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Methods & Techniques

Methods for measuring the specific heat capacity

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The specific heat capacity of a substance is a very useful thermodynamic quantitiy. All internal degrees of freedom contribute to it and can be determined as a function of magnetic field and temperature. Especially superconducting and magnetic phase transitions show up as anomalies in the specific heat capacity.

heat capacity
The figure shows two silver sample holders to which the sample, a heater, and a thermometer are thermally coupled by thin wires of well defined thermal conductivity. The device can be screwed to the cold finger of a dilution refrigerator and be cooled to about 10 mK. Visible are also the electrical leads to the thermometers and heaters.

The main method for measuring the specific heat capacity is the so called semiadiabatic heat pulse technique. The sample along with a tiny heater and a small thermometer is connected to the sample holder by a "weak thermal link". The platform is cooled to a final temperature Tbase given by the temperature of the cold finger, the thermal conductance of the weak link and the residual heat leak. When a well defined heat puls ΔQ is applied, the sample along with the so called addendum (heater and thermometer) is heated to a slightly higher temperature Tbase + ΔT and relaxes back to Tbase with a time constant τ = Rthermal · C. The specific heat capacity at Tbase + ½ΔT is given by c = ΔQ/ΔT. The procedure is repeated at various temperatures Tbase and the specific heat capacity is thus determined as a function of T.