Ashkin-Teller criticality and pseudo first-order behavior in a frustrated Ising model on the square lattice

TL;DR

This paper investigates the phase transition in a frustrated square-lattice Ising model, revealing Ashkin-Teller criticality with continuously varying exponents and identifying a pseudo first-order behavior in a specific parameter range.

Contribution

It demonstrates Ashkin-Teller criticality in the frustrated Ising model and clarifies the nature of the phase transition, including the pseudo first-order behavior for certain coupling ratios.

Findings

Critical exponents vary continuously between 4-state Potts and Ising models.

Transition is first-order for g<0.67, lower than previously thought.

Pseudo first-order behavior observed for 0.67<g<1.

Abstract

We study the challenging thermal phase transition to stripe order in the frustrated square-lattice Ising model with couplings J1<0 (nearest-neighbor, ferromagnetic) and J2>0 (second-neighbor, antiferromagnetic) for g=J2/|J1|>1/2. Using Monte Carlo simulations and known analytical results, we demonstrate Ashkin-Teller criticality for g>= g*, i.e., the critical exponents vary continuously between those of the 4-state Potts model at g=g* and the Ising model for g -> infinity. Thus, stripe transitions offer a route to realizing a related class of conformal field theories with conformal charge c=1 and varying exponents. The transition is first-order for g<g*= 0.67(1), much lower than previously believed, and exhibits pseudo first-order behavior for g* < g < 1.