Optical Spectroscopy on Correlated Electron Materials

Licentiatavhandling, 2010

Strongly correlated materials poses a great challenge to our microscopic understanding of matter, mainly due to the inability of perturbative methods to predict the properties of such materials. This inability stems from strong correlations between electronic, lattice and spin degrees of freedom, leading to a variety of complex and exotic phenomena such as (unconventional) superconductivity, metal-insulator transitions, non-Fermi liquid behavior and colossal magnetoresistance. In this thesis, optical spectroscopy investigations on two kinds of strongly correlated matter is presented: materials exhibiting colossal magnetoresistance (manganites) and superconductivity (iron-pnictides). The research has been done from a fundamental point of view, with the goal to advance our understanding of the fascinating interplay between electrons, phonons and spin fluctuation that is encountered in these materials. The electron-phonon coupling in the newly discovered iron-pnictide class of high temperature superconductors have been studied using Raman spectroscopy assisted by theoretical calculations with the goal to elucidate the role of phonons as the mediating boson between electrons in a Cooper-pair. Our results indicate that the phononic spectra can, to good accuracy, be explained by neglecting completely the electron-phonon coupling. This provides further credentials to the widespread opinion that magnetic fluctuations constitute the attractive force responsible for the destabilization of the Fermi-sea leading to the condensation into superconducting Cooper-pairs. Further analysis shows that, through F doping on the O site, ejecting electrons into the quasi-two dimensional Fe-As plane actually leads to hole-doping at the Fe site, in accord with previous first principle calculations. The fundamental dynamics between the electrons, lattice and spins in two manganite samples have been studied using sub-picosecond time resolved pump-probe reflectance spectroscopy: The parent compound LaMnO3 and the doped La0.7Ca0.3MnO3 exhibiting colossal magnetoresistance. Even though LaMnO3 is a semiconductor and La0.7Ca0.3MnO3 is a double-exchange metal at low temperatures, the observed femtosecond dynamics are very similar. We introduce a phenomenological model that can explain all observed features in both samples using a combination of thermal (>1ps) and non-thermal (<1ps) effects. In addition to explaining the low-temperature dynamics in these samples, a new way of performing transmission measurements of the bandgap in semiconducting samples is found.