Effects of Strong Correlations in Complex Oxides Investigated by Raman Spectroscopy
Doctoral thesis, 2007
Our global environment is under excessive strain and we are facing a possible global climate change. There is a urgent need today for finding both alternative energy sources and new techniques for the transport and storage of energy in order to save our planet in its present status. One important route to improve our environment is to find a superconductor that works at room temperature. During the last decades a huge effort in science has been put on the high temperature superconducting complex oxides since they for a long time have been regarded to have the best potential to achieve superconductivity at room temperature. This thesis focus is on complex oxides, including superconductivity and related phases in the sense of structure, magnetism and charge dynamics. The main tool of investigation is Raman scattering spectroscopy. From doping and temperature dependent investigations of a quasi one-dimensional spin ladder I show the emergence and fall of a charge density wave phase. The magnetism and its relation to superconductivity has been studied in artificial superlattices of alternating superconducting and isolating layers, the results support a picture of possible coexisting antiferromagnetic and superconducting order parameters. Further, our investigations of the magnetic excitation in a oxychloride compound reveal a delicate interplay between the spin coupling parameters stemming from the two interpenetrating magnetic substructures. In an isotropic superconducting pyrochlore I follow the structural transitions that precurse the superconducting state, a refined picture for the course of events upon lowering the temperature and an order-disorder-order picture emerge from the results. Another related complex oxide that has been studied is a Fe-Cr solid solution with perovskite structure. Here, the exceptional higher order Raman scattering is assigned to a Franck-Condon type of resonance.