Minimal diffeomorphisms with $L^1$ Hopf differentials

We prove that for any two Riemannian metrics $\sigma_1, \sigma_2$ on the unit
disk, a homeomorphism $\partial\mathbb{D}\to\partial\mathbb{D}$ extends to at
most one quasiconformal minimal diffeomorphism $(\mathbb{D},\sigma_1)\to
(\mathbb{D},\sigma_2)$ with $L^1$ Hopf differential. For minimal Lagrangian
diffeomorphisms between hyperbolic disks, the result is known, but this is the
first proof that does not use anti-de Sitter geometry. We show that the result
fails without the $L^1$ assumption in variable curvature. The key input for our
proof is the uniqueness of solutions for a certain Plateau problem in a product
of trees.