Charge Donation by Calcium into the t1g Band of C60

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Science  04 Dec 1992:
Vol. 258, Issue 5088, pp. 1638-1640
DOI: 10.1126/science.258.5088.1638


Photoemission spectra of compounds prepared by the reaction of C60 films with calcium show two distinct metallic phases, whereas alkali-doped C60 films have only one. In the first phase the bulk t1u band, derived from the lowest unoccupied molecular orbital of C60, is partially occupied. This is followed by an insulating phase that has the composition Ca3C60 in which the t1u band is filled and has properties analogous to those of K6C60. Continued exposure to calcium produces a second metallic phase in which electrons are donated into the t1g band. The superconductivity of Ca5C60 is associated with the t1g band.