Large-shift Raman scattering in insulating parent compounds of cuprate superconductors

We describe the use of large-shift Raman techniques to probe features near the optical conductivity peak in insulating high-Tc parent compounds, including the T'-phase materials (R₂{CuO}₄$, R=Gd, Eu, Sm, Nd, Pr), T-phase ${La}₂CuO₄, and 123-phase PrBa₂{Cu}₃O₆ and YBa₂{Cu}₃O₆. We have observed several features near 12 000 cm^-1 ({~}1.5 eV) with A2g, B1g, and A1g symmetry. At low temperatures, the largest feature with A2g symmetry splits in all the insulating cuprates into three distinct peaks: a strong main peak, and two weaker sidebands offset approximately 600 and 1500 cm^-1 higher in energy, respectively. A B1g feature coinciding in position with the optical conductivity peak is observed in all the T' phase materials, but not in the other insulators. An A1g peak is observed in Gd₂{CuO}₄$ and ${Eu}₂CuO₄ 40--50 cm^-1 lower in energy than the main A2g peak, and in YBa₂{Cu}₃O₆ around 2400 cm^-1 above the main A2g peak. All of these features have unusual, temperature-dependent resonance behavior with the incident photon energy. We propose explanations for many of the above observations involving copper dₓ²-y² to dxy hole transitions enabled by coupling to oxygen p_σ and p_π levels.