For other stoichiometries, both the lattice structure and electrical properties may be different. The hole-doped material remained superconducting, albeit with lower values of T c , for doping levels down to 1.7 and above 4.5 holes per molecule. For electron injection, the superconducting region was smaller, ranging from about 2.5 to 3.6 electrons per molecule. Doping with larger atoms expands the C 60 lattice. The wider spacing further reduces the overlap between the electronic bands of adjacent molecules and narrows the bandwidths.
T c varies inversely with the bandwidth, so that the larger the doping atom, the higher the T c .
Batlogg told us that he and his coworkers are trying to find a way to expand the hole-doped C 60 lattice, hoping to reach a higher T c . The peak in T c occurs at the same number of charges per molecule for both hole and electron doping. To gain further insight into what factors determine T c , the Bell Labs group explored its dependence on the density of electronic states. Nevertheless, comparing their data to the three-dimensional calculations, the researchers found that the peak in T c did not coincide with a peak in the density of states. They concluded that the density of states might not be the dominant determinant of T c .