How do superconductors work?
What is a phonon? The atoms of a crystalline solid form a regular lattice structure, which can be likened to an array of balls connected by springs, the springs representing forces between neighbouring atoms. Such structures can vibrate mechanically in various ways, either because of thermal motion or some external force. A sound wave travelling through a solid is an example of the latter. Quantum mechanics tells us that these vibrations can only gain or lose energy in discrete amounts and these are called ‘phonons’, by analogy with photons for light. Under ordinary conditions there is an enormous number of phonons or photons present and we do not see this ‘graininess’. However, careful experiments confirm the picture. What causes the lattice distortions in a superconductor? A conducting solid, such as a metal, contains electrons which can move relatively freely through the background of positively charged ion cores. As an electron moves it exerts a Coulomb attraction on neighbouring i
