We present infrared and Raman measurements of magnetite (Fe 3 O 4 ). This material is known to undergo a metal-insulator and a structural transition (Verwey transition) at T V = 120 K. At temperatures below T V , we observe a 1 strong gap-like suppression of the optical conductivity below 1000 cm −1 . The structural aspect of the Verwey transition demonstrates itself by the appearance of additional infrared-and Raman-active phonons. The frequencies of the infrared-active phonons show no significant singularities at the transition whereas their linewidths increase. The frequency and linewidth of the Ramanactive phonon at 670 cm −1 changes abruptly at the transition. For T < T V , we observe fine structures in the infrared and Raman spectra which may indicate strong anharmonicity of the system below the transition. Our estimate of the effective mass of the carriers above the transition to be m * ≈ 100m, where m is the electron mass. Our measurements favor a polaronic mechanism of conductivity and underline the importance of the electron-phonon interaction in the mechanism of the Verwey transition.

The transient optical conductivity of photoexcited 1T-TaS2 is determined over a three-order-of-magnitude frequency range. Prompt collapse and recovery of the Mott gap is observed. However, we find important differences between this transient metallic state and that seen across the thermally driven insulator-metal transition. Suppressed low-frequency conductivity, Fano phonon line shapes, and a midinfrared absorption band point to polaronic transport. This is explained by noting that the photoinduced metallic state of 1T-TaS2 is one in which the Mott gap is melted but the lattice retains its low-temperature symmetry, a regime only accessible by photodoping.

have been measured at room temperature. The frequencies of the Raman modes, obtained from first-principles calculations, for Bi 2 Ti 2 O 7 are presented for comparison. The spectra of the four samples are similar and agree well with the first-principles calculations. Each bismuth pyrochlore shows more than the six modes expected for the ideal pyrochlore structure. The analysis shows that many of the additional modes could be explained as the relaxation of the selection rules due to the displacive disorder. The Raman modes are assigned by reference to spectra of other pyrochlore materials, comparison to infrared data, and the ab initio calculations.

The electronic structure and equilibrium structure of magnetite ͑Fe 3 O 4 ͒ in the high temperature cubic Fd3m and low temperature monoclinic P2 / c unit cells have been computed using the Perdew-Wang generalized gradient approximation ͑GGA͒ to density functional theory ͑DFT͒ and the B3LYP hybrid density functional. The ground state for the GGA-DFT is an itinerant electron metallic state in the cubic unit cell and the ground state for the B3LYP functional is a charge ordered semiconducting state in the monoclinic unit cell. The equilibrium structure predicted by the B3LYP functional for Fe 3 O 4 in the P2 / c unit cell has been calculated with lattice parameters fixed at values obtained in recent x-ray diffraction work and with the lattice fully relaxed. Bond lengths obtained with lattice parameters fixed at experimental values are in excellent agreement with x-ray measurements ͓J. P. Wright et al., Phys. Rev. B 66, 214422 ͑2002͔͒. The degree of charge order, measured as disproportionation of charge on octahedral B sites, is considerably less than unity and in reasonable agreement with values from resonant x-ray diffraction measurements. However, conduction electrons are found to be fully localized on B1 and B4 sites in orbitally ordered t 2g states. This shows that they are formally Fe 2+ ions while Fe B2 and B3 sites are formally Fe 3+ sites. Therefore Verwey's original conjecture regarding charge localization in Fe 3 O 4 applies, even though the specific pattern of charge order is different. GGA-DFT and B3LYP density functionals were used to calculate phonons at the ⌫ point of the Brillouin zone. Phonon frequencies predicted for these crystal structures are compared to frequencies from infrared conductivity and Raman scattering experiments. Charge ordering causes symmetry breaking of force constants on symmetry lowering from the cubic Fd3m unit cell to the P2 / c unit cell. This produces frequency splitting of modes which are degenerate in the cubic unit cell and concentration of ion displacements in phonon eigenvectors on particular Fe octahedral B site chains, especially in the highest frequency bands.

The infrared reflectance of the transition metal chalcogenide 1T -TaS2 has been measured at temperatures from 30 K to 360 K over 30-45,000 cm −1 (4 meV-5.5 eV). The optical conductivity was obtained by Kramers-Kronig analysis. At 360K only modest traces of the phonon lines are noticeable. The phonon modes are followed by a pseudogap-like increase of the optical conductivity, with direct optical transitions observed at frequencies above 1eV. As the temperature decreases, the low frequency conductivity also decreases, phonon modes become more pronounced and pseudogap develops into a gap at 800 cm −1 (100 meV). We observe an anomalous frequency dependence of the 208 cm −1 infrared-active phonon mode. This mode demonstrates softening as the temperature decreases below the 180 K metal-to-insulator transition. The same mode demonstrates strong hysteresis of the frequency and linewidth changes, similar in its temperature behavior to the hysteresis in the dc-resistivity. We discuss a possible relation of the observed softening of the mode to the structural changes associated with the metal-to-insulator transition.