bei48482_FM

384 Chapter Ten

if undisturbed, and the electron scattering that leads to resistance in an ordinary

conductor does not occur.

A material with large-amplitude lattice vibrations may be only a fair conductor at

ordinary temperatures because electron scattering takes place frequently. However, the

same ease of lattice deformation means more strongly bound Cooper pairs at low tem-

peratures, and hence the material is more likely to be a superconductor then. Good

conductors, such as copper and silver, have small lattice vibrations at ordinary tem-

peratures, which means their lattices are unable to mediate the formation of Cooper

pairs at low temperatures and so they do not become superconducting. Such metals

as mercury, tin, and lead have large lattice vibrations at ordinary temperatures and so

are poorer conductors than copper and silver, but they are superconductors at low

temperatures.

Josephson Junctions

As we learned in Chap. 5, the wave nature of a moving particle allows it to tunnel

through a barrier that, in classical physics, it could not penetrate. Thus a small but de-

tectable current of electrons can tunnel through a thin insulating layer between two

metals. In 1962 Brian Josephson, then a graduate student at Cambridge University,

predicted that Cooper pairs could tunnel through what is now called a Josephson

junction,a thin insulating layer between two superconductors. The wave functions of

the Cooper pairs on each side of the junction penetrate the insulating layer with

exponentially decreasing amplitudes, just as the wave functions of individual electrons

would. If the layer is thin enough, less than 2 nm in practice, the wave functions over-

lap sufficiently to become coupled together, and the Cooper pairs they describe can

then pass through the junction. Josephson shared the 1975 Nobel Prize in physics for

his work.

In the dc Josephson effect,the current through a Josephson junction that has no

voltage across it is given by

dc Josephson effect IJImaxsin (10.27)

The small rectangle at the center of this photograph is a Josephson

junction 1.25 m wide.

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