𧲠Superconductivity
Below the critical temperature Tc, resistance drops to exactly zero and magnetic fields are expelled (Meissner effect). Explore BCS Cooper pairs, Type I/II phase diagrams, and Abrikosov flux vortices.
BCS Theory (Bardeen, Cooper, Schrieffer β 1957 Nobel Prize)
In a normal metal, electrons scatter off lattice vibrations (phonons), causing resistance. Below Tc, electrons form Cooper pairs via a phonon-mediated attraction: one electron distorts the lattice (leaving a positive polarization), which attracts a second electron. These pairs form a quantum condensate with a macroscopic wave function β they can flow without scattering.
The Meissner effect is the complete expulsion of magnetic flux from the interior of a superconductor. Surface currents spontaneously form to oppose any applied field. This is what levitates magnets above superconductors β it is not just perfect diamagnetism, but an active thermodynamic state.
Type I superconductors (like Hg, Pb) expel all
flux below Hc. Type II superconductors
(Nb, YBCO) allow flux to penetrate in quantized vortices between
Hc1 and Hc2 β enabling high-field magnets
and MRI. The BCS energy gap: 2Ξ β 3.52 k_B T_c.