Soft longitudinal modes in spin-singletCuGeO3

Abstract
We present results from a neutron-scattering study of lattice anomalies associated with the transition to the gapped spin-singlet state observed in the spin-1/2 linear-chain compound CuGeO3. Instead of the lattice dimerization along the chain direction (c axis) expected for a spin-Peierls system, we observe a remarkable softening of the longitudinal acoustic phonons in a direction (along the b axis) perpendicular to the chains. A spontaneous strain appears in the same direction below the magnetic transition temperature of 14 K.