Dispersion of a Hole in a Two-DimensionalCu3O4Plane: A Tale of Two Singlets

Polarization-dependent angle-resolved photoemission spectroscopy measurements of ${\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_4{\mathrm{Cl}}_2$ indicate the presence of two different Zhang-Rice singlets in the two-dimensional ${\mathrm{Cu}}_3{\mathrm{O}}_4$ plane of this insulating copper oxychloride. With the aid of model calculations, we show that one singlet is moving in the antiferromagnetically ordered cupratelike ${\mathrm{Cu}}_A{\mathrm{O}}_2$ subsystem and the other on the effectively paramagnetic sublattice formed by the extra ${\mathrm{Cu}}_B$ atoms. The dispersion of the former is thus determined by the superexchange integral $J_A$, and that of the latter by the hopping integral $t_B$. Consequently, as far as the role of the magnetic background is concerned, the dispersion relation of a single hole in this system represents simultaneously both the low and very high doping limits of a cuprate plane in a single structure.