Experimental measures of topological sector fluctuations in the F-model

The two-dimensional F-model is an ice-rule-obeying model, with a low-temperature antiferroelectric state and high-temperature critical Coulomb phase. Polarization in the system is associated with topological defects in the form of system-spanning windings which makes it an ideal system on which to observe topological sector fluctuations, as have been discussed in the context of spin ice and Berezinskii-Kosterlitz-Thouless (BKT) systems. In particular, the F-model offers a useful counterpoint to the BKT transition in that winding defects are energetically suppressed in the ordered state, rather than dynamically suppressed in the critical phase. In this paper we develop Lieb and Baxter's historic solutions of the F-model to exactly calculate relevant properties, several apparently for the first time. We further calculate properties not amenable to exact solution by an approximate cavity method and by referring to established scaling results. Of particular relevance to topological sector fluctuations are the exact results for the applied field polarization and the ``energetic susceptibility.'' The latter is a both a measure of topological sector fluctuations and, surprisingly, in this case, a measure of the order parameter correlation exponent. In the high-temperature phase, the temperature tunes the density of topological defects and algebraic correlations, with the energetic susceptibility undergoing a jump to zero at the antiferroelectric ordering temperature, analogous to the ``universal jump'' in BKT systems. We discuss how these results are relevant to experimental systems, including to spin-ice thin films, and, unexpectedly, to three-dimensional dipolar spin ice and water ice, where we find that an analogous ``universal jump'' has previously been established in numerical studies. This unexpected result suggests a universal limit on the stability of perturbed Coulomb phases that is independent of dimension and of the order of the transition. Experimental results on water ice Ih are not inconsistent with this proposition. We complete the paper by relating our new results to experimental studies of artificial spin-ice arrays.