Thermal conductivity down to 50 mK
Measuring thermal transport properties at temperatures below 1 K requires special experimental techniques. We apply a two-thermometer-one-heater method with RuO chip thermometers and a thin film resistor as heater. This setup is placed in a 3He/4He dilution refrigerator. Mechanical damping of vibrations from the pumps and appropriate electrical shielding and grounding are essential to restrict the heat input into the system.
An important issue for low-temperature thermal transport measurements is the thermal coupling of the sample to the thermometers, the heater, and the bath. This is achieved by evaporating gold pads as contacts directly onto the sample. The sample is clamped mechanically to the bath to further improve the thermal contact.
Another crucial point is the calibration of the sample thermometers, which is not reproducible for consecutive measurements, especially at very low temperatures (<100 mK). Therefore, we perform an in-situ calibration of the sample thermometers against the cold-finger thermometer for each temperature scan.
In so doing we are able to measure the thermal conductivity and the thermopower at temperatures down to about 30 mK.