NEW TOPOLOGIES IN THE PHASE DIAGRAM OF THE SEMI-INFINITE BLUME-CAPEL MODEL

The phase diagram of the Blume--Capel model on a semi--infinite simple cubic lattice with a (100) free surface is studied in the pair approximation of the cluster variation method. Six main topologies are found, of which two are new, due to the occurrence of a first order surface transition in the phase with ordered bulk, separating two phases with large and small surface order parameters. The latter is a new phase and is studied in some detail, giving the behaviour of the order parameter profiles in two typical cases. A comparison is made with the results of a low temperature expansion, where these are available, showing a great increase in accuracy with respect to the mean field approximation.Comment: RevTeX, 13 pages + 7 uuencoded PostScript figures (substituted raw with encoded PostScript

Surface reentrance in the semi-infinite spin-1 Ising models

The critical behavior of the semi-infinite Blume-Capel and Blume-Emery-Griffiths models is investigated in the pair approximation of the Cluster Variation Method. Equations for bulk and surface order parameters and n.n. correlation functions are given, from which analytical expressions for the second order bulk and surface critical temperatures are derived. The phase diagrams of the Blume-Capel model are classified, and the existence of a surface first order transition is discussed. This transition is shown to be, under certain conditions, slightly reentrant, and the behavior of the surface order parameters and correlation functions is given for such a case. The extension of our results to the Blume-Emery-Griffiths model is briefly discussed.Comment: 17 pages, 14 figures (PostScript, appended), POLFIS-TH.19/9

Multicritical Points And Reentrant Phenomenon In The BEG Model

The Blume - Emery - Griffiths model is investigated by use of the cluster variation method in the pair approximation. We determine the regions of the phase space where reentrant phenomenon takes place. Two regions are found, depending on the sign of the reduced quadrupole - quadrupole coupling strength $\xi$. For negative $\xi$ we find Para-Ferro-Para and Ferro-Para-Ferro-Para transition sequences; for positive $\xi$, a Para$_-$-Ferro-Para$_+$ sequence. Order parameters, correlation functions and specific heat are given in some typical cases. By-products of this work are the equations for the critical and tricritical lines.Comment: 14 pages, figures available upon reques

Binary Mixtures of Ferro- and Antiferromagnetic Bonds in Quantum Spin-1 Models: Critical Behavior

A quantum spin-1 model, describing a mixture of ferro- and antiferromagnetic bonds, is analyzed employing the effective Hamiltonian method extended to random systems. Uniform, staggered and spin-glass susceptibilities are introduced and Curie, Néel and spin-glass transition temperatures determined. The role anisotropy is examined