Basic Properties and Parameters of Superconductors 1)

1.1.1.1 Superconducting Transition and Loss of DC Resistance In the year 1908, Kamerlingh-Onnes [3], director of the Low-Temperature Laboratory at the University of Leiden, had achieved the liquefaction of helium as the last of the noble gases. At atmospheric pressure, the boiling point of helium is 4.2 K. It can be reduced further by pumping. The liquefaction of helium extended the available temperature range near the absolute zero point and Kamerlingh-Onnes was able to perform experiments at these low temperatures. At first, he started an investigation of the electric resistance of metals. At that time, the ideas about the mechanism of the electric conduction were only poorly developed. It was known that it must be electrons being responsible for charge transport. Also one had measured the temperature dependence of the electric resistance of many metals, and it had been found that near room temperature the resistance decreases linearly with decreasing temperature. However, at low temperatures, this decrease was found to become weaker and weaker. In principle, there were three possibilities to be discussed: