HEAT-CAPACITIES OF ONE-DIMENSIONAL HEISENBERG ANTIFERROMAGNETS UNDER HIGH MAGNETIC-FIELDS

Heat capacity measurements under magnetic fields are carried out on on e-dimensional Heisenberg antiferromagnets, a spin-Peierls (SP) and Hal dane gap system. In the SP system CuGeO3, the jump in the heat capacit y corresponding to SP transition is observed under fields up to 13 T. The obtained field-temperature phase diagram is in good agreement with the previous results of magnetic measurements. The heat capacity foll ows the exponential temperature dependence at low temperatures under z ero field, while it approaches T-3-dependence under external held. The heat capacity of Haldane gap system NINO is almost the same as that o f NENP, The energy gap at and above H-c is caused by the existence of the transverse moment.