Phonon spectra for epitaxial monolayer films of hexagonal boron nitride $\left(h-\mathrm{BN}\right)$ on Ni, Pd, and Pt (111) have been measured for the first time by using high resolution electron energy loss spectroscopy. On Ni(111), we have found two remarkable features in the observed dispersion curves: a level splitting at the crossing point of two phonon branches and degeneracy of two optical phonons with in-plane polarization at $\overline{\gamma }$, which means the rumpled structure of the $h-\mathrm{BN}$ film and the elimination of electric fields parallel to the film, respectively.

• Received 9 June 1997

DOI:https://doi.org/10.1103/PhysRevLett.79.4609