Magnon Sideband of Linear Chain Antiferromagnets

The temperature dependence of the intensity, frequency and line width of a magnon sideband is calculated for a linear chain antiferromagnet by means of classical approximation for spins, and comparison is made with the observed results on N(CH 3 ) 4 MnCl 3 (TMMC) and CsMnCl 3 ·2H 2 O(CMC). At low temperatures, the classical appoximation gives results corresponding to the Néel state for the antiferromagnetic ground state, so that certain discrepancy exists between the present results and that of the simple spin wave theory. At high temperatures, the results should be more reliable. In order ro see the validity of the classical approximation, the internal energy of the system is also calculated by means of a series expansion with respect to 1/ S . The variation of the internal energy as a function of temperature according to this method is compared with the results of various other approximations and also with the exact numerical results.