Enhancement and suppression of the transition temperature of a three-dimensionalXYferromagnet by control of vortex-loop fugacity

A previous vortex-loop scaling analysis for the three-dimensional XY model is extended to include an external loop-segment chemical potential {μ}={λ}kBT. The loop fugacity y₀ is suppressed, y₀{→}y₀{e}^{{{-}}2{{π}}{λ}}, for λ>0, enhancing the transition temperature{T}c(λ), in agreement with the Monte Carlo work of Kohring, Shrock, and Wills. One also gets the suppression of transition temperatures{T}c$(${e}²$) of lattice superconductors by electromagnetic charge ${e}²$, by mapping onto this loop-fugacity model. A possible approach to superconductor high-${T}c$ enhancement, by tailored suppression of topological excitations, is briefly conjectured.