Spectral and convex geometry of $α$-harmonic maps and sharp Morse index rigidity

Abstract

We develop a weighted scalar spectral geometric description of Sacks-Uhlenbeck $α$-harmonic maps into round spheres and identify its convex geometry realization for $α$-harmonic diffeomorphisms onto the round $2$-sphere. More precisely, we establish a one-to-one correspondence, modulo orientation preserving isometries of $\mathbb{S}^2$, between orientation preserving $α$-harmonic diffeomorphisms onto the round $2$-sphere and positive Codazzi tensors satisfying a determinant bound and a compatibility condition with the prescribed metric on $\mathbb{S}^2$. These tensors are the radii tensors of auxiliary strictly convex surfaces, and the weighted scalar forms in spectral geometry equal $-α$ times the second variations of the enclosed volumes with respect to the support functions. Using these spectral geometry and convex geometry results, we prove a sharp Morse index characterization of isometric equatorial parametrizations among $α$-harmonic $2$-spheres in the round $n$-sphere. We also prove that every $α$-harmonic diffeomorphism between unit round $2$-spheres is orthogonal for all $α>1$. Equivalently, the only admissible positive Codazzi tensor whose reconstructed metric on $\mathbb{S}^2$ has Gaussian curvature $1$ is $(κ+2)^{α-1}\bar{g}$, where $\bar{g}$ is the unit round metric.