Phonon spectra on ultrathin Pb films with scanning tunneling spectroscopy

After Blatt and Thomson's prediction [Phys. Rev. Lett. 10, 332 (1963)], several groups have reported the quantum size effect on transition temperature(Tc) as a function of thicknesses of atomically flat ultrathin Pb film. In those cases, Tc oscillation related to film thickness was attributed to oscillation of the density of states (DOS) near the Fermi energy. However, the Tc oscillation amplitude is much smaller than that derived from the DOS oscillation. One therefore would ask: What is the role of electron-phonon interaction? Also as reported by Qin, et. al. [Science 324, 1314 (2009)], when the film is only 2ML thick, the pseduomorphically strained film has lower Tc than the unstrained one, suggesting that interfacial phonons may play a role. To answer to above question, we perform layer-dependent scanning tunneling spectroscopy of Pb films on Si(111) at 2.3 K to observe the phonon related features in the tunneling spectra. Detailed analysis of thickness dependence of photon spectra will be reported.