Cold neutron inelastic scattering measurements of the spin-Peierls and antiferromagnetic excitations in Si-doped CuGeO3 single crystals

Cold neutron inelastic scattering experiments on four CuGe1-xSixO3 single crystals with x=0, 0.004, 0.01, and 0.02 have been performed to study the effects of Si doping on the antiferromagnetic (AFM) and spin-Peierls (SP) excitations with high energy resolution. The intrinsic energy width Γ of the AFM excitation measured along the c* ([001]) direction near the magnetic zone center (0, 1, 1/2) (q=0) varies as q2 for q≤ 0.032 A-1=0.06 qZB. This observation directly supports the recent theoretical predictions of Saito and Fukuyama. For pure CuGeO3 the intrinsic SP excitation energy width appears to be resolution-limited up to 0.085 A-1. Upon doping, the widths of the SP excitations increase dramatically, but they are weakly q-dependent in the range we investigated. At x=0.02 the SP phase is lost. The existence o a second gap in CuGeO3, first observed by Ain et al., is also confirmed. This second gap, which is quite sharp in the pure compound, is considerably broadened at Si concentrations as low as x=0.004. (author)