Decay of currents for strong interactions.

The decay of current autocorrelation functions is investigated for quantum systems featuring strong interactions. Here the term "interaction" refers to that part of the Hamiltonian causing the (major) decay of the current. On the time scale before the (first) zero crossing of the current, its relaxation is shown to be well described by a suitable perturbation theory in the lowest orders of the interaction strength, particularly if interactions are strong. In this description the relaxation is found to be rather close to a Gaussian decay and the resulting diffusion coefficient approximately scales with the inverse interaction strength. These findings are confirmed by numerical results from exact diagonalization for several one-dimensional transport models including spin transport in the Heisenberg chain with respect to different spin quantum numbers, anisotropy, next-nearest-neighbor interaction, or alternating magnetic field; energy transport in the Ising chain with tilted magnetic field; and transport of excitations in a randomly coupled modular quantum system. The impact of these results for weak interactions is discussed.