Broad doublet spectra in SrTiO3

Broad doublet (BD) spectra have been investigated in quantum paraelectric SrTiO3 by a 180degrees Brillouin-scattering experiment. Spectral analyses suggest that the second-sound scenario seems to be unfavorable for explaining the physical origin of BD. We find that the integrated intensity of BD shows quite strong anisotropy; the BD exists clearly around 27 K in the direction Theta = 0.4degrees deviated from [001](c) to [1 (1) over tilde0](c), whereas the BD is almost absent in the direction Theta = 21degrees. We also find that the integrated intensity of the longitudinal acoustic mode decreases as Theta increases, while that of the pure transverse acoustic mode increases. All these anisotropies can be explained by the conventional formula with Pockel's tensor, signifying that the concept of antiphase-domain walls is not needed for such explanation.