Matej Jošt

Photoemission and inverse photoemission studies of thin films of ${\mathrm{C}}_{60}$ and ${\mathit{C}}_{70}$ reveal the distribution of occupied and empty electronic states of these molecular solids. X-ray photoemission results also show the C 1s main line and features related to \ensuremath{\pi}-${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ shakeups, electron energy losses, and plasmons. Potassium doping produces changes that can be related to the occupation of states derived from the lowest unoccupied molecular orbitals of the fullerenes and band-structure effects. Important differences are observed upon K doping of ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$, particularly in states near the Fermi level, and these would be reflected in the electron-phonon coupling, superconductivity, and the phase diagram. Resistivity measurements for ${\mathrm{K}}_{\mathit{x}}$${\mathrm{C}}_{60}$ show a resistivity minimum for ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$ and a dependence on stoichiometry that is indicative of dispersed conducting micrograins in an insulating medium. Oxygen-exposure studies demonstrate that ${\mathrm{K}}_{\mathit{x}}$${\mathrm{C}}_{60}$ thin films are unstable.

Inverse-photoemission studies of the empty electronic states of solid ${\mathrm{C}}_{60}$ show ten features within 15 eV of the lowest empty state. Pseudopotential calculations identify the character of the empty states. Band calculations for fcc ${\mathrm{C}}_{60}$ demonstrate that dispersion of some of the states, due to intermolecular interaction, reproduces experimental results. Intensity variations of experimental features with incident electron energy reflect their \ensuremath{\pi} and \ensuremath{\sigma} character as well as their symmetries. Resonant coupling is observed between the inverse-photoemission decay channel involving the third feature and the radiative decay of a plasmon.

Photoemission studies of Li and Na fullerides show a band of alkali-metal-induced states that is fully below the Fermi level for a stoichiometry of ${\mathit{A}}_{2}$${\mathrm{C}}_{60}$ and inverse photoemission results show a splitting of the unoccupied bands. Equivalent results for the Rb fullerides show the formation of only a metallic phase, ${\mathrm{Rb}}_{3}$${\mathrm{C}}_{60}$. For all three fullerides, doping to saturation produces nonmetallic ${\mathit{A}}_{6}$${\mathrm{C}}_{60}$ phases.

Inverse photoemission studies of the unoccupied states of ${\mathrm{Bi}}_{2}{\mathrm{Ca}}_{1+x}{\mathrm{Sr}}_{2\ensuremath{-}x}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+y}$, with and without 10% Pb substitution for Bi, show a low density of states within 2 eV of the Fermi level and broad structures at 4, 9.6, and 12.6 eV that are associated with Bi $6p$, Ca $3d$, and Sr $4d$ empty-state bands. Plasmon losses are observed via their radiative decay at photon energies of 15 and 21.2 eV. Resonant inverse photoemission, using incident electron energies that excite O $2s$ shallow core levels, enhances emission from the unoccupied O $2p$ levels. Resonance results for $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ and ${\mathrm{Bi}}_{2}{\mathrm{Ca}}_{1+x}{\mathrm{Sr}}_{2\ensuremath{-}x}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+y}$ show similar oxygen distributions near ${E}_{F}$.

Photoemission and inverse photoemission studies of ${\mathrm{Ca}}_{\mathit{x}}$${\mathrm{C}}_{60}$ films show insulating character for x3\ifmmode\pm\else\textpm\fi{}0.3 as states derived primarily from the LUMO orbitals of ${\mathrm{C}}_{60}$ are occupied. Metallic character is observed at higher Ca content as a second band of states is partially occupied. This band has mixed character since it is derived from the LUMO+1 orbitals of ${\mathrm{C}}_{60}$ and the Ca s states. These states are responsible for superconductivity in ${\mathrm{Ca}}_{5}$${\mathrm{C}}_{60}$.