Momentum distribution function for the two-dimensional t-J model

We study the momentum distribution function n(k) for the two-dimensional t-J model by high-temperature expansions. Starting with a k-independent n(k) at T=∞, we observe the buildup of a sharp Fermi-surface-like feature as the temperature is lowered. For very small density of electrons, our results are clearly consistent with Luttinger’s theorem. At all fillings the contours of constant n(k) closely resemble those of the tight-binding model. No evidence for pocketlike Fermi surfaces is seen. Far from the Fermi surface n(k) has a particle-hole asymmetry similar to that found in the one-dimensional, U=∞ Hubbard model.