Interpretation of phonon anomalies in SrTiO3near 37K

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Abstract
The “extra” transverse acoustic (TA) mode reported by Courtens et al. [Phys. Scripta T49, 430 (1993)] and the loss of scattering intensity for the TA [100] phonon reported by Vacher et al. [Europhys. Lett. 17, 45 (1992)] for strontium titanate below 37K have previously been interpreted as evidence for a coherent quantum state or, more recently [E. Courtens, Workshop on Fundamental Experiments in Ferroelectricity, Williamsburg, Va., Feb 1995, in press] as evidence for second sound. In the present letter we point out that 37K is the exact temperature at which the ferroelectric soft mode (A2u + Eu) becomes accidentally degenerate with the antiferrodistortive soft mode (Eg). This crossing permits a sharp two-phonon difference scattering with acoustic phonon-like characteristics. A second crossing (Alg with A2u + u) occurs at 70K.