The Anisotropy of Phonon Propagation in Some Cubic and Trigonal Crystals
It is now well known that the propagation of acoustic phonons in the ballistic regime is anisotropic in all crystals. This is because the propagation is entirely governed by the elastic modulus tensor, which is a fourth rank tensor and hence can have the same symmetry elements as, say, a cubic crystal, and yet not be spherical.
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