Investigation of soliton bound states in the Raman spectrum of pure and doped spin-Peierls chains

We investigate the occurrence of singlet bound states in the Raman spectrum of dimerized spin 1/2 chains by Exact Diagonalization and Density Matrix Renormalization Group techniques. We predict that several bulk $\delta$-peaks could be observed in pure systems. Furthermore, we show that new low energy lines arise from non-magnetic impurity doping. These features are interpreted in terms of soliton-antisoliton and soliton-impurity bound states respectively. Energies and spectral weights associated with these bound states are sensitive to lattice relaxation effects. Our results are discussed in the context of the inorganic spin-Peierls compound CuGeO$_3$ and quantitatively compared to recent Raman experiments.