Finite-temperature Drude weight within the anisotropic Heisenberg chain

Finite-temperature Drude weight (spin stiffness) D(T) is evaluated within the anisotropic spin-1/2 Heisenberg model on a chain using the exact diagonalization for small systems. It is shown that odd-side chains allow for more reliable scaling and results, in particular, if one takes into account corrections due to low-frequency finite-size anomalies. At high T and zero magnetization D is shown to scale to zero approaching the isotropic point Delta = 1. On the other hand, for Delta > 2 at all magnetizations D is nearly exhausted with the overlap with the conserved energy current. Results for the T variation D(T) are also presented.