Quasi-one-dimensional spin-$\frac{1}{2}$ Heisenberg magnets in their ordered phase: Correlation functions

We study weakly coupled antiferromagnetic spin chains in their ordered phase by combining an exact solution of the single-chain problem with a random-phase-approximation analysis of the interchain interaction. A single chain is described by a quantum Sine-Gordon model and dynamical staggered susceptibilities are determined by employing the form-factor approach to quantum correlation functions. We consider both antiferromagnetic order encountered in quasi-one-dimensional materials such as ${\mathrm{KCuF}}_3$ and spin-Peierls order as found in ${\mathrm{CuGeO}}_3.$