Fractal structure of high-temperature graphs of O(N) models in two dimensions

Tricritical O(n) models in two dimensions

We show that the exactly solved low-temperature branch of the two-dimensional O(n) model is equivalent to an O(n) model with vacancies and a different value of n . We present analytic results for several universal parameters of the latter model, which is identified as a tricritical point. These results apply to the range n</=32 and include the exact tricritical point, the conformal anomaly, and a number of scaling dimensions, among which are the thermal and magnetic exponents, and the exponent associated with the crossover to ordinary critical behavior and to tricritical behavior with cubic symmetry. We describe the translation of the tricritical model in a Coulomb gas. The results are verified numerically by means of transfer-matrix calculations. We use a generalized ADE model as an intermediary and present the expression of the one-point distribution function in that language. The analytic calculations are done both for the square and the honeycomb lattice.

Geometric properties of the three-dimensional Ising and XY models

The fractal structure of high-temperature graphs of the three-dimensional Ising and XY models is investigated by simulating these graphs directly on a cubic lattice and analyzing them with the help of percolation observables. The Ising graphs are shown to percolate right at the Curie critical point. The diverging length scale relevant to the graphs in the vicinity of the percolation threshold is shown to be provided by the spin correlation length. The fractal dimension of the high-temperature graphs at criticality is estimated to be D=1.7349(65) for the Ising and D=1.7626(66) for the XY models.

Invaded cluster algorithm for a tricritical point in a diluted Potts model

The invaded cluster approach is extended to the two-dimensional Potts model with annealed vacancies by using the random-cluster representation. Geometrical arguments are used to propose the algorithm which converges to the tricritical point in the two-dimensional parameter space spanned by temperature and the chemical potential of the vacancies. The tricritical point is identified as a simultaneous onset of the percolation of a Fortuin-Kasteleyn cluster and of a percolation of the "geometrical disorder cluster." The location of the tricritical point and the concentration of vacancies for q=1,2,3 are found to be in good agreement with the best known results. Scaling properties of the percolating scaling cluster and related critical exponents are also presented.

Exact characterization of O(n) tricriticality in two dimensions

We propose exact expressions for the conformal anomaly and for three critical exponents of the tricritical O(n) loop model as a function of n in the range -2<or=n<or=3/2. These findings are based on an analogy with known relations between Potts and O(n) models and on an exact solution of a "tri-tricritical" Potts model described in the literature. We verify the exact expressions for the tricritical O(n) model by means of a finite-size scaling analysis based on numerical transfer-matrix calculations.