When current flows through a copper wire, the wire warms up slightly; that heat is the result of electrons losing energy as they move through the material, which is what we call electrical resistance. Yet some materials, once cooled below a certain temperature, lose their resistance to every measurable value and, in practice, drop to exactly zero. This state is called superconductivity, and a current once started in a superconducting loop can circulate for years without any external power source. This article explains where this extraordinary behaviour comes from and why it appears only at very low temperatures.
Where Does Electrical Resistance Come From?
In an ordinary conductor, the electrons that carry the current move fairly freely through the crystal lattice formed by the atoms. That motion is not perfect, however: electrons scatter off lattice vibrations (thermal phonons), foreign atoms and defects, losing their direction and energy. The lost energy turns into heat, and that is resistance. As the temperature falls, lattice vibrations diminish and the resistance of most metals drops with them, but it never reaches zero; a "residual resistance" caused by defects always remains. Superconductivity is a sudden event, completely different from this gradual decline.
1911: Mercury's Resistance Vanishes Abruptly
Superconductivity was discovered in 1911 by the Dutch physicist Heike Kamerlingh Onnes. Onnes was the first person to liquefy helium, which allowed him to cool materials to temperatures very close to absolute zero (−273.15 °C). When he cooled solid mercury to about 4.2 kelvin (−269 °C), he found that its electrical resistance did not fall gradually but dropped abruptly to an immeasurably small value the moment a threshold was crossed. Onnes received the 1913 Nobel Prize in Physics for this discovery and for his work in low-temperature physics. In the following years, other metals such as lead, tin and niobium were found to behave the same way below their own critical temperatures.
The Meissner Effect: Superconductivity's Second Signature
For a long time superconductivity was thought of simply as "zero resistance." In 1933, Walther Meissner and Robert Ochsenfeld, working with tin and lead samples, found a far more striking property: as a material passes into the superconducting state, it expels the magnetic field from its interior. In this phenomenon, called the Meissner effect, the superconductor generates a counter-field with persistent currents at its surface that exactly cancels the applied field; the result is that the material behaves as a "perfect diamagnet." This is the key point that separates a superconductor from a merely very good conductor: a perfect conductor would trap a field that had already entered it, whereas a superconductor actively pushes it out. A magnet floating in the air above a superconducting plate is the best-known demonstration of this effect. The field is not driven entirely to zero; it seeps in from the surface over a very thin layer known as the London penetration depth, where it decays exponentially.
Cooper Pairs and BCS Theory
A microscopic explanation of superconductivity arrived only in 1957, with the theory developed by John Bardeen, Leon Cooper and John Robert Schrieffer, known by their initials as BCS (the trio received the 1972 Nobel Prize in Physics). At the heart of the theory is a surprising idea: two electrons, which normally repel each other, can weakly attract one another at very low temperature through the crystal lattice. As the first electron moves, it pulls the surrounding positive ions slightly toward itself and leaves behind a brief region of higher positive charge density; that region attracts a second electron. The slow response of the ions (retardation) lets this phonon-mediated attraction outweigh the direct repulsion between the electrons. The weakly bound pair of electrons formed in this way is called a Cooper pair. Because Cooper pairs behave as bosons as a whole, they all condense into the same quantum state and form a single, coherent "superfluid." Breaking this shared state requires crossing a certain energy threshold (the energy gap); at low temperature the lattice vibrations cannot supply that threshold, so the pairs flow without scattering — that is, without resistance.
Type I and Type II Superconductors
Superconductors are divided into two groups according to how they respond to a magnetic field. Type I superconductors (most pure metals) expel the field completely below a certain critical field and lose superconductivity all at once above it. Type II superconductors (alloys and ceramic compounds) remain in a "mixed state" between two critical field values: the magnetic field penetrates the material as thin flux filaments (vortices) while the surrounding region stays superconducting. This allows Type II materials to withstand much higher magnetic fields, and they are the kind used to build powerful magnets.
High-Temperature Superconductors and the Room-Temperature Goal
For decades every known superconductor worked only below 23 kelvin, which required expensive liquid helium. In 1986, Georg Bednorz and Alex Müller found much higher critical temperatures in copper-oxide-based ceramics (a Nobel Prize the following year). The yttrium-barium-copper-oxide (YBCO) compound discovered soon after became superconducting at 93 kelvin, meaning that cheap and abundant liquid nitrogen (77 K) could be used as the coolant. Whether these "high-temperature" superconductors can be fully explained by the BCS mechanism is still an open research question. In recent years, compounds such as sulfur and lanthanum hydrides have been shown to superconduct near room temperature under pressures of millions of atmospheres; however, a superconductor that works at room temperature and ordinary pressure has not yet been achieved.
Where Is Superconductivity Used?
Zero resistance and the ability to generate strong magnetic fields make superconductors indispensable in certain technologies. The MRI (magnetic resonance imaging) machines in hospitals get their strong, stable fields from superconducting coils. The Large Hadron Collider at CERN uses thousands of niobium-titanium magnets cooled to 1.9 kelvin to steer its particles. Magnetically levitating (maglev) trains ride without touching the track thanks to superconducting magnets. Loss-free power cables, extremely sensitive magnetic-field sensors (SQUIDs) and the qubits of quantum computers based on Josephson junctions also rely on this phenomenon.