Superconductivity is a set of physical properties observed in certain materials where electrical resistance vanishes and magnetic flux fields are expelled from the material. Any material exhibiting these properties is a superconductor. Unlike an ordinary metallic conductor, whose resistance decreases gradually as its temperature is lowered, a superconductor has a characteristic critical temperature (Tc) below which the resistance drops abruptly to zero.

History

The phenomenon of superconductivity was discovered in 1911 by Dutch physicist Heike Kamerlingh Onnes at Leiden University. While studying the properties of solid mercury at cryogenic temperatures using recently-produced liquid helium, he observed that the electrical resistance of mercury abruptly disappeared at 4.2 K (−268.95 °C; −452.11 °F) APS Physics: Discovery of Superconductivity. For this and other research, Onnes was awarded the Nobel Prize in Physics in 1913.

In 1933, German physicists Walther Meissner and Robert Ochsenfeld discovered that superconductors expel applied magnetic fields, a phenomenon that came to be known as the Meissner effect. This discovery demonstrated that superconductivity is more than just perfect conductivity and is a distinct state of matter Britannica: Superconductivity.

A comprehensive microscopic theory of superconductivity was finally proposed in 1957 by American physicists John Bardeen, Leon Cooper, and John Robert Schrieffer. Their BCS theory described superconductivity as a macroscopic quantum mechanics|quantum-mechanical effect involving electron pairs, known as Cooper pairs. The trio shared the 1972 Nobel Prize in Physics for their work The Nobel Prize in Physics 1972.

A major breakthrough occurred in 1986 when J. Georg Bednorz and K. Alex Müller at the IBM research laboratory in Zurich discovered superconductivity in a lanthanum-based cuprate perovskite material at a temperature of 35 K, significantly higher than any previous superconductor. This discovery of so-called high-temperature superconductors earned them the 1987 Nobel Prize in Physics and opened a new field of research.

Fundamental Properties

A material's transition to a superconducting state is defined by several key properties:

Zero Electrical Resistance

The most defining characteristic of a superconductor is the complete disappearance of direct current (DC) electrical resistance below its critical temperature (Tc). In a closed loop of superconducting wire, an induced current can persist indefinitely with no power source. This property enables the creation of extremely powerful and stable electromagnets.

The Meissner Effect

When a superconductor is placed in a weak external magnetic field and cooled below its Tc, it expels all magnetic flux from its interior. This phenomenon, the Meissner effect, is distinct from the behavior expected of a perfect classical conductor. It is a defining signature of the superconducting state and is responsible for the effect of magnetic levitation, where a magnet can be levitated above a superconducting material.

Critical Parameters

The superconducting state exists only under certain conditions. Three critical parameters define the boundary of this state:

• –Critical Temperature (Tc): The temperature below which a material becomes superconducting.
• –Critical Magnetic Field (Hc): The maximum external magnetic field strength that a superconductor can withstand before it ceases to be superconducting.
• –Critical Current Density (Jc): The maximum electrical current density that a material can carry in its superconducting state before resistance reappears.

Classification of Superconductors

Superconductors can be classified based on their physical properties, material composition, or operating temperature.

By Response to a Magnetic Field

• –Type I Superconductors: These materials, typically pure metals like lead and mercury, exhibit a very sharp transition to a superconducting state and completely exclude all external magnetic fields (a perfect Meissner effect). They have a single, relatively low critical magnetic field, which limits their practical applications in high-field magnets.
• –Type II Superconductors: These materials, including alloys and ceramic compounds like Niobium-titanium (NbTi) and Yttrium barium copper oxide (YBCO), have two critical magnetic fields, Hc1 and Hc2. Between these two field strengths, the material enters a "mixed state" or "vortex state," where it allows partial magnetic field penetration in the form of quantized flux tubes, or vortices, while still exhibiting zero electrical resistance. Because their upper critical field (Hc2) can be extremely high, Type II superconductors are used in applications requiring strong magnetic fields DOE Explains...Superconductivity.

By Operating Temperature

• –Low-Temperature Superconductors (LTS): These are conventional superconductors with critical temperatures below 30 K (−243.15 °C). They are generally well-described by BCS theory and require cooling with expensive liquid helium.
• –High-Temperature Superconductors (HTS): These materials have critical temperatures above 30 K. A practically significant subset of HTS materials are those that superconduct above 77 K (−196.15 °C), the boiling point of liquid nitrogen, which is far more abundant and cheaper than liquid helium. Most HTS materials are ceramic compounds like cuprates or iron-based superconductors, and the mechanism behind their superconductivity is not yet fully understood.

Applications

The unique properties of superconductors enable a range of advanced technologies.

• –Powerful Magnets: The ability to carry large currents without energy loss makes superconductors ideal for powerful electromagnets. They are essential components in Magnetic Resonance Imaging (MRI) machines for medical diagnostics, Nuclear Magnetic Resonance (NMR) spectrometers for chemical analysis, and particle accelerators like the Large Hadron Collider at CERN for fundamental physics research CERN: Superconductivity.
• –Transportation: Superconducting magnets are used to levitate and propel maglev trains, offering the potential for high-speed, low-friction transport.
• –Electronics and Sensors: Superconducting circuits can be used to create extremely sensitive detectors of magnetic fields known as SQUIDs (Superconducting Quantum Interference Devices). SQUIDs are used in medical imaging to measure faint magnetic fields produced by the brain (magnetoencephalography).
• –Energy and Power: Research is ongoing into using superconducting cables for lossless electricity transmission, which could dramatically improve the efficiency of power grids. Superconducting magnetic energy storage (SMES) systems offer a way to store large amounts of energy.
• –Quantum Computing: Superconducting circuits are a leading platform for building quantum bits, or qubits, the fundamental components of a quantum computer. The quantum nature of the superconducting state can be harnessed to store and manipulate quantum information.