A free boundary problem inspired by a conjecture of De Giorgi

Abstract

We study global monotone solutions of the free boundary problem that arises from minimizing the energy functional $I(u) = \int |\nabla u|^2 + V(u)$, where $V(u)$ is the characteristic function of the interval $(-1,1)$. This functional is a close relative of the scalar Ginzburg-Landau functional $J(u) = \int |\nabla u|^2 + W(u)$, where $W(u) = (1-u^2)^2/2$ is a standard double-well potential. According to a famous conjecture of De Giorgi, global critical points of $J$ that are bounded and monotone in one direction have level sets that are hyperplanes, at least up to dimension $8$. Recently, Del Pino, Kowalczyk and Wei gave an intricate fixed-point-argument construction of a counterexample in dimension $9$, whose level sets ``follow" the entire minimal non-planar graph, built by Bombieri, De Giorgi and Giusti (BdGG). In this paper we turn to the free boundary variant of the problem and we construct the analogous example; the advantage here is that of geometric transparency as the interphase $\{|u| < 1\}$ will be contained within a unit-width band around the BdGG graph. Furthermore, we avoid the technicalities of Del Pino, Kowalczyk and Wei's fixed-point argument by using barriers only.