In copper 2 is 100 times shorter than in silver. The cooling procedure is described in detail with the aid of Fig. 8. One of the difficult problems in these experiments is to measure the absolute temperature of the nuclei. It has been argued that negative temperatures are fictitious quantities because they do not represent true thermal equilibrium in the sample consisting of nuclei, conduction electrons, and the lattice. However, the experiments on silver, in particular, show conclusively that this is not the case.
Metals offer good models to investigate magnetism in general.
The nuclei are well localized, they are isolated from the electronic and lattice degrees of freedom at low temperatures, and the interactions between nuclei can be calculated from first principles. Therefore, these systems are particularly suitable for testing theory against experiments. The realm of negative spin temperatures offers new possibilities for studies of magnetism.