Physics below 1 K

Abstract

A temperature T can only be called if kT is small compared with some energy level spacing characteristic of the system. On this criterion 1° K is often not a low temperature, and the region below 1° K is one in which many events occur. The adiabatic demagnetization of paramagnetic bodies is the commonest technique for attaining these low temperatures, though other methods also exist. The main limitation on experiments is the small amount of power which can be absorbed by the working substances. At temperatures down to 3 × 10-2 ° K, a number of experiments have been performed, not only on the refrigerants but also on other substances. Results from several branches of physics, including solid state and nuclear physics, are briefly presented and discussed. Below 3 × 10-2 ° K only the refrigerants have been investigated; the lowest temperature yet reached is about 2 × 10-5 ° K.