Wigner measures and effective mass theorems

TL;DR

This paper extends the derivation of effective mass equations for electrons in crystals, including degenerate cases, using Wigner measures and Floquet-Bloch analysis, providing new operator-valued equations in higher dimensions.

Contribution

It generalizes effective mass theorems to degenerate critical points, introducing operator-valued equations and advancing the mathematical understanding of semi-classical electron dynamics.

Findings

Derived effective mass equations for non-degenerate critical points.

Established new operator-valued effective mass equations for degenerate critical points.

Applied Wigner measure theory to analyze dispersion effects in crystal electron dynamics.

Abstract

We study a semi-classical Schr{\"o}dinger equation which describes the dynamics of an electron in a crystal in the presence of impurities. It is well-known that under suitable assumptions on the initial data, the wave function can be approximated in the semi-classical limit by the solution of a simpler equation, the effective mass equation. Using Floquet-Bloch decomposition and with a non-degeneracy condition on the critical points of the Bloch bands, as it is classical in this subject, we establish effective mass equations for more general initial data. Then, when the critical points are degenerated (which may occur in dimension strictly larger than one), we prove that a similar analysis can be performed, leading to a new type of effective mass equations which are operator-valued and of Heisenberg form. Our analysis relies on Wigner measure theory and, more precisely, to its applications to the analysis of dispersion effects.