Thermal conductivity of QMG-YBa₂Cu₃O_x in magnetic fields up to 14T

The a-b plane thermal conductivity κ for a QMG-YBa₂Cu₃O_x has been investigated in the temperature range 20K < T < 120K under magnetic fields up to 14 T applied parallel to the c-axis. In zero field, κ strikingly increases below T_c and takes a maximum near T_c/2. The magnitude of this increase is about 45% of the value at T_c. Above T_c, κ exhibits slow decrease with increasing temperature, which suggests the importance of the phonon-phonon scattering. By applying magnetic fields, κ is remarkably reduced. The temperature dependence of κ can be explained in terms of the relaxation time approximation including Umklapp processes. The degradation of κ due to a magnetic field is discussed as a function of a scaling parameter to BCS energy gap.