Extinction of the Kapitza anomaly for phonons along the surface normal direction

We have succeeded in extinguishing the anomalous Kapitza transmission of phonons whose wave vector is normal to a surface treated by conventional means and handled in air. In the same experiments, phonons approaching the surface with oblique wave vectors are anomalously transmitted. We argue that these results demonstrate that the Kapitza anomaly is due to surface defects which couple to the phonon strain field.