Sensitivity of the Mott transition to noncubic splitting of the orbital degeneracy : application to NH3K3C60

Within dynamical mean-field theory, we study the metal-insulator transition of a twofold orbitally degenerate Hubbard model as a function of a splitting Delta of the degeneracy. The phase diagram in the U-Delta plane exhibits two-band and one-band metals, as well as the Mott insulator. The correlated two-band metal is easily driven to the insulator state by a strikingly weak splitting Delta<W of the order of the Kondo-peak width zW, where z<1 is the metal quasiparticle weight. The possible relevance of this result to the insulator-metal transition in the orthorhombic expanded fulleride NH3K3C60 is discussed.