Raman Scattering and Neutron Diffraction Studies of Manganites and Cuprates

Doctoral thesis, 2001

This thesis is devoted to experimental studies of two classes of strongly correlated transition metal oxides, namely cuprates and manganite perovskites. Manganites generally display complex phase diagrams, including the so-called colossal magnetoresistance effect. Some cuprates show high-temperature superconductivity, a phenomenon that is poorly understood in spite of many years of intense research. It is clear, though, that a delicate interplay between the atomic structure, magnetism and charge carriers is important in these materials. Experimental investigations have been done using Raman spectroscopy, a powerful technique for these studies since it is able to simultaneously probe vibrational, magnetic and electronic excitations. Neutron diffraction in combination with Reverse Monte Carlo modelling have been used to study both structural and magnetic ordering on a local scale. We demonstrate that an anomaly of the Ba-phonon mode in YBa2Cu4O8 is directly correlated to the opening of a pseudogap well above the superconducting critical temperature. A similar anomaly in the non-superconductor PrBa2Cu4O8 indicates that the pseudogap state is not a sufficient prerequisite for superconductivity. Following neutron diffraction studies of PrBa2Cu4O8 reveal a Ba-site instability, which we link to anomalies in the electronic properties. In the high-temperature superconductor YBa2Cu3O6+x, we investigate the influence of light irradiation on the Raman spectra and propose a simplistic model for photoinduced effects. We show effects of interactions between charge and lattice degrees of freedom in the manganites La0.5Ca0.5MnO3 and La1-xSrxMnO3. In the latter compound, we discovered a low-energy electronic excitation suggesting the existence of a two-component metallic state.