Allen-Cahn Solutions with Triple Junction Structure at Infinity

We construct an entire solution $U:\mathbb{R}^2\to\mathbb{R}^2$ to the elliptic system $$\Delta U=\nabla_uW(U),$$ where $W:\mathbb{R}^2\to [0,\infty)$ is a `triple-well' potential. This solution is a local minimizer of the associated energy $$\int \frac{1}{2}|\nabla U|^2+W(U)\,dx$$ in the sense that $U$ minimizes the energy on any compact set among competitors agreeing with $U$ outside that set. Furthermore, we show that along subsequences, the `blowdowns' of $U$ given by $U_R(x):=U(Rx)$ approach a minimal triple junction as $R\to\infty$. Previous results had assumed various levels of symmetry for the potential and had not established local minimality, but here we make no such symmetry assumptions.