Magnon specific heat of CuGeO₃ in the spin-Peierls state

We have used ac calorimetry to measure the specific heat of CuGeO₃ below its spin-Peierls transition temperature in magnetic fields up to 7T, and have compared it to values calculated from a model triplet magnon dispersion curve. We find that the observed field dependence is much smaller than that calculated with Zeeman splitting of the magnon gap, but is consistent with that calculated assuming m=0 excitation (and mean-field, quadratic magnetic field dependence) only. This result implies weak coupling between thermal phonons and m=±1 magnons, so that population of the latter are frozen for times < 40 ms. A similar assumption accounts for the zero-field specific heat at temperatures above 4.5 K.