Semiconductor-to-metal transition in an ultrathin interface: Cs/GaAs(110)

Electronic excitation spectra (0≤ħω<4 eV) of an ultrathin Cs/GaAs(110) interface have been measured at various stages of development with electron-energy-loss spectroscopy. Spectral features which appear as a function of Cs coverage clearly identify the semiconductor-to-metal transition. The semiconducting interface observed up to one Cs layer is characterized as a highly correlated electronic system—a two-dimensional Mott insulator. Interfacial metallization upon Cs multilayer growth is identified by a metallic-excitation continuum and a collective excitation related to the Cs surface plasmon.