Multifractality of wave functions on a Cayley tree: From root to leaves
We explore the evolution of wave-function statistics on a finite Bethe lattice (Cayley tree) from the central site ("root") to the boundary ("leaves"). We show that the eigenfunction moments $P_q=N \left<|ψ|^{2q}(i)\right>$ exhibit a multifractal scaling $P_q\propto N^{-τ_q}$ with the volume (number of sites) $N$ at $N\to\infty$. The multifractality spectrum $τ_q$ depends on the strength of disorder and on the parameter $s$ characterizing the position of the observation point $i$ on the lattice. Specifically, $s= r/R$, where $r$ is the distance from the observation point to the root, and $R$ is the "radius" of the lattice. We demonstrate that the exponents $τ_q$ depend linearly on $s$ and determine the evolution of the spectrum with increasing disorder, from delocalized to the localized phase. Analytical results are obtained for the $n$-orbital model with $n \gg 1$ that can be mapped onto a supersymmetric $σ$ model. These results are supported by numerical simulations (exact diagonalization) of the conventional ($n=1$) Anderson tight-binding model.