  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.
