The low-temperature phase in the two-dimensional long-range diluted XY model
The critical behaviour of statistical models with long-range interactions exhibits distinct regimes as a function of $ρ$, the power of the interaction strength decay. For $ρ$ large enough, $ρ>ρ_{\rm sr}$, the critical behaviour is observed to coincide with that of the short-range model. However, there are controversial aspects regarding this picture, one of which is the value of the short-range threshold $ρ_{\rm sr}$ in the case of the long-range XY model in two dimensions. We study the 2d XY model on the {\it diluted} graph, a sparse graph obtained from the 2d lattice by rewiring links with probability decaying with the Euclidean distance of the lattice as $|r|^{-ρ}$, which is expected to feature the same critical behavior of the long range model. Through Monte Carlo sampling and finite-size analysis of the spontaneous magnetisation and of the Binder cumulant, we present numerical evidence that $ρ_{\rm sr}=4$. According to such a result, one expects the model to belong to the Berezinskii-Kosterlitz-Thouless (BKT) universality class for $ρ\ge 4$, and to present a $2^{nd}$-order transition for $ρ<4$.