Low-energy excitations in impurity substituted CuGeO3 - art. no. 134414

We report far-infrared reflectance measurements of Zn- and Si-doped CuGeO3 single crystals as a function of applied magnetic field at low temperature. Overall, the low-energy far-infrared spectra are extraordinarily sensitive to the various phase boundaries in the H-T diagram, with the features brin g especially rich in thr low-temperature dimerized state. Zn impurity subst itution rapidly collapses the 44 cm(-1) zone-boundary spin Peierls gap, alt hough broadened magnetic excitations are observed at the lightest doping le vel (0.2%) and a remnant is still observable at 0.7% substitution. In a 0.7 % Si-doped sample, there is no evidence of the spin gap. Impurity substitut ion effects on the intensity of the 98 cm(-1) zone-folding mode are strikin g as well. The lightly doped Zn crystals display an enhanced response, and even at intermediate doping levels, the mode intensity is larger than that in the pristine material. The Si-doped sample also displays an increased in tensity of the 98 cm(-1) mode in the spin Peierls phase relative to the pur e material. The observed trends are discussed in terms of the effect of dis order on the spin gap and 98 cm(-1) mode, local oscillator strength sum rul es, and broken selection rules.