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-[ital T][sub [ital c]] parent compounds, including the [ital T][prime]-phase materials ([ital R][sub 2]CuO[sub 4], [ital R]=Gd, Eu, Sm, Nd, Pr), [ital T]-phase La[sub 2]CuO[sub 4], and 123-phase PrBa[sub 2]Cu[sub 3]O[sub 6] and YBa[sub 2]Cu[sub 3]O[sub 6]. We have observed several features near 12 000 cm[sup [minus]1] ([similar to]1.5 eV) with [ital A][sub 2[ital g]], [ital B][sub 1[ital g]], and [ital A][sub 1[ital g]] symmetry. At low temperatures, the largest feature with [ital A][sub 2[ital 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[sup [minus]1] higher in energy, respectively. A [ital B][sub 1[ital g]] feature coinciding in position with the optical conductivity peak is observed in all the [ital T][prime] phase materials, but not in the other insulators. An [ital A][sub 1[ital g]] peak is observed in Gd[sub 2]CuO[sub 4] and Eu[sub 2]CuO[sub 4] 40--50 cm[sup [minus]1] lower in energy than the main [ital A][sub 2[ital g]] peak, and in YBa[sub 2]Cu[sub 3]O[sub 6] around 2400 cm[sup [minus]1] above the main [ital A][sub 2[ital g]]more » 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 [ital d][sub [ital x]][sup 2][minus][ital y][sup 2] to [ital d][sub [ital x][ital y]] hole transitions enabled by coupling to oxygen [ital p][sub [sigma]] and [ital p][sub [pi]] levels.« less