The unperturbed Hamiltonian refers to the band-edge spin-orbit system (for k =0). At the band edge, the conduction band Bloch waves exhibits s-like symmetry, while the valence band states are p-like (3-fold degenerate without spin). Let us denote these states as | S ⟩ ⟩ , and | X ⟩ ⟩ , | Y ⟩ ⟩ and | Z ⟩ ⟩ respectively. These Bloch functions can be pictured as periodic repetition of atomic orbitals, repeated at intervals corresponding to the lattice spacing. The Bloch function can be expanded in the following manner: