Zero Resistance Below Tc
In 1911 Heike Kamerlingh Onnes cooled mercury to 4.2 K and watched its electrical resistance vanish entirely — not just decrease, but drop to unmeasurably zero. This phase transition is one of the most dramatic in all of physics: below the critical temperature, electrons form Cooper pairs that glide through the lattice without scattering. A current set flowing in a superconducting ring persists for years without decay.
The BCS Theory
Bardeen, Cooper, and Schrieffer explained superconductivity in 1957 through a quantum many-body theory. An electron distorts the positive ion lattice, creating a region of slightly higher positive charge density that attracts a second electron. This phonon-mediated attraction binds electrons into Cooper pairs with opposite momenta and spins. The pairs condense into a collective ground state separated from excited states by an energy gap, making scattering impossible at low temperatures.
Magnetic Field Effects
Applied magnetic fields compete with the superconducting state. In type-I superconductors, the Meissner effect completely expels flux until the critical field Bc is reached, causing an abrupt transition to normal state. Type-II superconductors allow partial flux penetration through quantized vortices in a mixed state between lower critical field Bc1 and upper critical field Bc2. This simulation shows how the effective Tc decreases with increasing field and visualizes Cooper pair density.
From MRI to Quantum Computing
Superconducting magnets generate the powerful, stable fields needed for MRI scanners and particle accelerators like the Large Hadron Collider, which uses 1,232 NbTi dipole magnets cooled to 1.9 K. SQUID sensors detect magnetic fields a billion times weaker than Earth's, enabling magnetoencephalography. Today, superconducting transmon qubits operating at 15 millikelvin form the basis of leading quantum computers from IBM and Google.