8.9 SUPERCONDUCTIVITY
8.9.1 Zero Resistance and the Meissner Effect
In 1911 Kamerlingh Onnes at the University of Leiden in Holland observed that
when a sample of mercury is cooled to below 4.2 K, its resistivity totally vanishes
and the material behaves as a superconductor, exhibiting no resistance to current
flow. Other experiments since then have shown that there are many such substances,
not simply metals, that exhibit superconductivity when cooled below a critical
temperature Tc that depends on the material. On the other hand, there are also many
conductors, including some with the highest conductivities such as silver, gold, and
copper, that do not exhibit superconductivity. The resistivity of these normal
conductors at low temperatures is limited by scattering from impurities and crystal
defects and saturates at a finite value determined by the residual resistivity. The two
distinctly different types of behavior are depicted in Figure 8.44. Between 1911 and
1986, many different metals and metal alloys had been studied, and the highest