Random-phase-approximation analysis of orbital- and magnetic-fluctuation-mediated superconductivity in a two-band Hubbard model

We have applied a generalized random-phase-approximation analysis to a two-band Hubbard model for the cuprate superconductors which treats magnetic and orbital (quadrupolar) fluctuations on an equal footing and which allows for significant itinerant character of the copper d electrons. The two bands represent the lowest copper orbitals. We qualitatively reproduce the known results of antiferromagnetic order at half filling with an adjoining region of d-wave-symmetry superconductivity in the single-band limit. When the value of the band splitting is comparable to the hopping integral of the x2-y2 orbitals, we find a qualitatively new region of superconductivity at increased hole doping which is clearly associated with orbital (quadrupolar) fluctuations and a significant x2-y2→3z2-r2 charge transfer but no concomitant charge or magnetic order. The pairs are very spread out in momentum and energy space. At the optimal temperature for the new instability, the Fermi surface has two sheets, and d-wave and s-wave pair states are nearly degenerate. We discuss the possible phenomenological relevance of these results to photoemission and other data for the cuprates