Plasma excitation in misfit-layer compounds

Plasmons in the misfit-layer compounds MTS₃ (M=Pb, Sn, Bi, or Sm; T=Ti, V, or Nb), which consist of alternatively stacked MS and TS₂ layers of about 6 A{} thickness, are studied by means of reflection-electron-energy-loss spectroscopy (REELS). The incident-energy dependence of the excitation energy is investigated by the measurements of the REELS spectra in the range 100--2000 eV in both the pulse-counting mode and the voltage-modulation mode. The volume plasmon, the interface plasmon, and the optical and acoustic plasmons of the misfit-layer compounds are discussed. At high and intermediate incident energies, an interface plasmon, such as a plasmon observed at a boundary of two semi-infinite media, or an optical plasmon is generated. At low incident energy, only a volume plasmon of a TS₂ layer is generated and plasma oscillations within an MS layer are suppressed, decaying into single-particle excitations. No clear evidence for acoustic plasmons has been obtained, although unidentified structures are often observed.