Abstract: Crystals or structures with incompatible translational periodicities are described as incommensurate. Such structures exhibit unique dynamical properties associated with the fact that the phase relationship between incommensurate elements can be shifted without changing the energy of the overall system. This allows for highly supersonic acoustic-like waves called phasons, or super lubricity (near zero friction) in the sliding of incommensurate surfaces. Using inelastic neutron scattering and thermal conductivity measurements, we establish that phasons in the piezoelectric mineral fresnoite make a major contribution to thermal conductivity by propagating many times further and faster than acoustic phonons. The phason contribution to thermal conductivity is maximum near room temperature, where it is the single largest contributing degree of freedom, while phonons still dominate at cryogenic temperatures. This work shows how excitations unique to incommensurate lattices can dominate transport properties and must therefore be considered when attempting to model thermal transport behavior in such systems.
M. E. Manley1, *, A. F. May1, B. L. Winn2, D. L. Abernathy2, R. Sahul3, R. P. Hermann1
1Material Science and Technology Division, Oak Ridge National Lab, Oak Ridge, TN 37831, USA