Comparison of photoemission and resistivity in alkaline-earth compounds of C 60

Abstract

Photoemission spectra of Ca x C 60, Sr x C 60, and Ba x C 60 show successive filling of the t lu -derived LUMO and the t lg -derived LUMO + 1 bands, for 0 < x < 6. For x < 2 compounds of Ca and Ba exhibit a well defined Fermi cut-off; compounds of Sr do not. In this range of composition, the resistivity remains activated, suggesting that the metallic character in photoemission is due to a surface phase. Stoichiometric compounds with formula A 3C 60 differ in both structure and electronic properties. The Ca compound retains the fcc stacking of C 60, and is an insulator. The Sr compound transforms partially to the A15 structure but remains an insulator. The Ba compound has the A15 structure and is weakly metallic. For 3 < x < 6 metallic behavior is weak in the Ca and Sr compounds, but well defined in the Ba compound, indicating that the LUMO + 1 band is filled progressively by Ba. This is in accord with the superconductivity of these systems, which is found in compounds with x near 5, when the Fermi level is in the t lg band.