 1D arrangements of metal centers (Fe, Cu), regularly spaced by organic linkers such as trimesitylic acid, open new possibilities for the study of low\u2010dimensional magnetism

    Landscape statistics of the low autocorrelated binary string problem

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    The statistical properties of the energy landscape of the low autocorrelated binary string problem (LABSP) are studied numerically and compared with those of several classic disordered models. Using two global measures of landscape structure which have been introduced in the Simulated Annealing literature, namely, depth and difficulty, we find that the landscape of LABSP, except perhaps for a very large degeneracy of the local minima energies, is qualitatively similar to some well-known landscapes such as that of the mean-field 2-spin glass model. Furthermore, we consider a mean-field approximation to the pure model proposed by Bouchaud and Mezard (1994, J. Physique I France 4 1109) and show both analytically and numerically that it describes extremely well the statistical properties of LABSP

    Conduction of Ultracold Fermions Through a Mesoscopic Channel

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    In a mesoscopic conductor electric resistance is detected even if the device is defect-free. We engineer and study a cold-atom analog of a mesoscopic conductor. It consists of a narrow channel connecting two macroscopic reservoirs of fermions that can be switched from ballistic to diffusive. We induce a current through the channel and find ohmic conduction, even for a ballistic channel. An analysis of in-situ density distributions shows that in the ballistic case the chemical potential drop occurs at the entrance and exit of the channel, revealing the presence of contact resistance. In contrast, a diffusive channel with disorder displays a chemical potential drop spread over the whole channel. Our approach opens the way towards quantum simulation of mesoscopic devices with quantum gases

    The Optical System for the Large Size Telescope of the Cherenkov Telescope Array

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    The Large Size Telescope (LST) of the Cherenkov Telescope Array (CTA) is designed to achieve a threshold energy of 20 GeV. The LST optics is composed of one parabolic primary mirror 23 m in diameter and 28 m focal length. The reflector dish is segmented in 198 hexagonal, 1.51 m flat to flat mirrors. The total effective reflective area, taking into account the shadow of the mechanical structure, is about 368 m2^2. The mirrors have a sandwich structure consisting of a glass sheet of 2.7 mm thickness, aluminum honeycomb of 60 mm thickness, and another glass sheet on the rear, and have a total weight about 47 kg. The mirror surface is produced using a sputtering deposition technique to apply a 5-layer coating, and the mirrors reach a reflectivity of ∌\sim94% at peak. The mirror facets are actively aligned during operations by an active mirror control system, using actuators, CMOS cameras and a reference laser. Each mirror facet carries a CMOS camera, which measures the position of the light spot of the optical axis reference laser on the target of the telescope camera. The two actuators and the universal joint of each mirror facet are respectively fixed to three neighboring joints of the dish space frame, via specially designed interface plate.Comment: In Proceedings of the 34th International Cosmic Ray Conference (ICRC2015), The Hague, The Netherlands. All CTA contributions at arXiv:1508.0589
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