Dopant-bound spinons in Cu_{1-cusive Greek chi}Zn_{cusive Greek chi}GeO₃

Polarized inelastic light scattering experiments on Cu_{1-cusive Greek chi}Zn_{cusive Greek chi}GeO₃ (0 ≤ cusive Greek chi ≤ 0.045) single crystals show for cusive Greek chi ≠ 0 a new distinct mode at nearly half the energy of the singlet response below the spin-Peierls transition. The temperature, magnetic field, polarization, and doping dependences of this mode are similar to those of the singlet bound state. The data are interpreted in terms of a spinon-assisted light scattering process. Position and form of the peak provide strong evidence for the presence of dopant-bound spinons in Cu_{1-cusive Greek chi}Zn_{cusive Greek chi}GeO₃.