Infrared Optical Absorption Spectra in Cuprate Two-Leg Spin-Ladder Systems with a Cyclic Four-Spin Interaction

On the basis of a resonating valence-bond description, we investigate the excitation spectrum caused by the Raman process in cuprate two-leg spin ladders with a cyclic four-spin interaction. Owing to a cyclic four-spin interaction, the S</it> = 0 three-triplet excitation energy is reduced and becomes smaller than the S</it> = 0 two-triplet excitation energy around the end of the Brillouin zone. Possible interpretations for the change in the lowest-lying mode are discussed. We show that such a change in the lowest-lying mode yields the shoulder structure at the lower edge in the phonon-assisted optical absorption spectrum.