Optic modes in PbMg$_{1/3}$Nb$_{2/3}$O$_{3}$ relaxor by hyper-Raman scattering

Hyper-Raman scattering (HRS) experiment is performed in three different single crystals of PbMg$_{1/3}$Nb$_{2/3}$O$_{3}$ (PMN) relaxor [1]. The relative intensities of the band near 250 cm$^{-1}$ in various polarization geometries are fully compatible with the HRS tensor of the $F_{2u} `$silent' mode of the parent $O_{h}$ cubic structure. The temperature dependence of the three $F_{1u }$polar modes was investigated between 20 K and 800 K. High-resolution data reveal that the lowest $F_{1u}$ mode is splitted up to the highest investigated temperatures. On cooling, the low frequency component strongly decreases in frequency and exhibits clear soft mode behaviour with a crossover from a vibrational to a relaxational behaviour around $T_{d} \quad \approx $ 620 K [2]. This observation gives a new insight into the low-frequency dynamics of PMN-type relaxor systems. \\[4pt] [1] A. Al-Zein \textit{et al.}, PRB \textbf{78}, 134113 (2008)~; B. Hehlen \textit{et al.}, PRB \textbf{75}, 052104 (2007). \\[0pt] [2] A. Al-Zein \textit{et al.}, submitted to PRL.