Spin bags and quasiparticles in doped La2CuO4

Limited configuration-interaction calculations beyond Hartree-Fock are reported for an ab initio derived eight-band effective Hamiltonian for ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$. These results clearly implicate two important types of configurations, as well as the role of apical-O ${\mathrm{p}}_{\mathrm{z}}$ states, in the nature and dispersion of quasiparticle states associated with one hole doped into the parent insulator. The two types of configurations are spatially inhomogeneous ``spin bags'' centered about a flipped spin relative to the host antiferromagnetic order, and ``doped N\'eel'' configurations that leave the host spin order intact. Taken together these complementary parts are suggestive of the local singlet of Zhang and Rice. It is, however, the latter doped N\'eel configurations that carry those apical-O ${\mathit{p}}_{\mathit{z}}$ components that are primarily responsible for very shallow quasiparticle dispersion from k=(\ensuremath{\pi}/2,\ensuremath{\pi}/2), where the first holes are doped, towards (\ensuremath{\pi},0).