Strong spin triplet contribution of the first removal state in the insulating regime of Bi2Sr2Ca1− xYxCu2O8+δ

The experimental dispersion of the first removal state in the insulating Bi2Sr2Ca1− xYxCu2O8+δ regime is found to differ significantly from that of other parent materials: oxyclorides and La2CuO4. For Y contents of 0.92≥x≥0.55 due to nonstoichiometric effects in the Bi-O layers, the hole concentration in the CuO2 layers is almost constant and, on the contrary, the crystal lattice parameters a, b, c change very strongly. This (a, b) parameter increase and c parameter decrease results in an unconventional three peak structure at (0, 0), (π/2, π/2), (π, π) for x=0.92. We can describe the experimental data only beyond the framework of the three-band p-d-model involving the representations of a new triplet counterpart for the Zhang-Rice singlet state.