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
recorded critical temperature was about 23 K in a niobium-germanium compound
(NbsGe) whose superconductivity was discovered in the early 1970s. In 1986 Bednorz
and Miiller, at IBM Research Laboratories in Zurich, discovered that a copper
oxide-based ceramic-type compound La-Ba-Cu-O, which normally has high resistivity,
becomes superconducting when cooled below 35 K. Following this Nobel
prize-winning discovery, a variety of copper oxide-based compounds (called cuprate
ceramics) have been synthesized and studied. In 1987 it was found that yttrium barium
copper oxide (Y-Ba-Cu-O) becomes superconducting at a critical temperature
of 95 K, which is above the boiling point of nitrogen (77 K). This discovery was particularly
significant because liquid nitrogen is an inexpensive cryogent that is readily
liquified and easy to use compared with cryogent liquids that had to be used in the