Phase diagrams of soluble multi-spin glass models

We include p-spin interactions in a spherical version of a soluble mean-field spin-glass model proposed by van Hemmen. Due to the simplicity of the solutions, which do not require the use of the replica trick, we are able to carry out a detailed investigation of a number of special situations. For p larger or equal to 3, there appear first-order transitions between the paramagnetic and the ordered phases. In the presence of additional ferromagnetic interactions, we show that there is no stable mixed phase, with both ferromagnetic and spin-glass properties.Comment: To appear in Physica

Correlated disordered interactions on Potts models

Using a weak-disorder scheme and real-space renormalization-group techniques, we obtain analytical results for the critical behavior of various q-state Potts models with correlated disordered exchange interactions along d1 of d spatial dimensions on hierarchical (Migdal-Kadanoff) lattices. Our results indicate qualitative differences between the cases d-d1=1 (for which we find nonphysical random fixed points, suggesting the existence of nonperturbative fixed distributions) and d-d1>1 (for which we do find acceptable perturbartive random fixed points), in agreement with previous numerical calculations by Andelman and Aharony. We also rederive a criterion for relevance of correlated disorder, which generalizes the usual Harris criterion.Comment: 8 pages, 4 figures, to be published in Physical Review

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'' $\phi^3$ 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

Mario Schönberg and the introduction of Fock space in classical statistical physics

Há cerca de cinqüenta anos, numa série pioneira de trabalhos, Mario Schönberg utilizou métodos de segunda quantização para generalizar o teorema de Liouville, introduzindo a idéia de indistinguibilidade entre partículas clássicas. O espaço de Fock, que era considerado um atributo paradigmático dos sistemas quânticos, foi utilizado com rigor matemático e consistência física para construir um formalismo da mecânica estatística clássica descrevendo um sistema com número variável de partículas. Abordagens semelhantes foram redescobertas ao longo das últimas três décadas, em particular no contexto de modelos estocásticos, incluindo processos irreversíveis em redes de spins e reações químicas. Apresentamos uma descrição da teoria de Schönberg, estabelecendo conexões com desenvolvimentos mais recentes. O nosso trabalho é uma contribuição pedagógica, enfatizando a consistência física da utilização da representação número de ocupação em contextos clássicos.About fifty years ago, in a pioneering series of articles, Mario Schönberg used methods of second quantization in order to generalize the Liouville theorem and to introduce the idea of indistinguishability of particles in a classical context. The Fock space, which was a paradigmatic attribute of quantum systems, was used with mathematical rigor and consistency in order to construct a classical statistical formalism for describing systems with a variable number of particles. Similar treatments have been rediscovered along the last three decades, in particular in the context of stochastic models, including irreversible processes in spin lattices and chemical reactions. We present a description of Schönberg's theory, and establish some connections with more recent developments. This work is a pedagogical contribution, with emphasis on the physical consistency of using the occupation number representation in classical contexts