SPIN-1/2 FRUSTRATED HEISENBERG-MODEL AT FINITE TEMPERATURE

We analyzed the thermodynamic properties of the two-dimensional spin-1/2 frustrated antiferromagnetic Heisenberg model. Using a Lanczos-like technique to avoid the diagonalization of the complete Hamiltonian matrix, we calculate the specific heat, uniform susceptibility, structure factor, and dimer susceptibility for different values of the frustration parameter, J2/J1. At low temperatures and intermediate values Of J2/J1, the enhancement of the column correlation function shows that in this regime the dimer state may be the ground state. Although the behavior of the magnetic structure factor at J2/J1 almost-equal-to 0.5 can be roughly described by a dimer state, this kind of state cannot explain an observed quasidegeneracy at J2/J1 almost-equal-to 0.6 of the values of the structure factor between q=(pi,pi), (0,pi), and (pi/2,pi). We also calculated the twisted susceptibility at nonzero momentum. For q = (0,pi/2) we found an enhancement of a factor 2 at J2/J1 almost-equal-to 0.6 with respect to the infinite-temperature limit.