Abstract:This text considers the discrete height functions associated with the Berezinskii--Kosterlitz--Thouless transition (BKT) at slope zero. Our main results are as follows.
* Sharpness: If the model is localised, then the two-point function (covariance) decays exponentially fast in the distance between the points.
* Effective temperature gap: If the model is delocalised, then the variance grows at least as $c\log n$, where $n$ is the distance to the boundary and $c>0$ a universal constant not depending on the temperature. Thus, the effective temperature must jump from $0$ to at least $c$ at the transition point; values in the interval $(0,c)$ are forbidden.
* Delocalisation at the transition point: The delocalised phase includes the transition point, in the sense that it is a closed set in the phase diagram in the appropriate topology.
These results contribute to the understanding of the regime at and around the transition point which remained largely unexplored. In a follow-up paper, the sharpness derived here is used to establish that the localisation-delocalisation transition is equivalent to the BKT transition in the dual XY and Villain models.
From: Piet Lammers [view email]
[v1]
Fri, 25 Nov 2022 20:17:39 UTC (1,199 KB)
[v2]
Tue, 18 Apr 2023 17:18:06 UTC (950 KB)
[v3]
Thu, 30 Apr 2026 10:26:46 UTC (587 KB)
[v4]
Sat, 25 Jul 2026 09:40:38 UTC (606 KB)
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