Frustration-Induced $1/6$-Vortex-Paired Phase in a Kagome XY Model

Abstract

Geometric frustration can fractionalize vortices in two-dimensional XY systems, but a given frustrated model usually hosts only a single type of fractional vortex. Here we study a kagome XY model in which an antiferromagnetic coupling $J_1$ and a second-harmonic coupling $K$ favor two competing frustrated ground-state manifolds. For $K/J_1<1/9$, the ground states map onto those of the kagome three-state Potts antiferromagnet and support $1/3$ vortices; for $K/J_1>1/9$, they map onto those of the kagome Ising antiferromagnet and support $1/2$ vortices. At $K/J_1=1/9$, the two manifolds become degenerate, and bends in the domain walls between them carry $1/6$-vortex excitations. Tensor-network calculations show that a finite-temperature phase with quasi-long-range order only in $e^{6iθ}$ emerges from this degenerate point, separating the $1/3$- and $1/2$-vortex-paired phases. This $1/6$-vortex-paired phase has no counterpart in either parent manifold, showing that competing frustrated manifolds provide a route to new fractional-vortex-paired phases.