Results of a molecular-dynamics study of the discrete sine-Gordon (SG) chain are reported, emphasizing φ-φ, sinφ-sinφ, and cosφ-cosφ dynamic correlation functions (φ is the SG field variable). Correlations at the temperature kBT0.29EK (EK is the continuum SG kink-soliton energy) are interpreted in terms of elementary linear and nonlinear modes---kinks, breathers, single-, and multiphonons. The validity of "ideal-gas" approximations is assessed and corrections from lattice discreteness and mode-mode interactions are discussed. Finally, the relevance of our results to planar ferromagnetic chains (e.g., CsNiF₃) in an easy-plane applied magnetic field is assessed.
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Kerr et al. (1981) studied this question.