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-${\mathit{T}}_{\mathit{c}}$ parent compounds, including the T’-phase materials (${\mathit{R}}_2$ ${\mathrm{CuO}}_4$, R=Gd, Eu, Sm, Nd, Pr), T-phase ${\mathrm{La}}_2$ ${\mathrm{CuO}}_4$, and 123-phase ${\mathrm{PrBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_6$ and ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_6$. We have observed several features near 12 000 ${\mathrm{cm}}^{\mathrm{-}1}$ (∼1.5 eV) with ${\mathit{A}}_{2\mathit{g}}$, ${\mathit{B}}_{1\mathit{g}}$, and ${\mathit{A}}_{1\mathit{g}}$ symmetry. At low temperatures, the largest feature with ${\mathit{A}}_{2\mathit{g}}$ symmetry splits in all the insulating cuprates into three distinct peaks: a strong main peak, and two weaker sidebands offset approximately 600 and 1500 ${\mathrm{cm}}^{\mathrm{-}1}$ higher in energy, respectively. A ${\mathit{B}}_{1\mathit{g}}$ feature coinciding in position with the optical conductivity peak is observed in all the T’ phase materials, but not in the other insulators. An ${\mathit{A}}_{1\mathit{g}}$ peak is observed in ${\mathrm{Gd}}_2$ ${\mathrm{CuO}}_4$ and ${\mathrm{Eu}}_2$ ${\mathrm{CuO}}_4$ 40–50 ${\mathrm{cm}}^{\mathrm{-}1}$ lower in energy than the main ${\mathit{A}}_{2\mathit{g}}$ peak, and in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_6$ around 2400 ${\mathrm{cm}}^{\mathrm{-}1}$ above the main ${\mathit{A}}_{2\mathit{g}}$ 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 ${\mathit{d}}_{\mathit{x}}^2$-${\mathit{y}}^2$ to ${\mathit{d}}_{\mathit{x}\mathit{y}}$ hole transitions enabled by coupling to oxygen ${\mathit{p}}_{\mathrm{\sigma }}$ and ${\mathit{p}}_{\mathrm{\pi }}$ levels.