  When two piezoelectric solids are placed in close proximity, acoustic waves
(phonons) can "tunnel" across a vacuum gap transmitting energy between the two
solids. Here, we demonstrate analytically that not only is such a phenomenon
possible, but that a simple resonance condition exists for which complete
transmission of the incoming wave is possible, physically corresponding to the
excitation of leaky surface waves. This result is derived for an arbitrary
anisotropic crystal symmetry and orientation. We also show that the complete
transmission condition can be related to the surface electric impedance and the
effective surface permittivity of the piezoelectric material, making it
possible to be determined experimentally. In addition, we present numerical
results for the maximum power transmittance of a slow transverse wave,
tunneling between identical ZnO crystals, as function of all possible crystal
orientations. The results show a large range of orientations for which complete
tunneling can be achieved.
