Role of the van Hove singularity on the critical temperature of doped fullerenes

We investigate the role played by van Hove singularity (vHs) on the optimiz ation of the value of T-c in doped fullerense C60K3 by employing a first-pr inciples self-consistent full-potential linear muffin-tin orbital method in local-density approximation. For C-60, the computed band structure shows a n insulating behavior with a direct band gap at the symmetry point X. The v alence band originates from C-60 molecule h(u) states whereas the conductio n band originates from the molecular t(1u) states. In C60K3 the two types o f K(1) and K(2) atoms occupy tetrahedral and interstitial positions, respec tively. The band structure is very nearly similar to that of pure C-60. The three extra K electrons fill the t(1u) band up to half, Making C60K3 condu cting. The K-induced states appear mostly in the conduction-band region. We observe a saddle point leading to vHs in the vicinity of the symmetry poin t L slightly shifted towards the Gamma point, very near the Fermi level. Th e saddle point lies exactly at the Fermi level for a lattice constant of 14 .51 Angstrom (a 5% dilation) for which the highest value of T-c may be dete cted in the experiments.