The role of band-to-band coupling in generating warped bands can be understood by considering the α2 and ζ2 terms in Eq. ( ). The two terms originate from the A0,z and A0,0 functions, respectively, and are therefore found in the diagonal components of the Hamiltonian. As a result, the two terms together modulate the curvatures of the bands directly as expressed in Eq. ( ). Using second-order k • p perturbation theory, it can be seen that 21