Metal-semimetal transitions in the extended Hubbard model on the fcc lattice: Implications for superconducting fullerides

Abstract

We present results from a Hartree-Fock analysis of the extended Hubbard model on the face-centered-cubic lattice. For positive U there are three metallic states at half-filling. As a result of frustration, the ordinary single-band metallic state undergoes a second-order transition (with increasing U) to a spin-density-wave semimetal at small V and to a charge-density-wave (CDW) semimetal at larger V. The semimetals give way to insulators at larger U. In an earlier paper we have shown that a similar CDW semimetal exists for U<0, a state that is stabilized by V and becomes superconducting at low temperatures. In the positive U case, the CDW semimetal would require additional retarded interactions (that are always present) to become superconducting. Combining these results, a phase diagram is obtained showing that the CDW semimetal occupies a substantial region of the parameter space. Next, we extend our mean-field analysis away from half-filling in the negative-U case to study the behavior of the CDW-superconducting state across the metal-insulator transition. We find that the superconducting transition temperature Tc has a minimum slightly above half-filling. In either direction Tc increases rapidly as the CDW is weakened and acquires maximum values at the semimetal-metal transition where the direct CDW gap vanishes. Implications for alkali fullerides are discussed.

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