Optical observation of the interplay between magnetic and elastic energy in a spin-Peierls system

We report infrared reflectance measurements of GeCu${\mathrm{O}}_3$ single-crystal samples as a function of temperature and applied magnetic field. Upon passing through the 14-K spin-Peierls transition, both a softening of the $B_{3u}$ shearing mode near 300 ${\mathrm{cm}}^{-1\mathrm{}}$ and a modification of the structure near 44 ${\mathrm{cm}}^{-1\mathrm{}}$ is observed. Application of a magnetic field in the low-temperature phase modifies the aforementioned $B_{3u}$ shearing mode and causes an unusual structure to develop in the reflectance ratio near 44 ${\mathrm{cm}}^{-1\mathrm{}}$, which moves in a systematic way with increasing field. Above the 12.5-T dimerized→magnetic phase boundary, only the center feature of this structure remains. The field dependence of the dips in the reflectance ratio display a Zeeman-like splitting, and we attribute the disappearance of the reflectance ratio dips to the 12.5-T transition to the magnetic phase.