Multifractality of wave functions on a Cayley tree: From root to leaves

Abstract

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 $P_q=N⟨|\psi {|}^{2q}(i)⟩$ exhibit a multifractal scaling $P_q\propto N^{-{\tau }_q}$ with the volume (number of sites) $N$ at $N\to \infty$. The multifractality spectrum ${\tau }_q$ 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 ${\tau }_q$ 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 $\sigma$ model. These results are supported by numerical simulations (exact diagonalization) of the conventional ($n=1$) Anderson tight-binding model.

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• How periodic driving stabilises and destabilises Anderson localisation on random trees