SUPERCONDUCTIVITY Defined as the absence of electrical resistance: R=0 V I V=IR, so V=0

Absence of Friction Since P=I 2 R, R=0 implies no power dissipation Current flow without heat production opens up a huge range of possible applications: –Power transmission –Electromagnets (main present application) –Windings in motors Why not get rid of copper?

Low Temperatures Required! Discovered in 1911 when He was first liquefied,and the resistance of Hg was measured down to 2K. R T TcTc Sudden disappearance is indicative of a collective phenomenon

T c (K) YEAR

Microscopic origin of resistance in normal, nonsuperconducting metals Scattering from disorder of various kinds Collisions generate the heat that dissipates energy

Collective Pairing of the Electrons Inhibits Scattering The size of the pairs varies: in low-T superconductors it is about 1000 A, but in high-T superconductors only about 20 A. The collective nature implies that if one pair breaks, then the binding energy is reduced for all pairs. This is why there is no scattering. Superconductivity is a macroscopic quantum effect!

Origin of Attractive Interaction First note that the attraction is extremely unexpected – like charges should repel! Because of the very slow motion of the positive ions, there can be a tiny region of attraction in spacetime This explanation doesn’t work in high-T superconductors - Can spins do a similar trick??

Meissner Effect Not only is R=0, but also B=0: a superconductor shields its interior from magnetic fields