A possible scenario for two soft branches in PMN

Abstract

The acoustic–optic mode coupling model proposed earlier as a framework for interpretation of the apparent vertical soft optic mode branch dispersion in lead-based perovskite relaxors has been tentatively extended by taking into account the two-component nature of the lowest frequency optic phonon mode. Inelastic scattering spectral function and the course of the apparent phonon dispersion branches were calculated for parameters relevant to Pb(Mg1/3Nb2/3)O3 (PMN) relaxor. Dynamical parameters of the zone-centre soft mode components in the problematic region around the Burns temperature have been described with the help of the recent hyper-Raman scattering study [A. Al-Zein, J. Hlinka, J. Rouquette, and B. Hehlen, Soft mode doublet in PbMg1/3Nb2/3O3 relaxor investigated with hyper-Raman scattering, Phys. Rev. Lett. 105 (2010), p. 017601] on PMN. We show that the vertical dispersion (waterfall anomaly) is primarily due to the coupling of the transverse acoustic branch to the lower frequency component of the TO1 phonon branch. As in the inelastic neutron scattering experiments, in this model the temperature interval in which the vertical dispersion (waterfall anomaly) appears is roughly restricted by the freezing and Burns temperatures.