Dirac cone dispersion of lamb waves in plates

Propagating waves with infinite phase velocity and non-zero group velocity exist in simple homogenous isotropic plates. These waves have conical dispersion at zero wavenumber and occur for discrete values of Poisson's ratio. This anomalous dispersion is a result of degeneracies between longitudinal and transverse thickness resonances, of like symmetry. Here we demonstrate these so called Dirac point or conical point waves by cooling an aluminum plate in order to tune Poisson's ratio to a degenerate case. Linear dispersion is measured for forward and backward propagating waves proximate to zero wavenumber. In addition, we show that conical point waves exhibit angle independent mode conversion from the free edge of a plate. We propose that simple elastic plates can be used to study Dirac cone dispersion in other systems and that the unusual behavior of such waves in plates may find use in acoustic devices or nondestructive testing.