Coupling of GHz phonons to ferroelastic domain walls in SrTiO3.

: We study the linear and nonlinear acoustic response of SrTiO3 across its ferroelastic transition at Ta=105  K by time domain Brillouin scattering. Above Ta we observe that for a strain amplitude of ∼0.18% the sound velocity for compressive strain exceeds the tensile strain velocity by 3%. Below Ta we find a giant slowing down of the sound velocity by 12% and attribute this to the coupling of GHz phonons to ferroelastic twin domain walls. We propose a new mechanism for this coupling on the ultrafast time scale, providing an important new test ground for theories used to simulate atomic motion in domain forming crystals.