Superconductivity
A macroscopic quantum state of matter where certain materials exhibit exactly zero electrical DC resistance and complete expulsion of magnetic fields below a critical temperature T_c.
Governing formula
R = 0 (for T < T_c), B = 0 (Meissner expulsion)
SI unit
K (Critical Temperature T_c)
In depth
Discovered by Heike Kamerlingh Onnes in 1911 in liquid mercury. BCS theory explains low-temperature superconductivity as the pairing of conduction electrons into Bosonic 'Cooper pairs' via lattice phonon interactions, flowing without scattering.
Examples in the real world
Niobium-titanium (NbTi) superconducting magnets producing intense magnetic fields in MRI scanners and CERN accelerators.