Phonon scattering at interfaces

• The heat-pulse technique was first demonstrated for solids by von Gutfeld and Nethercot in 1964.
• In 1969, Taylor, Maris, and Elbaum noted that for all crystals a large difference in the intensities of the longitudinal and transverse pulses depended on the propagation directions. This effect was called phonon focusing.
• Phonon focusing occurs because elastic waves in anisotropic media have a group velocity that in general is not parallel to the wavevector.
  • the direction of energy propagation is not normal to the wavefronts. In particular, waves with quite different k direction can have nearly the save Vg. So that the energy flux associated with those waves bunches along certain crystalline direction.
  • As a consequence of the noncollinear k and Vg, an isotropic distribution of wavevectors emanating from a point source of heat can produce a highly anisotropic distribution of energy flux.
  • Hensel and Dynes found an extreme anisotropy in the nonequilibrium phonon flux emanating from a point source. (measured by a narrow bolometer)

The kapitza problem

• Kapitza’s experiments in 1941, sharp temperature gradient at the interface of a heated solid immersed in liquid He.
• Khalatnakov later realized that this thermal boundary resistance could be explained from a simple acoustic mismatch theory.
  • aoustic impedances Z= rhonu (rho is the density, nu is the speed of sound)
  • however, based on the acoustic impedance, he predict that the average transmission coefficient between solid and liquid should be approximately 1%, whereas the measured thermal boundary resistances corresponded to a transmission coefficient of approximately 50%.
  • [Wyatt and Page] employed ballistic heat pulses to continuous scan the incident angle of phonons at solid/liquid-He interface.

[Wyatt and Page] https://daringfireball.net/projects/markdown/

solid/solid interfaces

• total internal reflection at sepcially prepared solid/solid interfaces has been observed.