From free electron theory of metals, we know that by decreasing the temperature of a perfect metal, the thermal vibrations and electron scattering of ions (or atoms) decrease and hence the electrical resistance of the substance decreases. One would expect that the electrical resistance of a substance may become zero as its temperature reaches 0 K. Based on this point, experiments were conducted on many substances by scientists. In 1911, Kammerling Onnes found that the electrical resistance of pure mercury vanishes suddenly at 4.2 K as shown in Fig. 9.1. This temperature is called its superconducting transition temperature and denoted as TC. The superconducting transition temperature of a few important ...