Theory of a single Oxygen hole propagating in Sr_2CuO_2Cl_2: the spin of the quasiparticles in CuO_2 planes

Abstract

Recent photoemission experiments have measured E vs. k for a single hole propa- gating in antiferromagnetically aligned Sr_2CuO_2Cl_2. Comparisons with (i) the t - t' - J model, for which the carrier is a spinless vacancy, and (ii) a strong-coupling version of the three-band Emery model, for which the carrier is a S = 1/2 hole moving on the Oxygen sublattice, have demonstrated that if one wishes to describe the quasiparticle throughout the entire first Brillouin zone the three-band model is superior. Here we present a new variational wave func- tion for a single Oxygen hole in the three-band model: it utilizes a classical representation of the antiferromagnetically ordered Cu-spin background but explicitly includes the quantum fluctuations of the lowest energy doublet of the Cu-O-Cu bond containing the Oxygen hole. We find that this wave function leads to a quasiparticle dispersion for physical exchange and hopping parame- ters that is in excellent agreement with the measured ARPES data. We also obtain the average spin of the Oxygen hole, and thus deduce that this spin is only quenched to zero at certain wave vectors, helping to explain the inadequacy of the t - t' - J model to match the experimentally observed disper- sion relation everywhere in the first Brillouin zone.