Researchers have observed waves of atomic rearrangements, known as phasons, propagating supersonically through a vibrating crystal lattice — a discovery that may dramatically improve heat transport in insulators and enable new strategies for heat management in future electronics devices. It provides a shortcut through the material — a way to send the energy of pure atomic motion at a speed that's higher than what is achievable with phonons (atomic vibrations). The shortcut may open possibilities in heat management of nanoscale materials.
Neutron scattering was used to measure phasons with velocities about 2.8 times and about 4.3 times faster than the natural “speed limits” of longitudinal and transverse acoustic waves, respectively. Insulators are necessary in electronic devices to prevent short circuits, but without free electrons, thermal transport is limited to the energy of atomic motion. Hence, understanding the transport of heat by atomic motion in insulators is important.
The neutrons were scattered in fresnoite, a crystalline mineral so named because it was first found in Fresno, California. It is promising for sensor applications through its piezoelectric property, which allows it to turn mechanical stress into electrical fields. Fresnoite has a flexible framework structure that develops a competing order in the structure that does not match the underlying crystal order, like an overlay of mismatched tiles. Phasons are excitations associated with atomic rearrangements in the crystal that change the phase of waves describing the mismatch in the structure.
Phase differences accumulate in a lattice of wrinkles called solitons. Solitons are solitary waves that propagate with little loss of energy and retain their shape. They can also warp the local environment in a way that allows them to travel faster than sound.
Next, the researchers will explore other crystals that, like fresnoite, can rotate phasons. Strain applied with an electric field may be able to change the rotation; changes in temperature also may vary properties.