Below a critical temperature $T_c$, certain metals lose all DC resistance and expel magnetic fields from their interior — the Meissner effect. Microscopically (BCS theory, 1957), electrons near the Fermi surface bind into Cooper pairs via phonon-mediated attraction. Pairs are bosonic and condense into a coherent macroscopic ground state with an energy gap $\Delta$ to excitations:

The London equation $\nabla^2 \mathbf B = \mathbf B / \lambda_L^2$ shows fields decay exponentially on the London penetration depth $\lambda_L$. Type-II superconductors (most non-pure materials) admit quantized magnetic flux vortices, each carrying $\Phi_0 = h/2e$ — the "2e" is direct evidence of electron pairing.

Cuprate high-$T_c$ superconductors (1986) reach $T_c$ above the boiling point of liquid nitrogen (77 K). The exact pairing mechanism remains debated. Recent room-temperature claims (LK-99, etc.) have not been independently reproduced.

Interactive: Meissner expulsion vs temperature