Effects of in-chain and off-chain substitutions on spin fluctations in the spin-Peierls compound CuGeO3

The effect of in-chain and off-chain substitutions on one-dimensional (1D) spin fluctuations in the spin-Peierls compound ${\mathrm{CuGeO}}_{3}$ has been studied using Raman scattering in order to understand the interplay between defect induced states, enhanced spin-spin correlations, and the ground state of low-dimensional systems. In-chain and off-chain substitutions quench the spin-Peierls state and induce 3D antiferromagnetic order at T\ensuremath{\leqslant}5 K. Consequently the suppression of a 1D gap-induced mode as well as a constant intensity of a spinon continuum are observed at low temperatures. A 3D two-magnon density of states now gradually extends to higher temperatures T\ensuremath{\leqslant}60 K compared with pure ${\mathrm{CuGeO}}_{3}$. This effect is more pronounced in the case of off-chain substitutions (Si) for which a N\'eel state occurs over a larger substitution range, starting at very low concentrations. Besides, additional low energy excitations are induced. These effects, i.e., the shift of a dimensional crossover to higher temperatures are due to an enhancement of the spin-spin correlations induced by a small amount of substitutions. The results are compared with recent Monte Carlo studies on substituted spin ladders, pointing to a similar instability of coupled, dimerized spin chains and spin ladders upon substitution.