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 Δ=1. On the other hand, for Δ>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.