Relaxation-coupled order-parameter oscillation in a transverse Ising system

A novel technique, which employs ωχ’/χ’ ’ of the longitudinal susceptibility in place of the shape function χ’ ’/ωχ(0), reveals a coupling of the heavily overdamped soft mode to a relaxation mode in the transverse Ising ferromagnet ${\mathrm{LiTbF}}_4$. Our analysis by exact Mori-Langevin equations suggests that this contribution to the central peak, which becomes temperature dependent only below ${\mathrm{T}}_{\mathrm{c}}$, arises from a dynamical coupling to the transverse polarization.