Superconductivity

Cooper Pairs

In normal conductors, electrons scatter off vibrating atoms in the crystal lattice. Each collision transfers energy from the electron to the lattice, generating heat. This is resistance. Electrons lose momentum, current decays, and energy dissipates. Every wire you have ever touched is warm partly because of this process.

Below the critical temperature, something remarkable happens. Electrons form Cooper pairs: two electrons with opposite spin and opposite momentum, bound together through lattice vibrations called phonons. The mechanism is subtle. One electron moves through the lattice and slightly distorts it, pulling positive ions toward its path. This creates a brief region of higher positive charge density. A second electron, some distance away, is attracted to that region. The binding energy is tiny, easily overwhelmed by thermal vibrations at room temperature, but at sufficiently low temperatures it is enough.

These pairs are not like two tennis balls glued together. Electrons in a Cooper pair can be hundreds of nanometers apart, with millions of other electrons between them. What binds them is a quantum correlation, not proximity. Destroying one pair means disturbing the lattice vibrations that support all the other pairs. This collective protection is what makes superconductivity robust once established.

Why Pairs Flow Without Resistance

Individual electrons are fermions, particles with half-integer spin that obey the Pauli exclusion principle. No two fermions can occupy the same quantum state. This is why electrons in atoms fill orbitals in strict order and why ordinary metals have resistance: electrons compete for available states and scatter when disturbed.

Cooper pairs change this picture entirely. Two fermions bound together form a boson, a particle with integer spin. Bosons face no exclusion constraint. They can all pile into exactly the same quantum state simultaneously. Below the critical temperature, all Cooper pairs condense into a single macroscopic quantum state known as the BCS ground state, named after Bardeen, Cooper, and Schrieffer who explained it in 1957.

i The pair’s two half-integer spins add to a whole number; exclusion checks nothing else. ×

In this condensate, scattering one pair would require breaking it apart, which costs energy equal to the superconducting energy gap. Small perturbations from lattice vibrations or impurities simply do not carry enough energy to do this. Current flows without any mechanism to slow it down. Resistance is not merely small. It is forbidden by quantum mechanics.

Meissner Effect

A superconductor does more than conduct without resistance. It actively expels the magnetic field from its interior. This is the Meissner effect, and it is not simply a consequence of zero resistance. Place a material in a magnetic field and then cool it below the critical temperature. Field lines that were passing through are pushed out. Superconducting currents spontaneously arise on the surface, generating an opposing field that cancels the internal flux.

i The dashed lines are flux that got in anyway, and in Type II it steadies the levitation. ×

In Type I superconductors, the expulsion is complete – every field line is pushed out. But most practical superconductors are Type II, which allow a few magnetic flux lines to penetrate through narrow channels called vortices. These pinned flux lines actually make levitation more stable. Instead of a magnet sliding off to the side, flux pinning locks it in place – hovering rigidly in midair with no energy input.

This creates one of the most visually striking demonstrations in physics: magnetic levitation. A permanent magnet placed above a cooled superconductor floats indefinitely, locked in position by pinned flux. No engines, no electromagnets, no tricks. Just quantum mechanics holding a magnet in the air.

i Boiling nitrogen at 77 kelvin pours over the rim; before 1986 this needed liquid helium. ×

Two Types

Not all superconductors behave the same way in magnetic fields. Type I superconductors have a single critical field strength. Below it, the Meissner effect is complete and all flux is expelled. Above it, superconductivity is destroyed entirely and the material returns to normal. Pure metals like lead, mercury, and tin are Type I. Their critical fields are relatively low, limiting practical applications.

Type II superconductors are far more interesting. They have two critical fields. Below the first, the behavior is identical to Type I with complete flux expulsion. Between the first and second critical fields, something unusual happens: magnetic flux penetrates the material in quantized vortices, tiny tubes of normal material each carrying exactly one quantum of magnetic flux. The surrounding material remains superconducting. These Abrikosov vortices arrange themselves in a triangular lattice, a beautiful geometric pattern that emerges spontaneously from quantum mechanics.

This mixed state allows Type II superconductors to tolerate much stronger magnetic fields while maintaining zero resistance in the bulk. Every practical superconducting magnet uses Type II materials. Magnetic resonance imaging machines that image living tissue rely on niobium-titanium coils carrying enormous currents in fields that would destroy any Type I superconductor. Particle accelerators, fusion reactors, and maglev trains all depend on this remarkable ability to coexist with magnetic flux.

i Most of the hardware around the coil is there to keep it cold, not to carry the current. ×

High-Temperature Superconductors

For 75 years after Onnes, superconductivity seemed confined to temperatures near absolute zero. Then in 1986, Georg Bednorz and Karl Alexander Müller discovered a copper oxide ceramic that superconducted at 35 kelvin. Still cold by everyday standards, but dramatically warmer than anything before. They received the Nobel Prize in 1987, one of the fastest awards in history, reflecting how profoundly their discovery changed the field.

Within months, related compounds pushed critical temperatures above 77 kelvin, the boiling point of liquid nitrogen. This mattered enormously for practical applications because liquid nitrogen is cheap and abundant compared to liquid helium. Suddenly superconducting devices became accessible to laboratories worldwide rather than just a handful of cryogenics specialists.

In 2015, hydrogen sulfide was shown to superconduct at 203 kelvin, but only while squeezed to 155 gigapascals, about 1.5 million times atmospheric pressure and well inside the range found in Earth’s core. Lanthanum hydride reached 250 kelvin in 2019 under similar crushing conditions. That pressure is the catch. These materials superconduct almost at room temperature, but only inside a diamond anvil cell, in samples smaller than a grain of sand. Two claims from one group of getting past that limit have since collapsed: a 2020 result at 288 kelvin was retracted in 2022 over data-integrity concerns, and a 2023 result claiming near-ambient pressure was retracted the same year. Room-temperature superconductivity at ordinary pressure remains elusive. The mechanism behind high-temperature superconductivity is not fully understood either. Cooper pairing likely plays a role, but the attractive interaction between electrons is not conventional phonon exchange. Understanding these materials remains one of the most active and challenging areas of condensed matter research.

Josephson Junctions

Bring two superconductors almost together, separated by a barrier only a few atoms thick, and the paired electrons tunnel straight across it – a supercurrent flows with no voltage at all. This is the Josephson junction, predicted by Brian Josephson in 1962 while he was still a graduate student. Wire two junctions into a loop and you get a superconducting quantum interference device (SQUID), the most sensitive magnetometer ever built. It can register fields a billion times weaker than Earth’s, faint enough to map the electrical currents of a working brain.

The junction has a deeper trick. Because the whole circuit is a single coherent quantum state, its electrical configuration behaves like one quantum particle: it can tunnel out of a trapped state, and its energy comes in discrete, quantized levels. In the mid-1980s, John Clarke, Michel Devoret, and John Martinis demonstrated exactly that in a circuit large enough to hold in the hand – macroscopic quantum tunneling and quantized energy in an object you can see. Those levels are now the working parts of superconducting qubits, the heart of the quantum computers Google and IBM run today. The discovery earned the 2025 Nobel Prize in Physics.

Quantum Mechanics Made Visible

Superconductivity is quantum mechanics made visible. In most of physics, quantum effects hide at atomic scales, too small to see or touch. Superconductors are different. Billions of electrons lock into a single quantum state that spans an entire material, carrying current without resistance, expelling magnetic fields, behaving as one coherent entity you can hold in your hand. It is the clearest demonstration that quantum mechanics is not just a theory about tiny particles. It is the operating system of reality, and under the right conditions, it reveals itself at human scale.