Multiband Hamiltonians of the Luttinger-Kohn Theory and Ellipticity Requirements

Modern applications require a robust and theoretically strong tool for the realistic modeling of electronic states in low dimensional nanostructures. The $k \cdot p$ theory has fruitfully served this role for the long time since its creation. During last two decades several problems have been detected in connection with the application of the $k \cdot p$ approach to such nanostructures. These problems are closely related to the violation of the ellipticity conditions for the underlying system, the fact that until recently has been largely overlooked. We demonstrate that in many cases the models derived by a formal application of the Luttinger-Kohn theory fail to satisfy the ellipticity requirements. The detailed analysis, presented here on an example of the $6 \times 6$ Hamiltonians, shows that this failure has a strong impact on the physically important properties conventionally studied with these models.