  We investigate the interplay between geometrical frustration and strong
electron correlation based upon the pyrochlore Hubbard model. In the
half-filling case, using the perturbative expansion in terms of electron
correlation, we show that the self-energy shows a divergent behavior leading
the system into the Mott insulating state, in which quantum disordered spin
liquid without magnetic long-range order realizes. In the hole-doped case, we
obtain heavy-fermion-like Fermi liquid state. We also calculate the neutron
cross section which is well consistent with recent neutron scattering
experiments for itinerant pyrochlore systems.
