Surface phonons onBi2Sr2CaCu2O8+δ

We report measurements of surface optical phonons on ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_{8+\mathrm{\delta }}$ with high-resolution electron-energy-loss spectroscopy (HREELS). In addition to peaks near 50 and 80 meV (403 and 645 ${\mathrm{cm}}^{\mathrm{-}1}$), which have been previously observed, our loss spectra exhibit a peak at 26 meV (210 ${\mathrm{cm}}^{\mathrm{-}1}$). Loss spectra were measured at temperatures from 45 to 146 K, and the temperature dependence of the peaks was found to be weak. The 50 and 80 meV peaks shift to lower frequency by ∼1.5 meV over this temperature range. All three peaks are attributed to surface optical phonons. The identification of particular bulk modes corresponding to the surface modes observed with HREELS is discussed.