The puzzle of bicriticality in the XXZ antiferromagnet

TL;DR

This paper resolves the discrepancy between theoretical predictions and experimental/simulation results for the phase diagram of the XXZ antiferromagnet, showing that observed bicritical behavior is due to finite-size effects near a fluctuation-driven first order transition.

Contribution

The authors generalize the condition for tetracritical points and use new renormalization-group relations to explain why experiments and simulations observe bicritical diagrams instead of the predicted tetracritical or triple point.

Findings

Experiments and simulations only see bicritical diagrams due to finite-size effects.

The true asymptotic behavior involves a fluctuation-driven first order transition.

A similar explanation may apply to other systems with competing order parameters.

Abstract

Renormalization-group theory predicts that the XXZ antiferromagnet in a magnetic field along the easy Z-axis has asymptotically either a tetracritical phase-diagram or a triple point in the field-temperature plane. Neither experiments nor Monte Carlo simulations procure such phase diagrams. Instead, they find a bicritical phase-diagram. Here this discrepancy is resolved: after generalizing a ubiquitous condition identifying the tetracritical point, we employ new renormalization-group recursion relations near the isotropic fixed point, exploiting group-theoretical considerations and using accurate exponents at three dimensions. These show that the experiments and simulations results can only be understood if their trajectories flow towards the fluctuation-driven first order transition (and the associated triple point), but reach this limit only for prohibitively large system sizes or correlation lengths. In the crossover region one expects a bicritical phase diagram, as indeed is observed. A similar scenario may explain puzzling discrepancies between simulations and renormalization-group predictions for a variety of other phase diagrams with competing order parameters.