Optical spectra and their temperature dependence have been investigated for a prototypical double-exchange ferromagnet ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{MnO}}_3$ ( $x=0.175\mathrm{}$). The optical conductivity spectrum above $T_c$ is characterized by interband transitions between the exchange-split bands and gradually changes into intraband excitations with decrease of temperature, accompanied by conspicuous transfer of spectral weight over a wide energy region of 0–3 eV. The results indicate a large variation of the electronic structure with the spin polarization of the conduction carriers.

We report that the organic salt (EDO-TTF) ₂ PF ₆ with ¾-filled-band (¼-filled in terms of holes), which forms an organic metal with strong electron and lattice correlation, shows a highly sensitive response to photoexcitation. An ultrafast, photoinduced phase transition from the insulator phase to the metal phase can be induced with very weak excitation intensity at near room temperature. This response makes the material attractive for applications in switching devices with room-temperature operation. The observed photo-induced spectroscopic change shows that this photoinduced phase transition process is caused by the cooperative melting of charge ordering assisted by coherent phonon generation.

Optical conductivity spectra and their variation with temperature and doping level x have been investigated for single crystals of ${\mathrm{La}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Sr}}_{\mathrm{x}}$ ${\mathrm{MnO}}_3$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 ${\mathrm{T}}_{\mathrm{c}}$ is characterized by interband transitions between the exchange-split conduction bands, and it gradually changes into that of intraband excitations below ${\mathrm{T}}_{\mathrm{c}}$. The energy scale (up to ≈2 eV) of the spectral weight transfer is determined by the effective Hund's-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.