Spin-fluctuation-induced superconductivity controlled by orbital fluctuation

A microscopic Hamiltonian reflecting the correct symmetry of f orbitals is proposed for discussing superconductivity in heavy-fermion systems. In the orbitally degenerate region in which not only spin fluctuations (SF) but also orbital fluctuations (OF) show considerable development, cancellation between SF and OF destabilizes the d_x²-y²-wave superconductivity. On entering the non-degenerate region by increasing the crystalline electric field, d_x²-y²-wave superconductivity mediated by antiferromagnetic SF emerges from the suppression of OF. We argue that the present scenario can be applied to the recently discovered superconductors CeTIn₅ (T = Ir, Rh, and Co).