Magnetic critical behavior of two-dimensional random-bond Potts ferromagnets in confined geometries

TL;DR

This study numerically investigates the critical behavior of 2D random-bond Potts ferromagnets in confined geometries, revealing how disorder influences phase transitions and scaling dimensions across a range of Q values.

Contribution

It applies conformal invariance techniques to analyze disorder effects in 2D Potts models above and below Q=4, providing new insights into critical behavior in confined geometries.

Findings

Disorder induces second-order transitions for Q>4.

Scaling dimensions are accurately determined using conformal mapping.

Connectivity transfer matrix identifies disordered fixed point regimes.

Abstract

We present a numerical study of 2D random-bond Potts ferromagnets. The model is studied both below and above the critical value $Q_c=4$ which discriminates between second and first-order transitions in the pure system. Two geometries are considered, namely cylinders and square-shaped systems, and the critical behavior is investigated through conformal invariance techniques which were recently shown to be valid, even in the randomness-induced second-order phase transition regime Q>4. In the cylinder geometry, connectivity transfer matrix calculations provide a simple test to find the range of disorder amplitudes which is characteristic of the disordered fixed point. The scaling dimensions then follow from the exponential decay of correlations along the strip. Monte Carlo simulations of spin systems on the other hand are generally performed on systems of rectangular shape on the square lattice, but the data are then perturbed by strong surface effects. The conformal mapping of a semi-infinite system inside a square enables us to take into account boundary effects explicitly and leads to an accurate determination of the scaling dimensions. The techniques are applied to different values of Q in the range 3-64.