A Variational Singular Perturbation Problem Motivated by Ericksen’s Model for Nematic Liquid Crystals

Abstract

We study the asymptotic behavior, when $\varepsilon \rightarrow 0$, of the minimizers $\{u_\varepsilon \}_{\varepsilon >0}$ for the energy

$$\begin{aligned} E_\varepsilon (u)=\int _{\Omega }\Big (|\nabla u|^2+\big (\frac{1}{\varepsilon ^2}-1\big )|\nabla |u||^2\Big ), \end{aligned}$$

over the class of maps $u\in H^1(\Omega ,{{\mathbb {R}}}^2)$ satisfying the boundary condition $u=g$ on $\partial \Omega$, where $\Omega$ is a smooth, bounded and simply connected domain in ${{\mathbb {R}}}^2$ and $g:\partial \Omega \rightarrow S^1$ is a smooth boundary data of degree $D\ge 1$. The motivation comes from a simplified version of the Ericksen model for nematic liquid crystals with variable degree of orientation. We prove convergence (up to a subsequence) of $\{u_\varepsilon \}$ towards a singular $S^1$–valued harmonic map $u_*$, a result that resembles the one obtained in Bethuel et al. (Ginzburg–Landau Vortices, Birkhäuser, 1994) for an analogous problem for the Ginzburg–Landau energy. There are however two striking differences between our result and the one involving the Ginzburg–Landau energy. First, in our problem, the singular limit $u_*$ may have singularities of, degree strictly larger than one. Second, we find that the principle of “equipartition” holds for the energy of the minimizers, i.e., the contributions of the two terms in $E_\varepsilon (u_\varepsilon )$ are essentially equal.