Spin Thermal Conductivity of the Haldane Chain CompoundY2BaNiO5

We have measured the thermal conductivity of the spin $S=1$ chain compound ${\mathrm{Y}}_2{\mathrm{BaNiO}}_5$. Analyzing the anisotropy of the thermal transport allows us to identify a definite spin-mediated thermal conductivity ${\kappa }_s$ along the chain direction. The calculated spin-related energy diffusion constant $D_E(T)$ shows a broad peak around 120 K. Close to room temperature, $D_E(T)$ approaches the theoretically predicted high-temperature value, while scattering of spin excitations by magnetic impurities seems to be the major limiting factor of ${\kappa }_s$ at low temperature.