On Possible Coexistence of Superconductivity and Charge Density Wave in Hole-doped C60

Superconductivity in C60 materials is modeled in terms of an intramolecular, nonretarded attraction. It was shown previously that, at intermediate coupling, the model possesees a state in which s-wave superconductivity coexists with a charge density wave, the latter stabilized by intersite repulsion. The CDW causes Tc to decrease near half filling. We argue that hole-doped C60, for which Tc peaks away from half filling, is a possible candidate for this state. But the electron-doped C60 and A3C60 are conventional superconductors, stabilized against a CDW by metallic screening, which is treated in a parameter-free fashion. PACS: 74.10 +v, 74.20 -z, 74.70 Wz, 74.72 -h About a decade ago, superconductivity with surprisingly high Tc’s (30−40K) was observed in A3C60, where A is an alkali atom [1, 2]. Recently much higher Tcs (52K−117K) have been seen in hole-doped C60 [3, 4]. Within the BardeenCooper-Schrieffer (BCS) picture, origin of such high Tc’s has been attributed to special properties C60, such as, high density of states (DOS) and large phonon frequency. However, variation of Tc with the density n does not follow the DOS. Also, in the electron-doped C60 and A3C60 the maximum of Tc occurs near half filling. But in the hole-doped material, Tc peaks at a density of n = 3 − 3.5 holes/molecule, and decreases rapidly toward half filling (5 holes/molecule) [3]. Such a behavior has been predicted previously on theoretical grounds[5, 6]. The sharp decline in Tc is caused by the appearance of a charge density wave (CDW) near half filling which, due to the frustration effects of the lattice, can coexist with superconductivity. Here we examine the conditions for the existence of such a state in C60. The possibility of a CDW instability in C60 was recognized early by Zhang, Ogata and Rice[8]. As discussed below, it becomes more likely as Tc increses. The key issue is the effect of Coulomb repulsion which opposes superconductivity, but favors the CDW. We will focus mostly on the normal state, and consider realistic potentials. We find that if there is no metallic screening, the