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 optimization of the value of T-c in doped fullerense C60K3 by employing a first-principles self-consistent full-potential linear muffin-tin orbital method in local-density approximation. For C-60, the computed band structure shows an insulating behavior with a direct band gap at the symmetry point X. The valence band originates from C-60 molecule h(u) states whereas the conduction band originates from the molecular t(1u) states. In C60K3 the two types of K(1) and K(2) atoms occupy tetrahedral and interstitial positions, respectively. 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 conducting. 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 point L slightly shifted towards the Gamma point, very near the Fermi level. The 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 detected in the experiments.