Uniqueness of Dirac-harmonic maps from a compact surface with boundary

Abstract

As a commutative version of the supersymmetric nonlinear sigma model, Dirac-harmonic maps from Riemann surfaces were introduced fifteen years ago. They are critical points of an unbounded conformally invariant functional involving two fields, a map from a Riemann surface into a Riemannian manifold and a section of a Dirac bundle which is the usual spinor bundle twisted with the pull-back of the tangent bundle of the target by the map. As solutions to a coupled nonlinear elliptic system, the existence and regularity theory of Dirac-harmonic maps has already received much attention, while the general uniqueness theory has not been established yet. For uncoupled Dirac-harmonic maps, the map components are harmonic maps. Since the uniqueness theory of harmonic maps from a compact surface with boundary is known, it is sufficient to consider the uniqueness of the spinor components, which are solutions to the corresponding boundary value problems for a nonlinear Dirac equation. In particular, when the map components belong to $W^{1,p}$ with $p>2$, the spinor components are uniquely determined by boundary values and map components. For coupled Dirac-harmonic maps, the map components are not harmonic maps. So the uniqueness problem is more difficult to solve. In this paper, we study the uniqueness problem on a compact surface with boundary. More precisely, we prove the energy convexity for weakly Dirac-harmonic maps from the unit disk with small energy. This yields the first uniqueness result about Dirac-harmonic maps from a surface conformal to the unit disk with small energy and arbitrary boundary values.