Metastability on the hierarchical lattice
  We study metastability for Glauber spin-flip dynamics on the $N$-dimensional hierarchical lattice with $n$ hierarchical levels. Each vertex carries an Ising spin that can take the values $-1$ or $+1$. Spins interact with an external magnetic field $h>0$. Pairs of spins interact with each other according to a ferromagnetic pair potential $\vec{J}=\{J_i\}_{i=1}^n$, where $J_i>0$ is the strength of the interaction between spins at hierarchical distance $i$. Spins flip according to a Metropolis dynamics at inverse temperature $\beta$. In the limit as $\beta\to\infty$, we analyse the crossover time from the metastable state $\boxminus$ (all spins $-1$) to the stable state $\boxplus$ (all spins $+1$). Under the assumption that $\vec{J}$ is non-increasing, we identify the mean transition time up to a multiplicative factor $1+o_\beta(1)$. On the scale of its mean, the transition time is exponentially distributed. We also identify the set of configurations representing the gate for the transition. For the special case where $J_i = \tilde{J}/N^i$, $1 \leq i \leq n$, with $\tilde{J}>0$ the relevant formulas simplify considerably. Also the hierarchical mean-field limit $N\to\infty$ can be analysed in detail. 
Frank den Hollander and Oliver Jovanovski
math.PR