On form-factor expansions for the XXZ chain in the massive regime

We study the large-volume-L limit of form factors of the longitudinal spin operators for the XXZ spin-1/2 chain in the massive regime. We find that the individual form factors decay as L-n, n being an even integer counting the number of physical excitations-the holes-that constitute the excited state. Our expression allows us to derive the form-factor expansion of two-point spin-spin correlation functions in the thermodynamic limit L -> +infinity. The staggered magnetisation appears naturally as the first term in this expansion. We show that all other contributions to the two-point correlation function are exponentially small in the large-distance regime.