Inelastic neutron scattering study of the soft phonon branch in deuterated BCCD

Abstract

Inelastic neutron scattering on deuterated BCCD revealed a transverse acoustic branch of Λ 3 symmetry coupled to a soft polarisation branch Py( k) of same symmetry. As the transition to the incommensurate state is approached from above (T>T 1 = 164 K), the lowest-frequency branch develops a broad dispersion minimum near k =0.32c ∗ . The soft mode is observed as an underdamped excitation down to 170K. The present results establish the displacive nature of the phase transition at T 1 and the P y- symmetry of the primary order parameter. A semi-quantitative scheme for the 14 lowest phonon branches along the c ∗ direction, in the normal phase, is suggested, based on the available neutron-scattering, infrared, Raman, and acoustic data.