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Title:Complex phase behavior of exotic oxides: optical spectroscopy studies
Author(s):Yoon, Seokhyun
Director of Research:Cooper, S. Lance
Department / Program:Physics
Subject(s):Raman scattering spectroscopy
spectroscopic ellipsometry
calcium ruthenium oxide
Abstract:We have used two optical spectroscopy techniques, Raman scattering spectroscopy and spectroscopic ellipsometry, to study complex phase transition behavior exhibited by the doped rare-earth manganese oxides, R1_xDxMn03 (0 :::; x :::; 1), and the layered calcium ruthenium oxide, Ca3Ru20 7 , in which there exist strong correlations among charge, lattice, spin, and/or orbital degrees of freedom. In the low doping regime of R1_xDxMn03 (0.2 :::; x :::; 0.5), where a paramagnetic insulator to ferromagnetic metal transition takes place with a large value of magnetoresistance at the transition, we observed that carriers are localized as small polarons in the high temperature paramagnetic phase, which contribute to the 'collision-limited' diffusive electronic Raman scattering response. The metal-insulator transition is characterized by a change in the low energy electronic Raman scattering response from the diffusive scattering response to a flat continuum response, which is associated with small-to-large polaron crossover and the increase of electron correlations. However, there is evidence that small polarons persist into the low temperature ferromagnetic metal phase, which supports the idea of phase separation in this compound. The magnetic field dependence of Raman spectra also indicates that the low temperature ferromagnetic phase is inhomogeneous. We observed signatures of the long-range charge ordering and short-range orbital ordering, respectively, on the dielectric functions of Bi1_xCaxMn03 (x > 0.5) over the large energy scales by using spectroscopic ellipsometry. Orbital ordering precedes the charge ordering and is affected by the charge ordering at the charge ordering temperature Teo, and exhibits few additional changes below Teo· We also observed anisotropy in the dielectric functions of this material, and resulting anisotropic charge dynamics with respect to the charge stripe direction. It is observed that the system is locally anisotropic, likely due to the short-range orbital order fluctuations, even at T » Teo' where it is isotropic in the large length scale. Notably, charge stripes gradually attain a long-range order below Teo· The anisotropy caused by the long-range charge ordering is also manifested in the lattice dynamics as anomalies in the phonon Raman scattering response. A symmetry selection rule is seen to be broken and certain phonon modes are observed to be 'activated' by the long-range charge ordering due to changes in the local environment caused by the anisotropic charge arrangement. We also found evidence that there exists strong magnetoelastic coupling associated with some Mn06-octahedra related phonon modes. In the distinctive T19 symmetry, which transforms like the spin-chirality operator s1 . Sz X s3, we observed a fluctuational quasielastic Raman scattering response. This unusual scattering response may be caused by the complex charge, spin, and orbital ordered structure and/ or the geometrical frustration in the material. More studies regarding this 'chiral' Raman response may lead us to a better understanding of the nature of the ordered structure in the system. Raman scattering study of Ca3Ru20 7 provided us valuable information regarding the paramagnetic metal to antiferromagnetic metal phase transition, followed by the antiferromagnetic metal to antiferromagnetic insulator transition. The out-of-phase c-axis oxygen vibration phonon mode is seen to be closely associated with the metalinsulator transition possibly by modulating the bandwidth of the system. We observed anomalous intensity, frequency, and width changes associated with the phonon mode across the metal-insulator transition. From the analysis of the electronic Raman scattering response, we could estimate the charge gap of energy ~c rv 760 cm-1 . Notably, the gap energy is seen to be much larger than the temperature scale involved in the metal-insulator transition, indicating the strong electronic correlations are associated with the metal-insulator transition. We could observe Raman scattering from two-magnon excitations in the antiferromagnetic phase of Ca3Ru20 7 . From the two-magnon scattering spectra, we could estimate the in-plane spin-spin exchange coupling constant J11 rv 8. 7 cm-1, which is reasonably compared to the estimation from the mean field calculation J"' 4.9 cm-1. Importantly, we found that Ca3Ru20 7 does not exhibit strong in-plane magnetic correlations above TN, unlike the layered high-Tc cuprate antiferromagnets.
Issue Date:2001
Dissertation / Thesis
Rights Information:© 2001 Seokhyun Yoon
Date Available in IDEALS:2012-05-10
Identifier in Online Catalog:4539124

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