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

We report infrared reflectance measurements of GeCuO3 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 cm(-1) and a modification of the structure near 44 cm(-1) 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 cm(-1), 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.