136 research outputs found
Antiferromagnetism in four dimensions: search for non-triviality
We present antiferromagnetism as a mechanism capable of modifying
substantially the phase diagram and the critical behaviour of statistical
mechanical models. This is particularly relevant in four dimensions, due to the
connection between second order transition points and the continuum limit as a
quantum field theory. We study three models with an antiferromagnetic
interaction: the Ising and the O(4) Models with a second neighbour negative
coupling, and the \RP{2} Model. Different conclusions are obtained depending
on the model.Comment: 4 pages LateX. Contribution to Lat9
Monte Carlo studies of antiferromagnetic spin models in three dimensions
We study several antiferromagnetic formulations of the O(3) spin model in
three dimensions by means of Monte Carlo simulations. We discuss about the
vacua properties and analyze the phase transitions. Using Finite Size Scaling
analysis we conclude that all phase transitions found are of first orderComment: 4 pages, 2 Postscript figures. Contribution to Lattice '9
Laser Shock Microforming of Thin Metal Sheets
Continuous and long-pulse lasers have been used for the forming of metal sheets in macroscopic mechanical applications. However, for the manufacturing of micro-electromechanical systems (MEMS), the applicability of such type of lasers is limited by the long-relaxation-time of the thermal fields responsible for the forming phenomena. As a consequence of such slow relaxation, the final sheet deformation state is attained only after a certain time, what makes the generated internal residual stress fields more dependent on ambient conditions and might make difficult the subsequent assembly process from the point of view of residual stresses due to adjustment. The use of ns laser pulses provides a suitable parameter matching for the laser forming of an important range of sheet components used in MEMS that, preserving the short interaction time scale required for the predominantly mechanic (shock) induction of deformation residual stresses, allows for the successful processing of components in a medium range of miniaturization, particularly important according to its frequent use in such systems. In the present paper, a discussion is presented on the physics of laser shock microforming and the influence of the different effects on the net bending angle. The experimental setup used for the experiments, sample fabrication and experimental results of influence of number of laser pulses on the net bending angle are also presented
Spin Glass Ordering in Diluted Magnetic Semiconductors: a Monte Carlo Study
We study the temperature-dilution phase diagram of a site-diluted Heisenberg
antiferromagnet on a fcc lattice, with and without the Dzyaloshinskii-Moriya
anisotropic term, fixed to realistic microscopic parameters for (IIB=Cd, Hg, Zn). We show that the dipolar Dzyaloshinskii-Moriya anisotropy
induces a finite-temperature phase transition to a spin glass phase, at
dilutions larger than 80%. The resulting probability distribution of the order
parameter P(q) is similar to the one found in the cubic lattice
Edwards-Anderson Ising model. The critical exponents undergo large finite size
corrections, but tend to values similar to the ones of the
Edwards-Anderson-Ising model.Comment: 4 pages plus 3 postscript figure
The antiferromagnetic phi4 Model, II. The one-loop renormalization
It is shown that the four dimensional antiferromagnetic lattice phi4 model
has the usual non-asymptotically free scaling law in the UV regime around the
chiral symmetrical critical point. The theory describes a scalar and a
pseudoscalar particle. A continuum effective theory is derived for low
energies. A possibility of constructing a model with a single chiral boson is
mentioned.Comment: To appear in Phys. Rev.
Hybrid Monte Carlo algorithm for the Double Exchange Model
The Hybrid Monte Carlo algorithm is adapted to the simulation of a system of
classical degrees of freedom coupled to non self-interacting lattices fermions.
The diagonalization of the Hamiltonian matrix is avoided by introducing a
path-integral formulation of the problem, in Euclidean space-time. A
perfect action formulation allows to work on the continuum euclidean time,
without need for a Trotter-Suzuki extrapolation. To demonstrate the feasibility
of the method we study the Double Exchange Model in three dimensions. The
complexity of the algorithm grows only as the system volume, allowing to
simulate in lattices as large as on a personal computer. We conclude
that the second order paramagnetic-ferromagnetic phase transition of Double
Exchange Materials close to half-filling belongs to the Universality Class of
the three-dimensional classical Heisenberg model.Comment: 20 pages plus 4 postscript figure
Morocco as a possible domestication center for barley: biochemical and agromorphological evidence.
The distribution of genetic variants of a group of low molecular weight, chloroform-methanol soluble proteins (CM proteins), among Moroccan and non-Moroccan accessions of Hordeum spontaneum and among selections from several Moroccan landraces of H. vulgare and cultivars of the same species with widespread European origin, suggests that domestication of barley might have taken place in Morocco. An agromorphological characterization of the H. spontaneum accessions further supports this hypothesis. The possible Moroccan origin of the French cultivar Hatif de Grignon and of several Spanish 6-rowed barleys is also presented
Configuración del Paisaje y Conservación de Hábitat Dunares: Las Dunas de Coterillo en la Región de Murcia (España)
El paisaje de Coterillo se caracteriza por hábitats que comparten el territorio con especies exóticas-invasoras y otras especies, que se plantaron a principios del siglo XX para fijar las dunas que invadían los estanques salineros. La metodología se ha basado muestreos de campo, análisis bibliográfico y del paisaje de cartografía histórica, del vuelo aéreo de 1957 y del satélite de 2016, para analizar la
evolución de los hábitats y de las especies exóticas-invasoras. En la actualidad hay 15 hábitats de interés comunitario, entre los que destacan los prioritarios 1510 Estepas salinas mediterráneas Limonietalia y 2250 Dunas litorales con Juniperus sp, con problemas de conservación por uso recreativo del territorio y competencia con especies exóticas-invasoras. Se analizan las acciones de gestión de la administración para recuperar los hábitats, entre las que destacan el control de especies exóticas-invasoras y ordenación del uso público. La prioridad es la recuperación de los hábitats originarios de dunas, por lo que se recomienda mantener las actuaciones emprendidas por la administración, en particular reforzar la sustitución de las especies arbóreas introducidas, sobre todo Pinus halepensis, por Juniperus turbinata y otras especies de dunas, así como reforzar las actuaciones para erradicar las especies exóticas invasoras.--
The landscape of Coterillo is characterized by habitats that share the territory with exotic-invasive species and other species, which were
planted in the early twentieth century to fix the dunes that invaded the salt ponds. The methodology has been based on field samplings,
bibliographic and landscape analysis of historical cartography, the 1957 aerial flight and the 2016 satellite, to analyze the evolution of
habitats and exotic-invasive species. At present there are 15 habitats of community interest, among which the priority ones are 1510
Mediterranean salt stepas Limonietalia and 2250 coastal dunes with Juniperus sp, with conservation problems due to recreational use
of the territory and competition with exotic-invasive species. The management actions of the administration are analyzed to recover
the habitats, among which the control of exotic-invasive species and public use management stand out. The priority is the recovery of
habitats originating from dunes, which is why it is recommended to maintain the actions undertaken by the administration, in particular
to reinforce the substitution of introduced tree species, especially Pinus halepensis, for Juniperus turbinata and other dune species, as
well as reinforce actions to eradicate invasive alien species.
Keywords: Coterillo; dunes; species; exotic-invasive; managemen
Effects of herbaceous covers and mineral fertilizers on the nutrient stocks and fluxes in a Mediterranean olive grove
Altres ajuts: acord transformatiu CRUE-CSICThe preservation of nutrient capital, soil fertility, and carbon (C) sequestration capacity in Mediterranean olive groves requires evaluation of agricultural practices beyond short-term productivity. We aim to contribute with a mechanistic understanding on the effects that the preservation of herbaceous cover and the use of chemical fertilizers have on the performance of olive trees and on the biogeochemical cycles of the agroecosystem. We compared nutrient fluxes and aboveground leafy stocks in an olive grove that had been organically managed for more than 60 years, in a treatment in which the annual spontaneous herbaceous cover was maintained (H), and after two years of shift to conventional management treatments in which the growth of herbaceous vegetation was avoided by the use of herbicides (NH), and where exclusion of the herbaceous cover is also combined with the supply of mineral fertilizers (NHF). Maintenance of herbaceous vegetation in H contributed to the retention of a high aboveground capital of C and nutrients, particularly nitrogen, (N), phosphorus (P) and potassium (K) that were about 2.9, 3.9 and 7.4 times greater than in NH, respectively. The permanence of herbaceous cover stimulated olive tree leaf litter decomposition rates by about 86 % and increased nutrient release. However, the H treatment led to a 37 % decrease in olive yield and lowered olive foliar N and P content as negative short-term effects. The addition of fertilizers (N, P, K, and Mg) in mineral and solid form in NHF resulted inefficient to improve olive tree nutritional status and olive production, and decelerated olive tree litter decomposition rates by 21 % and nutrient release. The nutrient retention in organic forms in the fast-growing species of herbaceous covers and the progressive nutrient release as litter decomposes may contribute to regulate and better adapt nutrient availability to the nutrient requirements of olive trees
The Harris-Luck criterion for random lattices
The Harris-Luck criterion judges the relevance of (potentially) spatially
correlated, quenched disorder induced by, e.g., random bonds, randomly diluted
sites or a quasi-periodicity of the lattice, for altering the critical behavior
of a coupled matter system. We investigate the applicability of this type of
criterion to the case of spin variables coupled to random lattices. Their
aptitude to alter critical behavior depends on the degree of spatial
correlations present, which is quantified by a wandering exponent. We consider
the cases of Poissonian random graphs resulting from the Voronoi-Delaunay
construction and of planar, ``fat'' Feynman diagrams and precisely
determine their wandering exponents. The resulting predictions are compared to
various exact and numerical results for the Potts model coupled to these
quenched ensembles of random graphs.Comment: 13 pages, 9 figures, 2 tables, REVTeX 4. Version as published, one
figure added for clarification, minor re-wordings and typo cleanu
- …