Twin motion faster than the speed of sound.

Twin growth is commonly thought to be bounded by the velocity of shear waves C(T) at which the information about this mechanical process travels in the material. Here, we report on experimental evidence of twin growth faster than the material's speed of sound. Driven by an electric field, needle twins in a ferroelectric crystal grew at intersonic speed, with an estimated average velocity close to square root(2) C(T). These results strengthen recent theoretical indications of intersonic dislocation motion, and contribute to the understanding of several twin motion-related processes.