P. Lemmens et al., EFFECTS OF IN-CHAIN AND OFF-CHAIN SUBSTITUTIONS ON SPIN FLUCTUATIONS IN THE SPIN-PEIERLS COMPOUND CUGEO3, Physical review. B, Condensed matter, 55(22), 1997, pp. 15076-15083
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.