Three New Phases in the Two-Dimensional Potts Model at Complex Temperature

Abstract

The complex-temperature phase structure of the two-dimensional Potts model has remained largely unresolved despite decades of study. Here we determine the complex-temperature phase diagram of the 3-state Potts model, and uncover three gapless phases beyond the familiar ferromagnetic and paramagnetic phases. This is done by combining tensor-network free energies with the Poisson equation to compute the Fisher zero density in the thermodynamic limit. The resulting zero distribution unifies and revises long-standing conjectures on the zero loci, and outlines each phase. One is a non-conformal gapless phase, with an exponentially growing number of low-lying transfer-matrix levels distributed across all momentum sectors. The other two are Luttinger-liquid phases with complex-valued Luttinger parameters related by the Kramers-Wannier duality. At the antiferromagnetic critical point, the Luttinger parameter equals 3, while its limit from the adjacent Luttinger-liquid phase is 6. These findings reveal rich phase structures and unconventional critical behavior in a classical textbook model.