Off-critical correlations in the Ashkin-Teller model

We use the exact scattering description of the scaling Ashkin-Teller model in two dimensions to compute the two-particle form factors of the relevant operators. These provide an approximation for the correlation functions whose accuracy is tested against exact sum rules.Comment: 8 pages, late

Universal Ratios and Correlation Functions

We review some recent results concerning the quantitative analysis of the universality classes of two-dimensional statistical models near their critical point. We also discuss the exact calculation of the two--point correlation functions of disorder operators in a free theory of complex bosonic and fermionic field, correlators ruled by a Painleve differential equation.Comment: 10 pages, JHEP Proceedings of the Workshop on Integrable Theories, Solitons and Duality, IFT-Unesp, Sao Paulo, Brasi

Correlators in integrable quantum field theory. The scaling RSOS models

The study of the scaling limit of two-dimensional models of statistical mechanics within the framework of integrable field theory is illustrated through the example of the RSOS models. Starting from the exact description of regime III in terms of colliding particles, we compute the correlation functions of the thermal, $\phi_{1,2}$ and (for some cases) spin operators in the two-particle approximation. The accuracy obtained for the moments of these correlators is analysed by computing the central charge and the scaling dimensions and comparing with the exact results. We further consider the (generally non-integrable) perturbation of the critical points with both the operators $\phi_{1,3}$ and $\phi_{1,2}$ and locate the branches solved on the lattice within the associated two-dimensional phase diagram. Finally we discuss the fact that the RSOS models, the dilute $q$-state Potts model at and the O(n) vector model are all described by the same perturbed conformal field theory.Comment: 22 pages, late

The Field Theory of the q->4+ Potts Model

The q-state Potts model in two dimensions exhibits a first-order transition for q>4. As q->4+ the correlation length at this transition diverges. We argue that this limit defines a massive integrable quantum field theory whose lowest excitations are kinks connecting 4+1 degenerate ground states. We construct the S-matrix of this theory and the two-particle form factors, and hence estimate a number of universal amplitude ratios. These are in very good agreement with the results of extrapolated series in q^(-1/2) as well as Monte Carlo results for q=5.Comment: 8 pages, 1 figure; late

Mapping between the Sinh-Gordon and Ising Models

The $S$-matrix of the Ising Model can be obtained as particular limit of the roaming trajectories associated to of the $S$-matrix of the Sinh-Gordon model. Using the form factors of the Sinh-Gordon, we analyse the correspondence between the operators of the two theories.Comment: 10 pages, LATEX file, (two figures not included in the text, to be requested separately) IC/93/143, ISAS/EP/93/8

Field theory of scaling lattice models. The Potts antiferromagnet

In contrast to what happens for ferromagnets, the lattice structure participates in a crucial way to determine existence and type of critical behaviour in antiferromagnetic systems. It is an interesting question to investigate how the memory of the lattice survives in the field theory describing a scaling antiferromagnet. We discuss this issue for the square lattice three-state Potts model, whose scaling limit as T->0 is argued to be described exactly by the sine-Gordon field theory at a specific value of the coupling. The solution of the scaling ferromagnetic case is recalled for comparison. The field theory describing the crossover from antiferromagnetic to ferromagnetic behaviour is also introduced.Comment: 11 pages, to appear in the proceedings of the NATO Advanced Research Workshop on Statistical Field Theories, Como 18-23 June 200

Susceptibility amplitude ratios in the two-dimensional Potts model and percolation

The high-temperature susceptibility of the $q$-state Potts model behaves as $\Gamma|T-T_c|^{-\gamma}$ as $T\to T_c+$, while for $T\to T_c-$ one may define both longitudinal and transverse susceptibilities, with the same power law but different amplitudes $\Gamma_L$ and $\Gamma_T$. We extend a previous analytic calculation of the universal ratio $\Gamma/\Gamma_L$ in two dimensions to the low-temperature ratio $\Gamma_T/\Gamma_L$, and test both predictions with Monte Carlo simulations for $q=3$ and 4. The data for $q=4$ are inconclusive owing to large corrections to scaling, while for $q=3$ they appear consistent with the prediction for $\Gamma_T/\Gamma_L$, but not with that for $\Gamma/\Gamma_L$. A simple extrapolation of our analytic results to $q\to1$ indicates a similar discrepancy with the corresponding measured quantities in percolation. We point out that stronger assumptions were made in the derivation of the ratio $\Gamma/\Gamma_L$, and our work suggests that these may be unjustified.Comment: 17 pages, late

Correlation Functions in the Two-dimensional Ising Model in a Magnetic Field at T=TcT=T_c

The one and two-particle form factors of the energy operator in the two-dimensional Ising model in a magnetic field at $T=T_c$ are exactly computed within the form factor bootstrap approach. Together with the matrix elements of the magnetisation operator already computed in ref.\,\cite{immf}, they are used to write down the large distance expansion for the correlators of the two relevant fields of the model.Comment: 18 pages, latex, 7 table

First order phase transitions and integrable field theory. The dilute q-state Potts model

We consider the two-dimensional dilute q-state Potts model on its first order phase transition surface for 0<q\leq 4. After determining the exact scattering theory which describes the scaling limit, we compute the two-kink form factors of the dilution, thermal and spin operators. They provide an approximation for the correlation functions whose accuracy is illustrated by evaluating the central charge and the scaling dimensions along the tricritical line.Comment: 21 pages, late

Finite temperature results on the 2d Ising model with mixed perturbation

A numerical study of finite temperature features of thermodynamical observables is performed for the lattice 2d Ising model. Our results support the conjecture that the Finite Size Scaling analysis employed in the study of integrable perturbation of Conformal Field Theory is still valid in the present case, where a non-integrable perturbation is considered.Comment: 9 pages, Latex, added references and improved introductio