Experimental Evidence of Breakdown of Simple Pseudopotential Theory for Lattice Dynamics of Beryllium

We present dispersion curves for phonons along $\Gamma \mathrm{KM}$ in Be. It is shown that the ordering of the branches corresponding to the phonon modes polarized in the basal plane at the symmetry point $K$ provides a simple and direct qualitative criterion for the presence of tensor forces. The existing pseudopotential calculations of the dynamics are inherently incapable of explaining these results. Generalizations required in the theory are mentioned.