Excitation of electrons between parallel or near-parallel one-electron bands in simple polyvalent metals constitutes a major source of the observed optical absorption. Much of the effect can be accounted for in a straightforward calculation of both real and imaginary parts of the conductivity, which does not require the constant-matrix-element assumption. In many cases, the magnitude and rounding of the absorption edges (singular in the absence of scattering) are quite sensitive to the phenomenological relaxation times (and hence to temperature) and to surface scattering. The sum rule for the (transverse) optical conductivity is related to the Fourier components of the weak periodic potential, and an expression is derived for the optical mass. The theory has been applied to study the optical properties of Al.

• Received 21 May 1970

DOI:https://doi.org/10.1103/PhysRevB.3.1898