Structure and Dynamics in Transition Metal Perovskites — An Optical Spectroscopy Study

Doktorsavhandling, 2012

Transition metal oxides with the perovskite structure exhibit a vast number of exotic properties due to the large number of competing degrees of freedom at the Fermi surface. Together with couplings between and correlations within these degrees of freedom, this class of materials is of high interest from both a practical and theoretical point of view. In this thesis, the properties of four different perovskite systems and their relation to structure and dynamics is investigated: La1-x CaxMnO3, exhibiting the colossal magnetoresistance effect; LaCoO3, undergoing spin-state transitions influenced by complicated electron-lattice interactions; BiFeO3 that is of the only known room-temperature multiferroic materials; and the proton conductor BaInO3H. Two kinds of optical spectroscopy methods have been used to investigate the structure and dynamics of these transition metal perovskites. Raman spectroscopy measures low-energy excitations and is, in combination with group theoretical selection rules, a sensitive probe of electronic, atomic, and magnetic structure. Ultrafast time-resolved spectroscopy is used to study, in real time, fundamental dynamics and interaction mechanisms on the femtosecond timescale. Utilizing time resolved pump-probe reflectance spectroscopy, thermal and lattice contributions to the spin-state transition are temporarily decouple, and the first explicit observation of the high-spin repulsion in LaCoO3, first conjectured by Goodenough in the 1950's, is presented. This opens up a novel avenue for investigating the spin-state transition in LaCoO3. With the same experimental technique, the electron-lattice, lattice-spin, and electron-spin dynamics are investigated in La1-xCaxMnO3. Incorporating a two-component heat diffusion mechanism it is shown that the rate-equation based model captures all essentials of the >1 ps dynamics. Raman spectroscopy is used to investigate the structural phase-diagram of isovalently substituted BiFeO3. The vibrational frequencies as a function of substitution provides unique input into the controversial phonon assignments of BiFeO3 and the pronounced second-order scattering is explained in terms of LO-modes activated by the Fröhlich interaction. Further, the first phonon assignment is presented and the structural transition during dehydration is investigated for BaInO3H. Complementing the studies on perovskites, the electron-phonon interactions are investigated in a iron-pnictide high temperature superconductor by combining Raman spectroscopy and phonon-phonon calculations. Our results point towards weak electron-phonon couplings and supports the view that a non-phononic glue is responsible for the Cooper pair condensation in iron-pnictides.