Energy Transport at Finite Temperatures in Isotropic Magnetic Chains withS=12

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Abstract
Energy transport is shown to be nondiffusive at finite temperatures in a magnetic chain of spin-½ particles with isotropic nearest-neighbor Heisenberg interactions in zero external field. In a uniform magnetic field, the Zeeman energy ensures that energy diffusion is reestablished at all temperatures if spin diffusion is present. At infinite temperatures the introduction of weak next-nearest-neighbor interactions also reestablishes energy diffusion. We use the Mori-Kawasaki expression for the diffusion constant in terms of the second and fourth moments of the time Fourier transform of the relaxation function. Investigation of the sixth moment indicates that the time derivative of the energy density exhibits diffusive behavior even though the energy density does not.