Spectral function at finite doping in thet-Jmodel

We investigate the effect of doping in the hole spectral function by exact diagonalization of a 16-site cluster with two holes. It is shown that for more than one hole the spectral function differs qualitatively from the one in the single-hole limit. In particular, we find no evidence for the formation of a Fermi surface having hole ‘‘pockets’’ localized around (±π/2,±π/2) in momentum space. On the contrary, a large low-energy peak appears in the spectral function of the hole propagator at k=(π,0) or (0,π) which is then the best candidate for the existence of those pockets. A (π/2,π/2) quasihole state is obtained as a small energy excitation of the two-hole system. We discuss some consequences for photoemission and optical-conductivity experiments in high-${\mathit{T}}_{\mathit{c}}$ materials at intermediate doping.