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Ground States of a Nonlinear Curl-Curl Problem in Cylindrically Symmetric Media

We consider the nonlinear curl-curl problem $\nabla\times\nabla\times U + V(x) U= Γ(x)|U|^{p-1}U$ in $\mathbb{R}^3$ related to the nonlinear Maxwell equations for monochromatic fields. We search for solutions as minimizers (ground states) of the corresponding energy functional defined on subspaces (defocusing case) or natural constraints (focusing case) of $H(\mathrm{curl};\mathbb{R}^3)$. Under a cylindrical symmetry assumption on the functions $V$ and $Γ$ the variational problem can be posed in a symmetric subspace of $H(\mathrm{curl};\mathbb{R}^3)$. For a strongly defocusing case $\mathrm{esssup}\, Γ<0$ with large negative values of $Γ$ at infinity we obtain ground states by the direct minimization method. For the focusing case $\mathrm{essinf}\, Γ>0$ the concentration compactness principle produces ground states under the assumption that zero lies outside the spectrum of the linear operator $\nabla \times \nabla \times +V(x)$. Examples of cylindrically symmetric functions $V$ are provided for which this holds.