174 research outputs found
Wegner-Houghton equation in low dimensions
We consider scalar field theories in dimensions lower than four in the
context of the Wegner-Houghton renormalization group equations (WHRG). The
renormalized trajectory makes a non-perturbative interpolation between the
ultraviolet and the infrared scaling regimes. Strong indication is found that
in two dimensions and below the models with polynomial interaction are always
non-perturbative in the infrared scaling regime. Finally we check that these
results do not depend on the regularization and we develop a lattice version of
the WHRG in two dimensions.Comment: 44 pages, 9 figures; some sections revised, refs. added; final
version to appear in Phys. Rev.
The Coulomb-Higgs transition of the three-parameter U(1)-Higgs model
We find a first order Coulomb--Higgs phase transition at moderately large
values of the coupling , and no evidence for a change of order at any
finite value of it.Comment: 3 pages, uuencoded compressed ps file. Contribution to Lattice '9
Antiferromagnetic O(N) models in four dimensions
We study the antiferromagnetic O(N) model in the F_4 lattice. Monte Carlo
simulations are applied for investigating the behavior of the transition for
N=2,3. The numerical results show a first order nature but with a large
correlation length. The limit is also considered with analytical
methods.Comment: 14 pages, 3 postscript figure
A Proposal of a Renormalization Group Transformation for Lattice Field Theories
We propose a new Real Space Renormalization Group transformation useful for
Monte Carlo calculations in theories with global or local symmetries. From
relaxation arguments we define the block-spin transformation with two tunable
free parameters, adapted to the system's action. Varying them it is possible to
place the fixed point very close to the simulation point. We show how the
method works in a simple model with global symmetry: the three dimensional XY
model.Comment: 26 pages, uuencoded compressed postscript single file, 8 figures
include
Evaluacion Economica Sectorial de la Inversion en Infraestructuras del Transporte: Aplicacion al Vector de Inversiones 1990-1998 en España
Para la evaluacion de una inversion en infraestructuras del transporte, existen varios enfoques. El enfoque intersectorial permite una desagregacion del impacto de la inversion. Se suele utilizar complementariamente a los enfoques 'agregados': se emplea como filtro la matriz de coeficientes tecnicos de una tabla input-output. Esta metodologia es inapropiada ante cambios en los coeficientes. Se propone un enfoque que reconcilie las informaciones que inciden en la tabla, lo que permitira un cambio controlado de los coeficientes. El enfoque se basa en un modelo matematico y unas etapas metodologicas. Se evaluo la inversion en infraestructuras del transporte en el periodo 1990-1998. There are several approaches in order to assess an investment in transport infrastructure. Intersectoral approach allows a desaggregation of the investment effect. It is usually employed in combination with other aggregated approaches: it is used as a filter the technical coefficients matrix of an input output table. This methodology is not appropriate if there are changes in the coefficients. We propose an approach that takes into account information that has influence over the table, which allows for a controlled change in the coefficients. This approach is based on a mathematical model and some methodological stages. It was evaluated the investment in transport infrastructure in period 1990 1998.Infraestructuras, Transporte, Input-Output, Ajuste-matematico
Do theoretical physicists care about the protein-folding problem?
The prediction of the biologically active native conformation of a protein is
one of the fundamental challenges of structural biology. This problem remains
yet unsolved mainly due to three factors: the partial knowledge of the
effective free energy function that governs the folding process, the enormous
size of the conformational space of a protein and, finally, the relatively
small differences of energy between conformations, in particular, between the
native one and the ones that make up the unfolded state.
Herein, we recall the importance of taking into account, in a detailed
manner, the many interactions involved in the protein folding problem (such as
steric volume exclusion, Ramachandran forces, hydrogen bonds, weakly polar
interactions, coulombic energy or hydrophobic attraction) and we propose a
strategy to effectively construct a free energy function that, including the
effects of the solvent, could be numerically tractable. It must be pointed out
that, since the internal free energy function that is mainly described does not
include the constraints of the native conformation, it could only help to reach
the 'molten globule' state. We also discuss about the limits and the lacks from
which suffer the simple models that we, physicists, love so much.Comment: 27 pages, 4 figures, LaTeX file, aipproc package. To be published in
the book: "Meeting on Fundamental Physics 'Alberto Galindo'", Alvarez-Estrada
R. F. et al. (Ed.), Madrid: Aula Documental, 200
Improved thermal isolation of silicon suspended platforms for an all-silicon thermoelectric microgenerator based on large scale integration of Si nanowires as thermoelectric material
Special suspended micro-platforms have been designed as a part of silicon compatible planar thermoelectric microgenerators. Bottom-up grown silicon nanowires are going to bridge in the future such platforms to the surrounding silicon bulk rim. They will act as thermoelectric material thus configuring an all-silicon thermoelectric device. In the new platform design other additional bridging elements (usually auxiliary support silicon beams) are substituted by low conductance thin film dielectric membranes in order to maximize the temperature difference developed between both areas. These membranes follow a sieve-like design that allows fabricating them with a short additional wet anisotropic etch step. © Published under licence by IOP Publishing Ltd.Peer ReviewedPostprint (published version
Interstitial lithium doping in SrTiO3
Strontium titanate (SrTiO3) has received much attention due to its wide range of potential applications including in electrochemical devices such as solid oxide fuel cells and capacitors. The stability and safety features of SrTiO3 led to the development of promising electrodes for Li-ion batteries. Here, we use density functional theory simulations to examine the incorporation of lithium from its gas-phase and bulk forms. The results show that a single Li atom is thermodynamically stable in bulk SrTiO3 with respect to its gas-phase and slightly unfavourable compared to its bulk. Multiple Li incorporation up to six is also considered and the incorporation is exoergic with respect to both gas-phase and bulk forms. Charge analysis confirmed the presence of Li+ ions in the lattice. Li incorporation turns the insulating nature of SrTiO3 into metallic and non-magnetic into magnetic. Lithium incorporation facilitates the formation of Sr, Ti and O vacancies. The loss of Li2O is exoergic suggesting that oxygen vacancy mediated-self diffusion will be promoted
The spin glass transition of the three dimensional Heisenberg spin glass
It is shown, by means of Monte Carlo simulation and Finite Size Scaling
analysis, that the Heisenberg spin glass undergoes a finite-temperature phase
transition in three dimensions. There is a single critical temperature, at
which both a spin glass and a chiral glass orderings develop. The Monte Carlo
algorithm, adapted from lattice gauge theory simulations, makes possible to
thermalize lattices of size L=32, larger than in any previous spin glass
simulation in three dimensions. High accuracy is reached thanks to the use of
the Marenostrum supercomputer. The large range of system sizes studied allow us
to consider scaling corrections.Comment: 4 pages, 4 Postscript figures, version to be published in Physical
Review Letter
Spin and chirality orderings of the one-dimensional Heisenberg spin glass with the long-range power-law interaction
The ordering of the one-dimensional Heisenberg spin glass interacting via the
long-range power-law interaction is studied by Monte Carlo simulations.
Particular attention is paid to the possible occurrence of the ``spin-chirality
decoupling'' for appropriate values of the power-law exponent \sigma. Our
result suggests that, for intermediate values of , the chiral-glass
order occurs at finite temperatures while the standard spin-glass order occurs
only at zero temperature.Comment: Proceedings of the Highly Frustrated Magnetism (HFM2006) conference.
To appear in a special issue of J. Phys. Condens. Matte
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