Figure 1.

(a) Schematic of a tunneling mosaic cell, with doubled circles denoting atomic tunneling displacements. The boundary's location is variable, as illustrated (in an exaggerated manner) by the blue and red lines. The domain wall distortion amounts to populating ripplon states on top of the structural transition energy ɛ, as shown in b. All transitions exemplified by solid lines involve tunneling between the intrinsic states. Those transitions are coupled linearly to the lattice distortion and contribute the strongest to the phonon scattering. The “vertical” transitions, denoted by the dashed line, are coupled to the higher order strain; they contribute only to the Rayleigh scattering (which is too weak to account for the plateau; ref. 17).