Semi-classical models for the schrödinger equation with periodic potentials and band crossings

The Bloch decomposition plays a fundamental role in the study of quantum mechanics and wave propagation in periodic media. Most of the homogenization theory developed for the study of high frequency or semi-classical limit for these problems assumes no crossing of the Bloch bands, resulting in a classical Liouville equation in the limit along each Bloch band. In this article, we derive semi-classical models for the Schrodinger equation in periodic media that take into account band crossings, which is important to describe quantum transitions between Bloch bands. Our idea is still based on the Wigner transform (on the Bloch eigenfunctions), but in taking the semi-classical approximation, we retain the off-diagonal entries of the Wigner matrix, which cannot be ignored near the points of band crossings. This results in coupled inhomogeneous Liouville systems that can suitably describe quantum tunneling between bands that are not well-separated. We also develop a domain decomposition method that couples these semi-classical models with the classical Liouville equations (valid away from zones of band crossings) for a multiscale computation. Solutions of these models are numerically compared with those of the Schrodinger equation to justify the validity of these new models for band-crossings.