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The dynamic structure factor, S(Q, ω), of vitreous silica, has been measured by inelastic X-ray scattering in the exchanged wavevector (Q) region Q=4÷16.5 nm −1 and up to energies ω=115 meV in the Stokes side. The unprecedented statistical accuracy in such an extended energy range allows to accurately determine the longitudinal current spectra, and the energies of the vibrational excitations. The simultaneous observation of two excitations in the acoustic region, and the persistence of propagating sound waves up to Q values comparable with the (pseudo-)Brillouin zone edge, allow to observe a positive dispersion in the generalized sound velocity that, around Q≈5 nm −1 , varies from ≈6500 to ≈9000 m/s: this phenomenon was never experimentally observed in a glass.

The phonon dispersions of Nd(1.86)Ce(0.14)CuO(4+delta) along the [xi,0,0] direction have been determined by inelastic x-ray scattering. Compared to the undoped parent compound, the two highest longitudinal phonon branches, associated with the Cu-O bond stretching and out-of-plane oxygen vibration, are shifted to lower energies. Moreover, an anomalous softening of the bond-stretching band is observed at about q = (0.2,0,0). These signatures provide evidence for strong electron-phonon coupling in this electron-doped high-temperature superconductor.

According to theoretical predictions, 1-3 the charges injected by doping into the Cu-O planes of high-T c superconductors may gather into fluctuating, one-dimensional structures (stripes) below a temperature T* >> T c . Their existence is confirmed indirectly by the magnetic order 4,5 attributed to the regions between the stripes, by lattice distortions detected in the Extended X-ray Absorption Fine Structure, 6 or by the opening at T* of "pseudogaps" in the density of states of the carriers. 7 However, the excitation of charge arrays in the polar Cu-O lattice should also produce strong dipole fluctuations that could be directly detected by infrared spectroscopy.The phase diagram of La 2-x Sr x CuO 4 (LSCO) is recalled in the inset of Fig. 3. The insulator-to-superconductor transition occurs 8 at x = 0.055 at T = 0, while x = 0.15 corresponds approximately to the optimum doping, where one achieves the highest-T c in LSCO. In order to explore the "stripe phase" 4 as extensively as possible, the optical conductivity σ(ω) of LSCO was determined between 295 and 30 K in seven single crystals with x = 0, 0.03, 0.05, 0.07, 0.10, 0.12, and 0.15 grown in different laboratories. The reflectivity R(ω) of the a-b planes, which include the Cu-O planes where superconductivity 1

We report on a systematic study of the structural, magnetic and transport properties of highpurity 1T-VS 2 powder samples prepared under high pressure. The results differ notably from those previously obtained by de-intercalating Li from LiVS 2 . First, no Charge Density Wave (CDW) is found by transmission electron microscopy down to 94 K. Though, ab initio phonon calculations unveil a latent CDW instability driven by an acoustic phonon softening at the wave vector q CDW ≈ (0.21,0.21,0) previously reported in de-intercalated samples. A further indication of latent lattice instability is given by an anomalous expansion of the V-S bond distance at low temperature.Second, infrared optical absorption and electrical resistivity measurements give evidence of non metallic properties, consistent with the observation of no CDW phase. On the other hand, magnetic susceptibility and NMR data suggest the coexistence of localized moments with metallic carriers, in agreement with ab initio band structure calculations. This discrepancy is reconciled by a picture of electron localization induced by disorder or electronic correlations leading to a phase separation of metallic and non-metallic domains in the nm scale. We conclude that 1T-VS 2 is at the verge of a CDW transition and suggest that residual electronic doping in Li de-intercalated samples stabilizes a uniform CDW phase with metallic properties.

We perform a comparative experimental and theoretical study of the temperature dependence up to 700 K of the frequency and linewidths of the graphite E1u and E2g optical phonons (~1590 and 1580 cm-1) by infra-red (IR) and Raman spectroscopy. Despite their similar character, the temperature dependence of the two modes is quite different, being, e.g., the frequency shift of the IR-active E1u mode is almost twice as big as that of the Raman active E2g mode. Ab initio calculations of the anharmonic properties are in remarkable agreement with measurements and explain the observed behavior.Comment: 5 pages, 4 figure

The mechanism and magnitude of fluorine incorporation in H-bearing forsterite were investigated through a combined experimental and theoretical approach. Forsterite samples were synthesized in a piston cylinder press at 2 and GPa, in hydrous conditions, with or without fluorine. High fluorine solubilities of 1715 and 1308 ppm F were measured by particle induced gamma-ray emission (PIGE) in forsterites synthesized at 2 and 4 GPa, respectively. In addition, first-principles calculations based on density functional theory were performed in order to investigate the coupled incorporation mechanisms of fluorine and water in forsterite. Our results demonstrate the close association of fluoride, hydroxyl groups and Si vacancies. Comparison of experimental and theoretical infrared absorption spectra enables assignment of the nine OH stretching bands (3500-3700 cm-1) observed in F-rich synthetic forsterite to clumped fluoride-hydroxyl defects in the forsterite crystal structure. Noteworthily, similar bands were previously recorded on some natural olivine with Mg/(Mg+Fe) molar ratio down to 0.86. Fluorine and water cycles are therefore strongly coupled through the nominally anhydrous minerals and the mantle fluorine budget can be entirely accommodated by these mineral phases.

The first measurements of the spectral distribution of infrared radiation emitted by an undulator are reported. They are compared with calculations including both velocity and acceleration terms. Measurements have been performed at the beam line SIRLOIN (Spectroscopie en InfraRouge Lointain). The agreement between the observations and this first exact numerical solution shows that the inclusion of the velocity term in the submillimeter frequency range is necessary. Moreover, structures due to undulator edges are observed in the far infrared and mid-infrared range, while the interference pattern due to redshifted harmonics of the undulator is dominating in the mid-infrared to near infrared.