The Ellipse Law: Kirchhoff Meets Dislocations

In this paper we consider a nonlocal energy I _α whose kernel is obtained by adding to the Coulomb potential an anisotropic term weighted by a parameter α∈ R. The case α = 0 corresponds to purely logarithmic interactions, minimised by the circle law; α = 1 corresponds to the energy of interacting dislocations, minimised by the semi-circle law. We show that for α∈ (0 , 1) the minimiser is the normalised characteristic function of the domain enclosed by the ellipse of semi-axes 1-α and 1+α. This result is one of the very few examples where the minimiser of a nonlocal anisotropic energy is explicitly computed. For the proof we borrow techniques from fluid dynamics, in particular those related to Kirchhoff’s celebrated result that domains enclosed by ellipses are rotating vortex patches, called Kirchhoff ellipses.