Wang Litian, Ryne Kent G
Ostfold University College, Engineering, 1757 Halden, Norway.
J Acoust Soc Am. 2007 Oct;122(4):1873-5. doi: 10.1121/1.2770546.
Acoustic wave propagation in elastic media is characterized by the slowness surface. The slowness surface consists of three sheets associated with three modes of wave propagation and the two outer sheets can have zero-curvature locally. It is shown that the outmost sheet can admit extraordinary zero-curvature and the slowness curve can appear as a straight line locally. Using the perturbation method, the conditions for the extraordinary zero-curvature are derived analytically without violating the thermodynamic condition for elastic media. The results can be applied to crystals with higher symmetry and to the study of phonon focusing and surface waves.
弹性介质中的声波传播由慢度曲面表征。慢度曲面由与三种波传播模式相关的三个片层组成，并且两个外部片层在局部可以具有零曲率。结果表明，最外层片层可以呈现异常零曲率，并且慢度曲线在局部可以表现为一条直线。使用微扰方法，在不违反弹性介质热力学条件的情况下，解析地推导了异常零曲率的条件。这些结果可应用于具有更高对称性的晶体以及声子聚焦和表面波的研究。
