The Challenge of Temperature Measurement Below 0.3 K in Very High Magnetic Fields

Recent experiments on exotic heavy Fermion superconductors, superconducting organic conductors, metal-to-insulator transitions in solids, and on the two-dimensional electron gas in semiconductor devices (quantum Hall effect) have needed a combination of low temperatures (T < 0.3 K) and very high magnetic fields (B ≤ 23 T). The low temperatures are achieved with 3He–4He dilution refrigeration, and the high fields are obtained with resistive water-cooled Bitter solenoids powered by large DC generators. In the range 23 ≤ B ≤ 30 T a hybrid magnet arrangement consisting of a superconducting outer solenoid and Bitter inner solenoid is used. Detailed studies of the temperature dependence of the properties of the systems under investigation have made necessary increasing accuracy in temperature determination to distinguish between sometimes subtle differences in theoretical predictions. Two principal difficulties which arise in these investigations are: heating due to changes and fluctuations in the magnet current, and a strong magnetic field dependence of most temperature sensors below 1.0 K. These two factors combine to change both the temperature of the system under investigation and the calibration of thermometer. Present methods of temperature determination (vapor pressure, paramagnetic salts, calibrated resistors, capacitive transfer standards) will be discussed, and plans for primary thermodynamic thermometers with known magnetic field dependence will also be presented.