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

Abstract

We describe the use of large-shift Raman techniques to probe features near the optical conductivity peak in insulating high- T c parent compounds, including the T’-phase materials ( R 2 CuO 4 , R=Gd, Eu, Sm, Nd, Pr), T-phase La 2 CuO 4 , and 123-phase PrBa 2 Cu 3 O 6 and YBa 2 Cu 3 O 6 . We have observed several features near 12 000 cm − 1 (∼1.5 eV) with A 2 g , B 1 g , and A 1 g symmetry. At low temperatures, the largest feature with A 2 g 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 B 1 g 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 1 g peak is observed in Gd 2 CuO 4 and Eu 2 CuO 4 40–50 cm − 1 lower in energy than the main A 2 g peak, and in YBa 2 Cu 3 O 6 around 2400 cm − 1 above the main A 2 g 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 d 2 x - y 2 to d x y hole transitions enabled by coupling to oxygen p σ and p π levels.