Superconductor

Superconductivity was first discovered in 1911 by Dutch physicist Heike Kamerlingh Onnes (1853–1926). After succeeding in liquefying helium (He), Onnes observed that the electrical resistance of a mercury filament dropped abruptly to an experimentally undetectable value at a temperature near -451.84°F (-268.8°C, 4.2K), the boiling point of helium. Onnes wrote: "Mercury has passed into a new state, which, because of its extraordinary electrical properties, may be called the superconductive state."

The temperature below which the resistance of a material = zero is referred to as the superconducting transition Figure 1. The curve of resistance R(Ω) versus temperature T (K) of a mercury filament (after H. Kamerlingh Onnes, 1911). Illustration by Hans & Cassidy. Courtesy of Gale Group. Figure 2. A type-II superconductor in various states under the application of magnetic field. Illustration by Hans & Cassidy. Courtesy of Gale Group. temperature or the critical temperature of that material, T_c. Another unique characteristic of superconductors is their diamagnetic property, which was discovered by German physicist W. Meissner (1882–1974), working with a graduate student, in 1933. When a superconducting object is placed in a weak magnetic field, a persistent super-current or "screening current" is set up on its surface. This persistent current induces a magnetic field that exactly mirrors or cancels the external field, and the interior of the superconductor remains field-free. This phenomenon is called the Meissner effect and is the basis of the ability to of superconducting objects to levitate magnets. (Levitation only occurs when the force of repulsion of the magnetic field, which is a function of the field's intensity, exceeds the weight of the magnet itself.)

Superconductors are categorized as type I (soft) and type II (hard). For type I superconductors (e.g., most pure superconducting elements, including lead, tin, and mercury), diamagnetism and superconductivity break down together when the material is subjected to an external magnetic field whose strength is above a certain critical threshold H_c, the thermodynamic critical field. For type II superconductors (e.g., some superconducting alloys and compounds such as Mb₃Sn), diamagnetism (but not superconductivity) breaks down at a first threshold field strength H_c1 and superconductivity persists until a higher threshold H_c2 is reached. These properties arise from differences in the ways in which microscopic swirls or vortices of current tend to arise in each particular material in response to an external magnetic field.

No unified or complete theory of superconductivity yet exists. However, the basic underlying mechanism for Figure 3. Increase in T_c with time since superconductivity was discovered. Illustration by Hans & Cassidy. Courtesy of Gale Group. superconductivity has been suggested to be an electron-lattice interaction. U.S. physicists John Bardeen (1908–1991), Leon Cooper (1930–), and Robert Schrieffer (1931–) derived a theory (termed the BCS theory, after their initials) in 1957, proposing that in the lattice of atoms comprising the material, pairing occurs between electrons with opposite momentum and spin. These electron pairs are called Cooper pairs, and as described by Schrieffer, they condense into a single state and flow as a totally frictionless "fluid." BCS theory also predicts that an energy gap—energy levels a discrete amount below those of normal electrons—exists in superconductors. English Brian Josephson (1940–), in 1962, proposed that Cooper pairs could tunnel from one superconductor to another through a thin insulating layer. Such a structure, called a Josephson junction, has for years been fabricated widely for superconducting electronic devices.