Variation of the charge dynamics in bandwidth- and filling-controlled metal-insulator transitions of pyrochlore-type molybdates

The systematics of the bandwidth- and filling-controlled metal-insulator transitions (MITs) have been investigated for R2 Mo2 O7 family (R=Nd, Sm, Eu, Gd, Dy, and Ho) by infrared spectroscopy. The substantial role of electron correlation in driving the MIT is verified. With changing the R ionic radius (r) or equivalently the one-electron bandwidth (W), the MIT occurs in a continuous manner at rc ≈ r (R=Gd). The T=0 K gap continuously vanishes as Δ (rc -r), while at the metallic side the linear decrease of Drude weight is followed toward rc. In the metallic compounds, some of the infrared-active phonon modes show remarkably large Fano asymmetry correlating with the Drude weight. These Mo-O-Mo bending modes strongly couple to the conduction electrons via effective modulation of the bandwidth. Even for r rc a minimal level of hole doping closes the correlation gap, for example, the barely insulating Gd2 Mo2 O7 is turned to an incoherent metal by 5% partial substitution of Gd3+ with Ca2+. However, even on further doping no coherent electronic states are formed, indicating the role of the disorder-induced localization effect besides the dominant correlation effects.