Abstract
Specific heat measurements in magnetic fields up to 5 T and at temperatures below 1 K are performed on single crystals of YBa 2Cu 3O 7−δ with a transition temperature of 89 K. The specific heat goes through a maximum with increasing magnetic field strength. This behavior and the temperature dependence of the zero field data, C = aT n + bT 3 and with n = 0.42, are characteristic for localized spins coupled via random Heisenberg antiferromagnetic exchange. The results are analyzed with the semi-empirical random exchange model of Bulaevskii. Only two free parameters are available, which are completely determined by the zero-field data. Using these values the field behavior is correctly predicted. Representative data of other groups are analyzed with the same model. Defect states that are not related to the high- T c mechanism are seen as an origin of this excess contribution to the lattice specific heat.
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