Large-Shift Raman Scattering in Insulating Cuprate Materials.

This thesis describes the use of large-shift ordinary Raman techniques to probe features near the charge-transfer gap in insulating cuprate materials, including the T ^' phase materials (rm R_2CuO_4, R = Gd, Eu, Sm, Nd, Pr), T phase rm La_2CuO_4, and 123 phase rm PrBa_2Cu_3CuO _6 and rm YBa_2Cu_3CuO _{6+x}. For all of these materials, several features are observed near 12000 cm^ {-1} ({~}1.5 eV) with rm A_{2g}, B_{1g}, and A_ {rm 1g} symmetry. At low temperatures, the largest feature with A_{rm 2g} symmetry splits in all the insulating cuprates into three distinct peaks: a strong main peak, and two weaker side-bands offset approximately 600 and 1500 cm^{-1} higher in energy respectively. A B_{rm 1g} feature coinciding in position with the optical conductivity peak is observed in all the T ^' phase materials, but not in the other insulators. An A_{rm 1g} peak is observed in rm Gd _2CuO_4 and rm Eu_2CuO _4 40-50 cm^{-1} lower in energy than the main A_ {rm 2g} peak, and in rm YBa_2Cu_3CuO_6 around 2400 cm^{-1} above the main A_{rm 2g} peak. All of these features have unusual, temperature-dependent resonance behavior with the incident photon energy, and they seem to weaken in rm YBa_2Cu_3CuO _{6+x} as x moves through the metal -insulator transition. Some residual A_ {rm 2g} scattering may remain in superconducting phases. We propose an explanation for many of the above observations involving a copper d _{xy} to d_{x ^2-y^2} orbital transitions. In this picture, the main A_{rm 2g} peak is the d-d transition energy, the first side-band results from coupled phonon scattering, and the second side-band results from unusual coupled two -magnon scattering.