Thermodynamic properties of a spin-1/2 Heisenberg model on the triangular lattice

By using the numerically exact diagonalization technique and a block-extended version of the finite-temperature Lanczos method, we study thermodynamic properties, such as entropy, specific heat, and uniform susceptibility, of a spin-1/2 Heisenberg model on the triangular lattice with the nearest-neighbor exchange interaction J and the four-spin exchange interaction J_c. Our calculations on small clusters containing up to 36 spins have found that, differently from the pure triangular-lattice (J_c=0) case, the temperature dependence of the specific heat exhibits a pronounced double-peak structure for finite and moderate J_c/J.