ac-Calorimetry at High Pressure and Low Temperature

Recent developments of the ac-calorimetric technique adapted for the needs of high pressure experiments are discussed. A semi-quantitative measurement of the specific heat with a Bridgman-type of pressure cell as well as a diamond anvil cell is possible in the temperature range 0.1 K< T < 10 K. The pressure transmitting medium used to ensure good pressure conditions determines to a great extent via its thermal conductivity the operating frequency and thus the accessible temperature range. Investigations with different pressure transmitting media for T > 1.5 K reveal for solid He a cut-off frequency which is considerably higher than for steatite. Experiments below 1 K and pressures above 10 GPa clearly show that the pressure dependence of the linear temperature coefficient of the specific heat can be measured. It is in qualitative agreement to a related quantity obtained quasi-simultaneously by electrical resistivity measurements on the same sample. The specific heat (C) is an important thermodynamic quantity. Its temperature dependence can deliver hints about microscopic energy scales and provides a powerful tool to identify phase transitions. In this respect temperature (T ) dependent measurements are an indispensable means not only for experimentalists. This has triggered the development of different and very sophisticated technical realizations to obtain C(T ) from the millikelvin range up to very high temperature. The available methods can be divided in two categories. Adiabatic techniques are considered as the most accurate way to estimate the absolute value of C(T ). They require sample masses of several grams and the subtraction of the addenda, i. e., the specific heat of sample holder and thermometer. Among the non-adiabatic (or dynamic) methods, ac-calorimetry is a suitable technique for samples with masses well below one milligram. The specific heat can be measured with very high sensitivity, despite the small masses. However, the absolute accuracy which can be achieved is less than for the adiabatic methods. Adiabatic techniques are used to detect pressure-induced phase transitions or to investigate the evolution of electronic properties as the unit cell volume is reduced. The sample masses needed demand large volume pressure cells, such as a piston-cylinder cell. With this technique the accessible pressure range is, however, limited to about 3.5 GPa. Very often it would be desirable for the pressure range to be extended. In this case an anvil-type of pressure cell is the only alternative. Such a high pressure tool demands a