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, ${\mathit{J}}_2$/${\mathit{J}}_1$. At low temperatures and intermediate values of ${\mathit{J}}_2$/${\mathit{J}}_1$, 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 ${\mathit{J}}_2$/${\mathit{J}}_1$≊0.5 can be roughly described by a dimer state, this kind of state cannot explain an observed quasidegeneracy at ${\mathit{J}}_2$/${\mathit{J}}_1$≊0.6 of the values of the structure factor between q=(π,π), (0,π), and (π/2,π). We also calculated the twisted susceptibility at nonzero momentum. For q=(0,π/2) we found an enhancement of a factor 2 at ${\mathit{J}}_2$/${\mathit{J}}_1$≊0.6 with respect to the infinite-temperature limit.