Effects of in-chain and off-chain substitutions on spin fluctuations in the spin-Peierls compound CuGeO_3

TL;DR

This study investigates how in-chain and off-chain substitutions affect spin fluctuations and magnetic ordering in CuGeO_3, revealing that substitutions suppress the spin-Peierls state and promote 3D antiferromagnetic order, with off-chain substitutions having a more pronounced effect.

Contribution

It provides new insights into how substitutions influence spin dynamics and magnetic phase transitions in low-dimensional quantum spin systems, supported by Raman scattering and Monte Carlo comparisons.

Findings

Substitutions quench the spin-Peierls state and induce 3D antiferromagnetic order.

Off-chain substitutions extend the 3D two-magnon density of states to higher temperatures.

Additional low energy excitations are induced by substitutions.

Abstract

The effect of in-chain and off-chain substitutions on 1D spin fluctuations in the spin-Peierls compound 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\leq 5 K. Consequently a 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\leq 60K compared with pure 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.