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We report that the organic salt (EDO-TTF)2PF6 with 3/4-filled-band (1/4-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.

The temperature-dependent electronic structure has been investigated for a single crystal of magnetite (Fe 3 O 4 ) by measurements of the optical-conductivity spectrum. Upon the charge-ordering ͑Verwey͒ transition (T V ϭ121 K), a clear opening of the optical gap ͑0.14 eV͒ is observed. Above T V , the optical conductivity spectra shows notable spectral weight transfer in the Fe 3d intersite transition region (0Ϫ2 eV), while preserving the pseudogap or polaronic feature, indicating highly diffuse charge dynamics.

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Correlated electron systems can undergo ultrafast photoinduced phase transitions involving concerted transformations of electronic and lattice structure. Understanding these phenomena requires identifying the key structural modes that couple to the electronic states. We report the ultrafast photoresponse of the molecular crystal Me4P[Pt(dmit)2]2, which exhibits a photoinduced charge transfer similar to transitions between thermally accessible states, and demonstrate how femtosecond electron diffraction can be applied to directly observe the associated molecular motions. Even for such a complex system, the key large-amplitude modes can be identified by eye and involve a dimer expansion and a librational mode. The dynamics are consistent with the time-resolved optical study, revealing how the electronic, molecular, and lattice structures together facilitate ultrafast switching of the state.

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