Hubbard Fermi surface in the doped paramagnetic insulator

We study the electronic structure of the doped paramagnetic insulator by finite temperature Quantum Monte-Carlo simulations for the 2D Hubbard model. Throughout we use the moderately high temperature T=0.33t, where the spin correlation length has dropped to < 1.5 lattice spacings, and study the evolution of the band structure with hole doping. The effect of doping can be best described as a rigid shift of the chemical potential into the lower Hubbard band, accompanied by some transfer of spectral weight. For hole dopings <20% the Luttinger theorem is violated, and the Fermi surface volume, inferred from the Fermi level crossings of the `quasiparticle band', shows a similar topology and doping dependence as predicted by the Hubbard I and related approximations.