Supersonic propagation of atomic motion by phasons in fresnoite*

Our recent inelastic neutron scattering measurements reveal that waves of atomic motion propagate at surprisingly high speeds of up to 4.3 times the speed of sound in the form of phasons in the piezoelectric mineral fresnoite (Ba₂TiSi₂O₈). Phasons are quasiparticles that exist because of an incommensurate modulation (or ‘wrinkles’) in the flexible framework structure of fresnoite. They are associated with atomic rearrangements that change the phase of waves describing the incommensurate modulation. Phasons are usually overdamped and move diffusively, and are thus much slower than phonons (vibrations involving the translation of atoms, rather than rearrangements). However, the phasons in fresnoite are found not to be overdamped owing to a characteristic rotation of the phasons away from the driving soft phonon, yielding the supersonic propagation. These supersonic phasons enhance thermal conductivity and channel lattice energy at speeds well beyond the limits of phonons.