OPTICAL EXPLORATION OF THE MAGNETIC-FIELD VERSUS TEMPERATURE PHASE-DIAGRAM OF CUGEO3

We report polarized optical reflectance and transmission studies of th e spin-Peierls compound, CuGeO3, as a function of temperature and magn etic field. Reflectance ratios (Delta R) are particularly sensitive to small changes in the electronic structure accompanying the uniform-to -dimerized and dimerized-to-incommensurate phase transitions. An inver ted peak at 1.47 eV, with polarization-dependent width and fine struct ure, appears in Delta R at both phase boundaries. We attribute this fe ature to an absorbance peak at the onset of the phonon-assisted Cu2+ d -d band. The field and temperature dependences of the Delta R feature indicate a correlation between the d-d band splitting and structural c hanges induced by the phase transitions.