Integral equations and long-time asymptotics for finite-temperature Ising chain correlation functions

This work concerns the dynamical two-point spin correlation functions of the transverse Ising quantum chain at finite (non-zero) temperature, in the universal region near the quantum critical point. They are correlation functions of twist fields in the massive Majorana fermion quantum field theory. At finite temperature, these are known to satisfy a set of integrable partial differential equations, including the sinh–Gordon equation. We apply the classical inverse scattering method to study them, finding that the 'initial scattering data' corresponding to the correlation functions are simply related to the one-particle finite-temperature form factors calculated recently by one of the persent authors. The set of linear integral equations (Gelfand–Levitan–Marchenko equations) associated with the inverse scattering problem then gives, in principle, the two-point functions at all space and time separations, and all temperatures. From these, we evaluate the long-time asymptotic expansion 'near the light cone', in the region where the difference between the space and time separations is of the order of the correlation length.