Phase transitions in two-component Bose-Einstein condensates with Rabi frequency (I): The De Giorgi conjecture for the local problem in $\mathbb{R}^{3}$

In this series of papers, we investigate coupled systems arising in the study of two-component Bose--Einstein condensates, and we establish classification results for solutions of De Giorgi conjecture type. In the first paper of the series, we focus on the local problem of the form $\Delta u = u(u^2+v^2-1) + v(\alpha uv - \omega)$, $\Delta v = v(u^2+v^2-1) + u(\alpha uv - \omega)$, and prove that positive global solutions in $\mathbb{R}^3$ satisfying $\partial u/\partial x_3 > 0 > \partial v/\partial x_3$ must be one-dimensional.