Relaxation time of anisotropic simple exclusion processes and quantum   Heisenberg models
  Motivated by an exact mapping between anisotropic half integer spin quantum Heisenberg models and asymmetric diffusions on the lattice, we consider an anisotropic simple exclusion process with $N$ particles in a rectangle of $\bbZ^2$. Every particle at row $h$ tries to jump to an arbitrary empty site at row $h\pm 1$ with rate $q^{\pm 1}$, where $q\in (0,1)$ is a measure of the drift driving the particles towards the bottom of the rectangle. We prove that the spectral gap of the generator is uniformly positive in $N$ and in the size of the rectangle. The proof is inspired by a recent interesting technique envisaged by E. Carlen, M.C. Carvalho and M. Loss to analyze the Kac model for the non linear Boltzmann equation. We then apply the result to prove precise upper and lower bounds on the energy gap for the spin--S, ${\rm S}\in \frac12\bbN$, XXZ chain and for the 111 interface of the spin--S XXZ Heisenberg model, thus generalizing previous results valid only for spin $\frac12$. 
Pietro Caputo and Fabio Martinelli