Low-frequency optical phonons in SrTiO3 under uniaxial stress

The transition from the quantum paraelectric state to the quantum ferroelectric state is well known to be caused by applying a uniaxial stress along the [010]c direction in SrTiO3. In order to clarify the dynamical mechanism of the transition, we have performed a 180° light-scattering experiment under stress at three temperatures of T = 5.0, 10.3, and 16.5 K in the frequency region from 1,000 GHz down to ∼ 60 GHz. We find that the ferroelectric B3u(x) soft mode, which originates from the polar Eu(x, y) mode of a Slater type, does not soften completely, not even at the critical stress σc. This finding indicates that some other transition mechanism, in addition to the condensation of the B3u(x) mode, must be present.