METAL-INSULATOR TRANSITIONS IN DEGENERATE HUBBARD MODELS AND AXC60

Analytic results for Mott-Hubbard metal-insulator transitions in N-fol d degenerate Hubbard models are obtained using the Gutzwiller approxim ation. It is found that for any commensurate filling with integer (x) electrons per site, there exists a metal-insulator transition at the c ritical correlation energy U(c)(N,x)={[square-root x(2N - x + 1) + squ are-root (x + 1)(2N - X)]2/(2N - x)}\epsilon(x)BAR\, where epsilonBAR is the energy per particle in the absence of correlation. U(c) increas es with x reaching the maximum at the half filling x = N. Therefore, i t is possible for a system to be metallic at half filling and insulati ng away from half filling. This provides an explanation for the unusua l metal-insulator transitions observed in fullerides A(x)C60.