Superconductivity

Superconductivity is a quantum mechanical phenomenon in which certain materials exhibit exactly zero electrical resistance and expel magnetic fields when cooled below a characteristic critical temperature. Unlike ordinary metallic conductors, whose resistance gradually decreases with falling temperature, a superconductor undergoes an abrupt phase transition into a state where electrical current can flow indefinitely without energy loss. This remarkable behaviour, first identified in 1911 by the Dutch physicist Heike Kamerlingh Onnes, is accompanied by the Meissner effect—the complete expulsion of magnetic flux from a material’s interior—which distinguishes superconductors from hypothetical perfect conductors in classical physics.
The discovery of high-temperature superconductors in 1986, particularly cuprate ceramics with critical temperatures above the boiling point of liquid nitrogen, revolutionised the field by enabling more practical laboratory applications. Since liquid nitrogen is inexpensive and readily accessible, materials such as YBCO allowed superconductivity to be achieved without the extreme cryogenic requirements of earlier systems.

Historical development

Superconductivity was first observed when Onnes cooled solid mercury using liquid helium and recorded a sudden disappearance of electrical resistance at around 4.2 K. Unbeknown to him at the time, he also observed superfluidity in helium, though its significance was not recognised. Subsequent discoveries identified superconductivity in other materials, including lead in 1913 and niobium nitride in 1941, each with higher critical temperatures.
A major conceptual advance occurred in 1933 when Walther Meissner and Robert Ochsenfeld demonstrated that superconductors actively exclude magnetic fields during the transition to the superconducting state. This showed that superconductivity was not merely an extension of perfect conductivity, but a qualitatively distinct thermodynamic phase.
In 1935, Fritz and Heinz London produced a phenomenological description encapsulated in the London equations, which explained the exponential decay of magnetic fields inside a superconductor. These equations reproduced the Meissner effect and provided the first theoretical framework for the phenomenon.
From the 1950s, further theoretical progress emerged. In 1950, Lev Landau and Vitaly Ginzburg developed the Ginzburg–Landau theory, which successfully described macroscopic superconducting properties and predicted the distinction between type I and type II superconductors. Experimental work demonstrating the isotope effect in the same year indicated that lattice vibrations—phonons—play a crucial role in the formation of the superconducting state.
The full microscopic explanation arrived in 1957 with the Bardeen–Cooper–Schrieffer (BCS) theory. It interpreted superconductivity as the formation of Cooper pairs, bound electron pairs that move collectively without resistance. Subsequent refinements by Bogolyubov and Gorkov further clarified the mathematical foundation and linked BCS theory with the earlier Ginzburg–Landau formulation. Although extremely successful for conventional superconductors, the applicability of such models to unconventional materials remains an area of active research.

Microscopic and macroscopic properties

Superconductivity involves unique quantum mechanical effects observable on macroscopic scales. The defining features include:

• Zero electrical resistance, enabling persistent currents in closed superconducting loops without any external power source.
• The Meissner effect, whereby a superconductor expels external magnetic fields, ensuring that magnetic flux cannot penetrate the material below its critical temperature.
• Flux quantisation, where magnetic flux in superconducting rings can exist only in discrete multiples of a fundamental quantum.
• Cooper pairing, the condensation of electron pairs mediated by phonons or other mechanisms, forming a coherent quantum state.

Superconductors are further categorised into type I and type II materials. Type I superconductors exhibit complete flux expulsion until a critical field is reached, whereas type II superconductors allow quantised vortices of magnetic flux to penetrate at intermediate field strengths. This behaviour makes type II materials especially valuable for high-field applications.

Practical materials and technological applications

The development of superconducting materials suitable for engineering applications progressed steadily through the twentieth century. One of the earliest technological devices exploiting superconductivity was the cryotron, a switching element invented in 1954 that used the contrasting critical fields of two superconductors.
Early attempts by Kamerlingh Onnes to construct superconducting electromagnets were limited by the materials’ low tolerance for magnetic fields. Advances occurred in the 1950s and 1960s. Researchers developed practical superconducting wires from niobium-based alloys, including brittle but powerful niobium–tin compounds and more ductile niobium–titanium alloys. These became essential for generating magnetic fields in the range of 10 to 20 tesla and are widely used in MRI systems and particle accelerator magnets.
By 2014, superconductivity-enabled technology accounted for billions of euros in global economic activity, with medical imaging dominating commercial use.

The Josephson effect

A major conceptual breakthrough came in 1962 when Brian Josephson predicted that a supercurrent could flow between two superconductors separated by a thin insulating barrier. This phenomenon, now known as the Josephson effect, underlies devices such as SQUIDs (superconducting quantum interference devices), which are capable of detecting extremely small magnetic fields with unparalleled sensitivity. The Josephson effect also plays a critical role in precision metrology, enabling highly accurate determination of the magnetic flux quantum and contributing to fundamental measurements of the Planck constant.

High-temperature superconductivity and open questions

The discovery of superconducting cuprates—ceramic materials with perovskite structures—in 1986 astonished researchers because their critical temperatures far exceeded expectations based on conventional BCS theory. The transition temperature rose further with the development of yttrium–barium–copper oxide (YBCO), which enabled stable superconductivity above the boiling point of liquid nitrogen. These “high-temperature” superconductors opened accessible paths for applications requiring reduced refrigeration costs.
Despite extensive study, the mechanism responsible for high-temperature superconductivity remains one of the central challenges in condensed matter physics. Although some features resemble conventional electron–phonon superconductors, evidence suggests that alternative interactions may be responsible.