Electron-Phonon Interactions in Transition Metal Oxides Studied by Resonance Raman Scattering

Doktorsavhandling, 2008

In this thesis, studies of strong electron-phonon interactions in a number of transition metal oxides are presented. Two different electron-phonon interaction mechanisms are identified; Franck-Condon scattering and infrared active longitudinal optical (IR LO) two-phonon activation. The main experimental technique used is temperature dependent resonance Raman scattering and the electron-phonon interactions are studied by the resonant effects they cause on the first and higher order Raman active phonon scattering. The mixed transition metal oxide LaFe$_{0.5}$Cr$_{0.5}$O$_{3}$ with orthorhombic perovskite structure has been studied in detail using resonance Raman scattering with incident photon energies between $\hbar\omega$=1.83 ($\lambda$=676 nm) and $\hbar\omega$=4.13 eV ($\lambda$=300 nm) and variable temperatures. It is established that the characteristic Franck-Condon multi-phonon scattering of a local oxygen breathing mode appears as the photon energy is tuned to the Fe-Cr charge transfer gaps. This interpretation is supported by results obtained from LaFe$_{1-x}$Cr$_{x}$O$_{3}$ (0$\leq$x$\leq$1) which show that the Franck-Condon resonance is critically sensitive to the presence of both Fe and Cr ions in the structure and by Sr-doping in La$_{1-x}$Sr$_{x}$Fe$_{0.5}$Cr$_{0.5}$O$_{3-\delta}$ which shows that the Franck-Condon resonance is sensitive to local effects. In addition to the Franck-Condon effect, Fr\"ohlich interaction induced IR LO two-phonon scattering is observed in the perovskites LaFeO$_{3}$ using $\lambda$=515 nm ($\hbar\omega$=2.41 eV) and LaFe$_{0.5}$Cr$_{0.5}$O$_{3}$ using $\lambda$=334 nm ($\hbar\omega$=3.71 eV). Interestingly, these different resonance effects can be made to mix either by an introduction of a small amount of Cr in LaFeO$_{3}$ or by incident photon energy tuning in LaFe$_{0.5}$Cr$_{0.5}$O$_{3}$ The results in the perovskites are complemented by observations in the spin ladder compound Sr$_{14}$Cu$_{24}$O$_{41}$ where an IR LO two-phonon resonance is activated for incident photon energies around the 1.8 eV charge transfer gap. In the spin ladder compound this resonance is linked to the formation of a charge density wave in the copper-oxide ladder layers below T=200 K. In combination, these observations illustrate the complexity of electron-phonon interactions in transition metal oxides and further establish resonance Raman scattering (in particular the higher order phonon scattering) as a sensitive probe of electron-phonon interactions and the relation between local and global effects in correlated material systems.