抄録
The conductivity tensor at finite frequencies for a narrow s-band is investigated on the basis of the Hubbard model. The frequency-dependent conductivity is expressed in terms of the Fourier transform of a retarded Green function. The Green function is calculated by the use of the equation of motion method. Like Hubbard’s first paper, decoupling approximations are made. The obtained frequency-dependent conductivity tensor expresses the optical absorption due to interband transition of electrons between two subbands split by the correlation effect. The spectrum of this optical absorption has some characteristic features different from the ordinary absorption due to interband transition. It is shown that Re σ(ω) has weak singularities like those for forbidden transitions. To demonstrate the result the expression of the conductivity is applied to the one-dimensional cosineband.
