Michael G. Mitch

Raman scattering and electron-phonon coupling in A3C60

Abstract Raman scattering measurements performed in situ in UHV on thin and ultrathin films of doped C60 are reported for variations in alkali-metal type, composition and temperature. The results emphasize the important role of disorder on Raman processes and the probing of electron-phonon coupling relevant to superconductivity. Detailed comparisons of free carrier phonon damping effects on selected radial and tangential intramolecular phonons are discussed in Rb3C60 and K3C60, using nonmetallic Na3Ca60 as a reference. Elevated temperature studies of Na3C60 suggest a transition to a more metallic state. Intennolecular Raman scattering associated with alkali-metal motion, weakly coupled to C60, clusters is reported. The effects of alkali size, lattice thermal expansion and disorder are discussed.

Raman Scattering of Alkali-Metal Fullerides

Raman scattering studies of alkali-metal doped ultrathin and thin film materials for 0 < x ≤ 3 provide information on charge transfer, electron-phonon coupling and disorder effects. Studies of variations in composition, alkali-metal type as well as thermal effects provide a means of determining the vibrational modes of significance for a BCS type mechanism for superconductivity. Thermal studies of Na3C60 provide evidence for a structural transition of importance for understanding the nonmetallic behavior and Raman scattering. Very weak, low frequency Raman measurements further indicate variations in intermolecular coupling associated with predominant motion of alkali-metal atoms.

Intermolecular Vibrations in Fullerene Systems

Intermolecular vibrations in a variety of fullerene systems are reported that provide information on C 60 -C 60 and alkali-metal - C 60 interactions and disorder effects. Low frequency Raman scattering in laser polymerized C 60 films are shown to exhibit additional modes not reported for bulk materials. This result is consistent with recent dimer calculations. Raman scattering and HREELS measurements are shown to provide complementary information on alkali-metal motions in K 3 C 60 . In FCC Na 6 C 60 both octahedral site disorder effects and reduced Na-C bond lengths account for changes in the Raman spectrum relative to Na 3 C 60 .

Photoemission Spectra of C60 Clusters in Metal and Nonmetal Systems

Changes in the weighted electronic density of states of C60 molecules deposited on a variety of substrates have been studied by HeI ultraviolet photoemission (UPS). Spectra of C60 deposited on polycrystalline Ag exhibit a C60, LUMO-derived band which appears to originate in interactions at the interface. C60 was also exposed to Rb, developing a LUMO-derived band and similar phase characteristics to those observed in KxC60 compounds. The effect of atomic H as an adsorbate was also investigated. While adsorbates generally show increased broadening of the πp and σp occupied states of C60 with increasing coverage, exposure to atomic H resulted in spectra where the distinct molecular features were completely lost, bearing a distinct resemblence to graphite. However, Raman spectroscopy indicates the essential C60 structure has not been changed.