The thermal conductivity of the spin-Peierls ͑SP͒ compound CuGeO 3 was measured in magnetic fields up to 16 T. Above the SP transition, the heat transport due to spin excitations causes a peak at ϳ22 K, while below the transition the spin excitations rapidly diminish and the heat transport is dominated by phonons; however, the main scattering process of the phonons is with spin excitations, which demonstrates itself in an unusual peak in at ϳ5.5 K. This low-temperature peak is strongly suppressed with magnetic fields in excess of 12.5 T. RAPID COMMUNICATIONS R2914PRB 58 YOICHI ANDO et al. RAPID COMMUNICATIONS R2916PRB 58 YOICHI ANDO et al.

The magnetic field dependence of the specific heat of Zn-doped single crystals of YBa 2 Cu 3 O 6.95 was measured between 2 and 10 K and up to 8 T. Doping levels of 0, 0.15%, 0.31%, and 1% were studied and compared. In particular we searched for the Schottky anomaly associated with the Zn-induced magnetic moments.

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An innovative microcalorimeter has been developed for measuring specific heat of very small microgram samples in the temperature range from 1.5 to 50 K and in magnetic fields up to 11 T. The device is built from a commercial sapphire temperature chip (Cernox), which is modified by means of standard microfabrication techniques and which is used as a sample holder, temperature sensor, and sample heater. Compared to existing microcalorimeters the simple design of our instrument allows a fabrication of the device in a few process steps by using facilities present in a standard laboratory clean room. As an illustrative example for the performance of our device, the specific heat of an underdoped (La1−xSrx)2CuO4 and CaRuO3 single crystal has been measured by means of the relaxation time method as well as the ac method.