A symmetry breaking phenomenon for anisotropic harmonic maps from a 2D annulus into $\mathbb S^1$

Abstract: In a two dimensional annulus $A_\rho={x\in \mathbb R^2: \rho<|x|<1}$, $\rho\in (0,1)$, we characterize $0$-homogeneous minimizers, in $H^1(A_\rho;\mathbb S^1)$ with respect to their own boundary conditions, of the anisotropic energy \begin{equation*} E_\delta(u)=\int_{A_\rho} |\nabla u|^2 +\delta \left( (\nabla\cdot u)^2-(\nabla\times u)^2\right) \, dx,\quad \delta\in (-1,1). \end{equation*} Even for a small anisotropy $0<|\delta|\ll 1$, we exhibit qualitative properties very different from the isotropic case $\delta=0$. In particular, $0$-homogeneous critical points of degree $d\notin \lbrace 0,1,2\rbrace$ are always local minimizers, but in thick annuli ($\rho\ll 1$) they are not minimizers: the $0$-homogeneous symmetry is broken. One corollary is that entire solutions to the anisotropic Ginzburg-Landau system have a far-field behavior very different from the isotropic case studied by Brezis, Merle and Rivi`ere. The tools we use include: ODE and variational arguments; asymptotic expansions, interpolation inequalities and explicit computations involving near-optimizers of these inequalities for proving that $0$-homogeneous critical points are not minimizers in thick annuli.