Analysis of singularities of area-minimizing currents, Part II: a uniform height bound, estimates away from branch points of rapid decay, and uniqueness of tangent cones

This is the second paper in a series developing a new framework for $n$-dimensional area-minimizing rectifiable currents $T$ of codimension $\geq 2$. In the present article we establish a new height estimate for $T$, which says that in a cylinder in the ambient space, the pointwise distance of $T$ to a union of non-intersecting planes is bounded from above, in the interior, \emph{linearly} by the $L^{2}$ height excess of $T$ relative to the same union of planes, whenever appropriate smallness-of-excess conditions are satisfied. We use this estimate and techniques inspired by the works \cite{Sim93}, \cite{Wic14}, \cite{KrumWic2} to establish a decay estimate for $T$ whenever, among other requirements, $T$ is significantly closer to a union of planes meeting along an $(n-2)$-dimensional subspace than to any single plane. Combined with Theorem~1.1 of Part~I, this implies two main results: (a) $T$ has a unique tangent cone at ${\mathcal H}^{n-2}$ a.e.\ point, and (b) the set of singular points of $T$ where $T$, upon scaling, does not decay \emph{rapidly} to a plane is countably $(n-2)$-rectifiable. In particular, concerning \emph{branch points} of $T$, the work here and in \cite{KrumWica} establishes the fact that rapid decay to a unique tangent plane is the generic behaviour, in the sense that at ${\mathcal H}^{n-2}$ a.e.\ branch point, $T$ decays to a unique tangent plane and has \emph{planar frequency} (or the order of contact with the tangent plane) bounded below by $1 + \alpha$ for some fixed $\alpha \in (0, 1)$ depending only on $n$, $m$ and a mass upper bound for $T$; the planar frequency exists, is uniquely defined and is finite by the approximate monotonicity of the (intrinsic) planar frequency function introduced in Part I.