= k·p perturbation theory
{title2=$H_{\mathbf k}=H_0+\hbar\mathbf k\cdot\mathbf p/m_0+\hbar^2k^2/(2m_0)$}
For a local periodic scalar potential, writing a Bloch wave as $e^{i\mathbf k\cdot\mathbf r}u_{n\mathbf k}$ gives the displayed cell-periodic Hamiltonian before additional spin-orbit terms. Momentum matrix elements mix zone-centre bands when k is varied. Perturbatively eliminating remote bands changes the <effective mass>; a multiband retained subspace describes degeneracy, nonparabolicity and coupled hole bands. Spin-orbit coupling and the chosen reference wavevector must be included consistently.
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