Optical Properties of Strongly Correlated Transition Metal Oxides
Simpson, Jeffrey Ray
Drew, Howard D.
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The strong electron interactions in transition metal oxides offer wide-ranging phenomena of interest to condensed matter physics and potential applications in novel electronic devices. This thesis focuses on optical studies performed on three such systems: colossal magnetoresistant (CMR) manganites, multiferroic LuMnO<sub>3</sub>, and dilute magnetic oxide (DMO) cobalt-doped TiO<sub>2</sub>. Optical measurements, using a variety of techniques, of thin film and bulk samples extend over a range of frequencies (0.002-5 eV) and temperatures (4-350 K) and determine the optical constants. Optical properties provide important insights into the electronic structure of these exciting systems and illustrate the value of optical measurements as a probe of novel materials. The pseudocubic manganites exhibit a rich phase diagram that includes, in addition to CMR, various types of magnetic, charge, and orbitally ordered phases. For the CMR manganites, the optical spectra and oscillator strength changes compare with models that require both double exchange and the dynamic Jahn-Teller effect in the description of the electronic structure. In the ferromagnetic state, results on the electronic scattering rate and mass enhancement refute the claims of an anomalously small Drude weight in these materials. Smaller rare-earth ions in the manganites (<i>e.g.</i>, Lu) result in crystallization into a hexagonal structure and a multiferroic ground state, in which ferroelectricity and antiferromagnetism occur simultaneously. A symmetry-allowed on-site Mn <i>d-d</i> optical transition blueshifts in the antiferromagnetic state resulting from Mn-Mn superexchange. TO phonon frequencies exhibit similar temperature dependent shifts arising from spin-phonon interactions. Further, these phonons dominantly contribute to the known anomaly below <i>T<sub>N</sub></i> of the quasi-static dielectric constant. Cobalt-doped TiO<sub>2</sub> has received recent attention as a new DMO displaying room temperature ferromagnetism. Optical conductivity of low-doped samples reveals an absence of absorption below an onset of interband transitions at 3.6 eV and a blue shift of the band edge with doping. The absence of below band gap absorption remains inconsistent with band calculations and suggests that strong on-site Coulomb interactions shift the optical transitions to energies above the gap.