Spin-phonon coupled modes in the incommensurate phases of doped CuGeO${}_3$

The doping effect of the folded phonon mode at 98 cm${}^{-1}$ was investigated on the Si-doped CuGeO${}_3$ by magneto-optical measurements in far-infrared (FIR) region under high magnetic field. The folded phonon mode at 98 cm${}^{-1}$ appears not only in the dimerized (D) phase but also in the dimerized-anitiferromagnetic (DAF) phase on the doped CuGeO${}_3$. The splitting was observed in the incommensurate (IC) phase and the antiferromagnetically ordered incommensurate (IAF) phase above $H_C$. The split-off branches exhibit different field dependence from that of the pure CuGeO${}_3$ in the vicinity of $H_C$, and the discrepancy in the IAF phase is larger than that in the IC phase. It is caused by the interaction between the solitons and the impurities.