The formation of bound states of holes in an antiferromagnetic spin-1/2 background is studied using numerical techniques applied to the t-J Hamiltonian on clusters with up to 26 sites. An analysis of the binding energy as a function of cluster size suggests that a two-hole bound state is formed for couplings larger than a ``critical'' value J/t${\mathrm{\ensuremath{\Vert}}}_{\mathit{c}}$ The symmetry of the bound state is
dx2-
y2. We also observed that its ``quasiparticle'' weight
Z2h (defined in the text), is finite for coupling J/t\ensuremath{\ge}J/t${\mathrm{\ensuremath{\Vert}}}_{\mathit{c}}$. Thus, in the region J/t\ensuremath{\ge}J/t${\mathrm{\ensuremath{\Vert}}}_{\mathit{c}}$ the bound state of two holes behaves like a quasiparticle with charge Q=2e, spin S=0, and
dx2-
y2 internal symmetry. The relation with recent ideas that have suggested the possibility of d-wave pairing in theories of the high-
Tc cuprates is briefly discussed.
The states of a single hole and of two holes in the planar quantum antiferromagnet are studied within the t-J model by performing the cumulant expansion in t/J and in the exchange anisotropy \ensuremath{\gamma}. Ground-state energies are calculated up to the n=12 order in t/J and to the m=6 order in \ensuremath{\gamma}. It is shown that results, which can be extended to smaller J/t, represent a very helpful supplement to exact diagonalization methods. In particular they yield finite-size corrections to previous results on the dispersion of single-hole states and to the binding energy of two holes, giving also more insight into the origin of binding.
The effective single-band Hamiltonian for holes in the two-dimensional quantum antiferromagnet, relevant for the
CuO2 layers in copper-oxide superconductors, is studied by the exact diagonalization of a finite-size system with 16 cells. Single- and two-hole ground-state properties are calculated. Pair-binding-energy and hole-density correlation functions indicate that two holes bind for moderate exchange interactions, even in the case of the extreme anisotropic-Ising limit.