This expression shows explicitly where the dominance of EDSR over the electron paramagnetic resonance comes from. The numerator m 0 c 2 ≈ ≈ 0.5 M e V of the second factor is a half of the Dirac gap while E G is of atomic scale, E G ≈ ≈ 1eV. The physical mechanism behind the enhancement is based on the fact that inside crystals electrons move in strong field of nuclei, and in the middle of the periodic table the product Z α α of the atomic number Z and the fine-structure constant α α is of the order of unity, and it is this product that plays the role of the effective coupling constant, cf. spin–orbit coupling. However, one should bear in mind that the above arguments based on effective mass approximation are not applicable to electrons localized in deep centers of the atomic scale. For them the EPR is usually the dominant mechanism.