Sensitivity of the Mott Transition to Non-cubic Splitting of the Orbital Degeneracy: Application to NH3 K3C60

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 NH3 K3C60 is discussed.