Dynamical lattice instability versus spin liquid state in a frustrated spin chain system

The low-dimensional s = 1/2 compound (NO)[Cu(NO sub(3)) sub(3)] has recently been suggested to follow the Nersesyan-Tsvelik model of coupled spin chains. Such a system shows unbound spinon excitations and a resonating-valence-bond ground state due to spin frustration. Our Raman scattering study of (NO)[Cu(NO sub(3)) sub(3)] demonstrates phonon anomalies as well as the suppression of a broad magnetic scattering continuum for temperatures below a characteristic temperature, T < T* [asymptotically =] 100 K. We interpret these effects as evidence for a dynamical interplay of spin and lattice degrees of freedom that might lead to an instability at lower temperatures.