EFFECTS OF IN-CHAIN AND OFF-CHAIN SUBSTITUTIONS ON SPIN FLUCTUATIONS 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 CuGeO3 has been st udied using Raman scattering in order to understand the interplay betw een defect induced states, enhanced spin-spin correlations, and the: g round state of low-dimensional systems. In-chain and off-chain substit utions quench the spin-Peierls state and induce 3D antiferromagnetic o rder at T less than or equal to 5 K. Consequently the suppression of a 1D gap-induced mode as well as a constant intensity of a spinon conti nuum are observed at low temperatures. A 3D two-magnon density of stat es now gradually extends to higher temperatures T less than or equal t o 60 K compared with purl CuGeO3. This effect is more pronounced in th e case of off-chain substitutions (Si) for which a Neel state occurs o ver 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 smal l amount of substitutions. The results are compared with recent Monte Carlo studies on substituted spin ladders, pointing to a similar insta bility of coupled, dimerized spin chains and spin ladders upon substit ution.