Surface phonons of the superconducting materials NbC(100) and TaC(100)

The dispersion curves of both the optical and acoustical surface phonons in the superconducting compounds NbC(100) and TaC(100) have been determined over the entire Brillouin zone of the Gamma-bar-M-bar symmetry axis by use of angle-resolved high-resolution electron-energy-loss spectroscopy. In contrast to the bulk phonon, no anomalies (dip) due to the electron-phonon coupling in the dispersion curves of the longitudinal phonon have been found at these surfaces. Conversely, the in-plane force constant between carbon and metal atoms is strongly enhanced there