MAGNON SPECIFIC-HEAT OF CUGEO3 IN THE SPIN-PEIERLS STATE

We have used ac calorimetry to measure the specific heat of CuGeO3 bel ow its spin-Peierls transition temperature in magnetic fields up to 7T , and have compared it to values calculated from a model triplet magno n 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 m ean-field, quadratic magnetic field dependence) only. This result impl ies weak coupling between thermal phonons and m=+/-1 magnons, so that population of the latter are frozen for times < 40 ms. A similar assum ption accounts for the zero-field specific heat at temperatures above 4.5 K. Copyright (C) 1996 Elsevier Science Ltd