Spurious solutions and boundary conditions in k center dot p theory

It is well known that the origin of one type of spurious solutions in multiband k(.)p theory is the failure to restrict the Fourier coefficients of the envelope functions to the first Brillouin zone. Often, the set of differential equations obtained is supplemented with interfacial boundary conditions derived by integrating the differential equations across the interface; however, this leads to a mathematically ill-posed problem as the envelope functions cannot simultaneously fulfill these boundary conditions and the requirement that the Fourier coefficients be restricted to the first Brillouin zone. We show, by way of an example, the origin of these spurious solutions and how to remove them