Variation of electronic structure f(0⩽x⩽0.3) as investigated by optical conductivity spectra

Optical conductivity spectra and their variation with temperature and doping level x have been investigated for single crystals of (Formula presented)(Formula presented)(Formula presented)n(0⩽x⩽0.3). For the low-doped insulating crystal (x=0.1) which shows a ferromagnetic insulating state at low temperature, the spectral weight of the optical conductivity increases only in the inner-gap region around 0.5 eV, but no Drude part emerges due to carrier localization effect. For x⩾0.17, where the low-temperature ferromagnetic metallic state shows up, the optical conductivity spectrum above (Formula presented) is characterized by interband transitions between the exchange-split conduction bands, and it gradually changes into that of intraband excitations below (Formula presented). The energy scale (up to ≈2 eV) of the spectral weight transfer is determined by the effective Hundșs-rule coupling energy. In the metallic phase, low-energy spectra arising from intraband excitations can be sorted into two parts: One is a nearly ω-independent broad structure (incoherent part), and the other a sharp coherent Drude peak with anomalously low spectral weight. This can hardly be reconciled with the simple double-exchange theory, but indicates that another degree of freedom (e.g., the orbital ordering and/or electron-lattice interactions) should be taken into account.