where σ σ x , σ σ y and σ σ z are the Pauli matrices related to the spin S of the electrons as S = 1 2 ℏ ℏ σ σ (here ℏ ℏ is the reduced Planck constant), and p x , p y and p z are the components of the momentum in the crystallographic directions [100], [010] and [001], respectively.