Dynamical properties of antiferromagnetic Heisenberg spin chains

The dynamical properties of spin-1 and spin-1/2 antiferromagnetic Heisenberg chains (AHC’s) are studied by diagonalizing exactly clusters of up to 18 and 26 sites, respectively. It is shown that the spin-1 AHC has a quasi-single-mode spectrum for momentum k≥0.3π, while the low-energy edge of the spin-1/2 AHC is dominated by a spin-wave continuum. The dispersion curve obtained for the spin-1 chain is in excellent agreement with recent experiments on Ni(${\mathrm{C}}_2$ ${\mathrm{H}}_8$ ${\mathrm{N}}_2$ ${)}_2$ ${\mathrm{NO}}_2$ ${\mathrm{ClO}}_4$. The size dependence of the low-energy spectral weights is also analyzed.