Thermodynamic Properties of the Planar t-J Model

Several thermodynamic quantities within the planar t-J model are calculated using the T>0 Lanczos method on clusters of up to 26 sites. The hole density c_h(μ,T) shows a non-Fermi-liquid behavior as a function of T and suggests a transition from small to large Fermi surfaces at c_h∼0.15. The specific heat reveals a maximum at the exchange energy scale up to c_h = 0.2, where it becomes almost T independent for T≳0.15t. At constant T the entropy has maximum for c_h∼0.15 with large values at low T, consistent with experiments on cuprates. In the underdoped regime the spin susceptibility χ₀(T) exhibits a maximum at finite T = T^*, with T^* decreasing with doping and disappearing for c_h>0.15.