Thermodynamic and diamagnetic properties of weakly doped antiferromagnets

The finite-temperature properties of weakly doped antiferromagnets as model ed by the two-dimensional t-J model and relevant to underdoped cuprates are investigated by numerical studies of small model systems at low doping. Tw o numerical methods are used: the world line quantum Monte Carlo method wit h a loop cluster algorithm and the finite-temperature Lanczos method, yield ing consistent results. The thermodynamic quantities-specific heat, entropy , and spin susceptibility-reveal a sizable perturbation induced by holes in troduced into a magnetic insulator, as well as a pronounced temperature dep endence. The diamagnetic susceptibility introduced by a coupling of the mag netic field to the orbital current reveals an anomalous temperature depende nce, changing character from diamagnetic to paramagnetic at intermediate te mperatures.