Vortex screening and the fate of the BKT transition with long-range couplings

Abstract

The stability of the Berezinskii--Kosterlitz--Thouless phase against long-range interactions remains an open problem. We address this question in the two-dimensional XY model with couplings decaying as $r^{-2-\sigma }$, by developing a real-space renormalization-group treatment near the Gaussian manifold of quasi-long-range ordered (QLRO) states. We retain the full many-body vortex interaction and show that, at leading order in the fugacity, screening by small vortex--antivortex pairs preserves its Gaussian-average representation without an expansion in the long-range coupling. Including spin waves perturbatively then yields a local stability criterion: a stable QLRO interval exists only for $\sigma >7/4$. We also investigate a Wilson--Fisher-like fixed point at finite coupling for $\sigma <7/4$, for which the retained flow predicts ${\nu }^{-1}\sim \sqrt{7/4-\sigma }$ as it merges with the Gaussian line.

arXiv comment7+6 pages, 2 figures