Tunneling for a class of Difference Operators: Complete Asymptotics

We analyze a general class of difference operators <tex>H_\varepsilon = T_\varepsilon +V_\varepsilo</tex> on <tex>\ell^2((\varepsilon\mathbb{Z})^d)</tex>, where
<tex>V_\varepsilon</tex> is a multi-well potential and <tex>\varepsilon</tex> is a small parameter. We derive full asymptotic expansions
of the prefactor of the exponentially small eigenvalue splitting due to interactions between two
“wells” (minima) of the potential energy, i.e., for the discrete tunneling effect. We treat both
the case where there is a single minimal geodesic (with respect to the natural Finsler metric
induced by the leading symbol<tex> h_0(x, ξ)</tex> of <tex>H_\varepsilon</tex>) connecting the two minima and the case where
the minimal geodesics form an k+1 dimensional manifold, k ≥ 1. These results on the tunneling
problem are as sharp as the classical results for the Schrödinger operator in [Helffer, Sjöstrand,
1984]. Technically, our approach is pseudodifferential and we adapt techniques from [Helffer,
Sjöstrand, 1988] and [Helffer, Parisse, 1994] to our discrete setting.