Giant phonon softening in the pseudogap phase of the quantum spin system TiOCl

Giant anomalies are found in the temperature dependence of Raman-active phonons in the quantum magnet TiOCl, suggesting the presence of an extended fluctuation regime. This regime coincides with a pseudogap phase identified in earlier NMR experiments. The observed large, local spin-gap is proposed to origin from coupled spin/lattice fluctuations. Below 100 K, in the long-range crystallographically distorted phase, a dimerized ground state with a smaller, global spin-gap of about 2Δ_spin ≈ 430 Êexists. This transition also marks a dimensionality cross-over of the system.