The structure and dehydration mechanism of the proton conducting oxide Ba$_{2}$In$_{2}$O$_{5}$(H$_{2}$O)$_{x}$ are investigated by means of variable temperature Raman spectroscopy together with inelastic neutron scattering. At room temperature, Ba$_{
2}$In$_{2}$O$_{5}$(H$_{2}$O)$_{x}$ is found to be fully hydrated ($x=1$) and to have a perovskite-like structure, which dehydrates gradually with increasing temperature and at around 600 $^{circ}$C the material is essentially completely dehydrated ($x=0$). The dehydrated material exhibits a brownmillerite structure, which is featured by alternating layers of InO$_{6}$ octahedra and InO$_{4}$ tetrahedra. The transition from a perovskite-like to a brownmillerite-like structure is featured by a hydrated-to-intermediate phase transition at $ca.$ 370 {deg}C. The structure of the intermediate phase is similar to the structure of the fully dehydrated material, but with the difference that it exhibits a non-centrosymmetric distortion of the InO$_{6}$ octahedra not present in the latter. For temperatures below the hydrated-to-intermediate phase transition, dehydration is featured by the release of protons confined to the layers of InO$_{4}$ tetrahedra, whereas above the transition also protons bound to oxygens of the layers of InO$_{6}$ are released. Finally, we found that the O-H stretch region of the vibrational spectra is not consistent with a single-phase spectrum, but is in agreement with the superposition of spectra associated with two different proton configurations. The relative contributions of the two proton configurations depend on how the sample is hydrated.
We investigate the low-temperature electron, lattice, and spin dynamics of LaMnO_3 (LMO) and La_0.7Ca_0.3MnO_3 (LCMO) by resonant pump-probe reflectance spectroscopy. Probing the high-spin d-d transition as a function of time delay and probe energy,
we compare the responses of the Mott insulator and the double-exchange metal to the photoexcitation. Attempts have previously been made to describe the sub-picosecond dynamics of CMR manganites in terms of a phenomenological three temperature model describing the energy transfer between the electron, lattice and spin subsystems followed by a comparatively slow exponential decay back to the ground state. However, conflicting results have been reported. Here we first show clear evidence of an additional component in the long term relaxation due to film-to-substrate heat diffusion and then develop a modified three temperature model that gives a consistent account for this feature. We confirm our interpretation by using it to deduce the bandgap in LMO. In addition we also model the non-thermal sub-picosecond dynamics, giving a full account of all observed transient features both in the insulating LMO and the metallic LCMO.
We present Raman measurements on the iron-pnictide superconductors CeFeAsO_{1-x}F_{x} and NdFeAsO{1-x}F_{x}. Modeling the Fe-As plane in terms of harmonic and a cubic anharmonic Fe-As interaction we calculate the temperature dependence of the energy
and lifetime of the Raman active Fe B_{1g} mode and fit to the observed energy shift. The shifts and lifetimes are in good agreement with those measured also in other Raman studies which demonstrate that the phonon spectrum is well represented by phonon-phonon interactions without any significant electronic contribution. We also estimate the anharmonic expansion from Fe (56->54) isotope substitution to Delta a=5.1 10^{-4}AA and Delta d_{Fe-As}= 2.510^{-4}AA and the shift of harmonic zero point fluctuations of bond lengths <Delta x^2><=3 10^{-5}AA^2, giving a total relative average decrease of electronic hopping integrals of |delta t|/t<= 2.0 10^{-4}. The results poses a serious challenge for any theory of superconductivity in the pnictides that does not include electron-phonon interactions to produce a sizable Fe-isotope effect.
