How Superconductivity Eliminates Electrical Resistance at Low Temperatures

A Current That Flows Forever

In 1911, Dutch physicist Heike Kamerlingh Onnes cooled mercury to 4.2 kelvins — just 4.2 degrees above absolute zero — and measured its electrical resistance. It vanished. Completely. He called it superconductivity. Persistent currents in superconducting rings have been measured running without detectable decay for years; theoretically, they would continue for longer than the age of the universe. This is not merely low resistance — it is zero resistance, a fundamentally different physical state arising from quantum mechanics.

Superconductivity underlies some of the most powerful technologies available: MRI magnets, particle accelerator magnets at CERN, maglev train prototypes, and the superconducting qubits at the heart of quantum computers. Understanding it requires visiting both the phenomenology of what superconductors do and the quantum mechanical explanation of why.

What Superconductors Do

Two defining phenomena characterise superconductors: zero electrical resistance and the Meissner effect.

• Zero resistance: Below the critical temperature T_c, DC electrical resistance drops to exactly zero. The material carries current with no energy dissipation — no heat generated, no voltage drop.
• Meissner effect: A superconductor expels magnetic fields from its interior as it transitions into the superconducting state. This is distinct from perfect conductivity; it is an active expulsion, not merely preventing field changes.
• Persistent currents arise in superconducting loops; once established, the current flows indefinitely without an external power source.
• A superconductor returns to normal resistance if the magnetic field, current, or temperature exceeds critical thresholds (H_c, J_c, T_c).

BCS Theory: Cooper Pairs

The microscopic explanation came in 1957 from John Bardeen, Leon Cooper, and John Robert Schrieffer — the BCS theory, for which they shared the Nobel Prize in Physics in 1972. The key insight: electrons in a lattice can attract one another through phonons — quantised vibrations of the crystal lattice.

When an electron moves through the lattice, it attracts nearby positive ions slightly, creating a momentary region of increased positive charge density. A second electron, arriving a short time later, is attracted to this region. The two electrons become weakly bound — a Cooper pair — with opposite momenta and opposite spins. The binding energy is tiny, typically fractions of a millielectronvolt, which is why conventional superconductors require extremely low temperatures to preserve it.

Type I and Type II Superconductors

Superconductors divide into two types based on their response to magnetic fields.

• Type I superconductors (elemental metals like mercury, tin, aluminium) completely expel magnetic fields up to a critical field H_c, then abruptly transition to the normal state.
• Type II superconductors (alloys and compounds) have two critical fields, H_c1 and H_c2. Below H_c1, full Meissner effect; between H_c1 and H_c2, magnetic flux penetrates in quantised vortices (Abrikosov vortices) while superconductivity persists; above H_c2, fully normal.
• Niobium-titanium wire used in MRI magnets carries fields up to 8–9 tesla in liquid helium coolant — only possible because it is Type II.
• The CERN Large Hadron Collider uses niobium-titanium magnets cooled to 1.9 K with superfluid helium to generate the 8.33-tesla fields that bend proton beams around the 27-kilometre ring.

High-Temperature Superconductors

BCS theory predicts a maximum T_c of roughly 30 K for phonon-mediated superconductivity. In 1986, Georg Bednorz and K. Alex Müller discovered a lanthanum barium copper oxide compound superconducting at 35 K, shattering the expected ceiling. Yttrium barium copper oxide (YBCO) followed in 1987, superconducting at 93 K — above the boiling point of liquid nitrogen (77 K) and thus coolable with a relatively cheap, abundant refrigerant.

Cuprate superconductors operate via a different mechanism from BCS pairing, still not fully understood after nearly four decades of study. The electrons in cuprates are strongly correlated; pairing is mediated by magnetic fluctuations rather than phonons, producing d-wave symmetry in the order parameter rather than the isotropic s-wave of BCS. This is an active research area.

Josephson Junctions and Quantum Computing

When two superconductors are separated by a thin insulating barrier, Cooper pairs tunnel through the barrier — the Josephson effect, predicted by Brian Josephson in 1962 and confirmed experimentally within a year. Josephson junctions are the fundamental building blocks of SQUIDs (Superconducting Quantum Interference Devices), the most sensitive magnetometers known, detecting fields as weak as 10⁻¹⁸ tesla.

Josephson junctions are also the basis of superconducting qubits — the transmon and related designs used by IBM, Google, and others in quantum computing. The transmon qubit uses the anharmonic energy levels of a Josephson junction-capacitor circuit to encode quantum information. Google's 2019 quantum supremacy demonstration and subsequent advances all relied on superconducting transmon qubits cooled to approximately 15 millikelvins in dilution refrigerators.

Room-Temperature Superconductivity

A room-temperature superconductor would transform energy infrastructure, enabling lossless transmission lines, compact motors, and practical magnetic levitation. In 2023, Sukbae Lee and Ji-Hoon Kim claimed room-temperature, ambient-pressure superconductivity in a material called LK-99 (a lead-apatite compound). The claim generated intense global attention but was not replicated; LK-99 showed diamagnetic properties attributable to a magnetic impurity, not superconductivity. Hydrogen-rich compounds under extreme pressures — near-room-temperature superconductors at 200 GPa — remain the frontier, but ambient-pressure room-temperature superconductivity remains an open goal.