Soft Mode Dynamics Above and Below the Burns Temperature in the Relaxor Pb(Mg_1/3Nb_2/3)O_3

Abstract

We report neutron inelastic scattering measurements of the lowest-energy transverse optic (TO) phonon branch in the relaxor Pb(Mg_1/3Nb_2/3)O_3 from 400 to 1100 K. Far above the Burns temperature T_d ~ 620 K we observe well-defined propagating TO modes at all wave vectors q, and a zone center TO mode that softens in a manner consistent with that of a ferroelectric soft mode. Below T_d the zone center TO mode is overdamped. This damping extends up to, but not above, the waterfall wave vector q_wf, which is a measure of the average size of the PNR.

4 pages, 4 figures; modified discussion of Fig. 3, shortened captions, added reference, corrected typos, accepted by Phys. Rev. Lett