Optical spectra for the ferromagnetic ground states have been investigated for single crystals of perovskite-type manganese oxides La1-cursive greek chiSrcursive greek chiMnO3 with finely varying hole-doping concentration (cursive greek chi) around the metal-insulator (MI) phase boundary (cursive greek chic ∼ 0.16). For such a fully spin-polarized state, orbital degree of freedom may play an important role in the MI transition, which is anticipated to be figured out in terms of optical spectroscopy. In the ferromagnetic ground state, the low-energy optical conductivity spectra are observed to vary from Drude-like (typically for cursive greek chi = 0.4) to incoherent broad feature (for cursive greek chi < 0.2) with decreasing cursive greek chi. The spectral-weight analysis shows that the kinetic energy of conduction electron scarcely changes with change of cursive greek chi when viewed on a large energy scale (about 1 eV) whereas the coherent response on a low energy scale (∼0.1 eV) is critically suppressed toward the insulating phase. Orbital correlation and its coupling with lattice dynamics are likely responsible for such an anomalous variation of charge dynamics. Far-infrared pseudo gap structure observed in the ferromagnetic metallic phase of cursive greek chi = 0.16 on the verge of MI transition suggests the charge localization affected by randomness and non-local interaction in the course of orbital and charge ordering.
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