Superconductor

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Superconductor

A superconductor is a material that exhibits zero resistance to the flow of electrical current and becomes diamagnetic (opaque to magnetic fields) when cooled to a sufficiently low temperature.

An electrical current will persist indefinitely in a ring of superconducting material; also, a magnet can be levitated (suspended in space) by the magnetic field produced by a superconducting, diamagnetic object. Because of these unique properties, superconductors have found wide applications in the generation of powerful magnetic fields, magnetometry, magnetic shielding, and other technologies. Many researchers are seeking to devise high-temperature superconductors—materials that superconduct at or above the boiling point of nitrogen (N₂), 77 K (Kelvin)—in order to carry large amounts of current without lapsing from the superconducting state. Such materials are already increasingly useful in power transmission and other applications.

Superconductivity history and theory

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 (Figure 1). 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 equals zero is referred to as the superconducting transition 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 Walther Meissner (1882−1974), working with a graduate student, in 1933. When a superconducting object is placed in a weak magnetic field, a persistent supercurrent, 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 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 (Figure 2).

No unified or complete theory of superconductivity yet exists. However, the basic underlying mechanism for superconductivity has been suggested to be an electron-lattice interaction. American physicists John Bardeen (1908−1991), Leon Niels Cooper (1930−), and John 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 physicist Brian David 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.

High-temperature superconductors

Before 1986, although a variety of superconductors had been discovered and synthesized, all had critical temperatures at or below the boiling point of He (e.g., Pb at−446.5°F [−265.8°C, 7.19 K] and Nb₃Sn at−426.91°F [−254.95°C, 18.05 K]). Since expensive refrigeration units are required to produce liquid He, this strictly limited the circumstances under which it was economical to apply superconductivity (Figure 3).

The first superconductor to be discovered having T_c> −320.8°F (−196.0°C, 77 K) (the boiling point of liquid N₂, which is much cheaper to produce than liquid He) was YBa₂Cu₃O₇ (T_c~−294°F [−181°C, 92 K]). The Y-Ba-Cu-O compound was discovered by American physicist C. W. Chu (1948−), working with a graduate student, in 1987 following the 1986 discovery by German physicist J. Georg Bednorz (1950−) and Swiss physicist K. Alexander Müller (1927−) of the La-Ba-Cu-O oxide superconductor (T _c = −394.6°F [−237.0°C, 36 K]). One year later, in 1988, bismuthbased (e.g., (Bi, Pb)₂Sr₂Ca₂Cu₃O₁₀, T _c = −261.4°F [−163.0°C, 110 K]) and thallium-based (e.g.,Tl₂Ba₂Ca₂Cu₃O₁₀, T _c = −234.4°F[−148.0°C, 125 K]) superconductors were successfully synthesized; their T _c ’s were some 20K higher than that of Y-Ba-Cu-O. Recently, mercury-based cuprates (HgBa₂Ca_n-1Cu_nO2_n+2+δ) have been shown to have T _c values higher than 130 K. These oxide superconductors are now classified as the high-temperature (or high-T _c) super-conductors (HTSCs).

All HTSCs so far discovered have an atomic structure that consists of thin planes of atoms, many of which consist of the compound copper dioxide (CuO₂). This compound is, so far, uniquely important to producing the property of high-temperature conductivity. Ironically, CuO₂ is a Mott insulator, meaning that at temperatures approaching absolute zero it begins to behave as an insulator (a substance having very high resistance) rather than as a conductor: yet at higher temperatures, embedded in an appropriate crystal matrix, it is key to the production of zero resistance (superconduction).

Current flow in the CuO₂ family of HTSCs has directional properties. That is, the critical current density, J_c—the largest current density that a superconductor can carry without lapsing into finite resistivity—along the CuO₂ plane direction is orders of magnitude higher than at right angles to it. For HTSCs to carry a large amount of current, this implies that individual crystalline grains in bulk conductors (e.g., wires or tapes) of HTSC material should be well aligned with the current transport direction. Significant grain misorientation and chemistry inhomogeneity at grain boundaries can form weak links between neighboring grains and thus lower local J_c values. Several bulk material manufacture technologies, such as the melt-powder-melt-growth (MPMG) method for Y-Ba-Cu-O and the oxide-powder-in-tube

(OPIT) process for Bi- and Tl-based superconductors, have been demonstrated to develop textured bulk structures. J_c values of 10⁶ to 10⁸ amperes per square centimeter at 77K have been achieved over small distances. For thin-film growth, dual ion beam sputtering (DIBS), molecular beam epitaxy (MBE), pulsed-laser deposition (PLD), and metal-organic chemical vapor deposition (MOCVD) have been shown to be successful methods. By optimizing processing temperature and pressure and using proper buffer layers, epitaxial HTSC films can be deposited on templates of other crystalline materials such as Si and MgO. With the integration of Si-based microelectronics processes, HTSC thin-film devices can be fabricated for a variety of applications.

Superconductivity applications

Superconductivity applications fall into two main areas—electromagnets (magnets whose magnetism depends on an externally-powered current passing through a winding) and electronics. In electromagnets, superconducting windings have much lower power consumption than do conventional copper windings, and thus are particularly attractive for high-field applications. Superconducting magnets can be used in

KEY TERMS

Critical current density, J_c —The largest current density that a superconductor can carry non-resistively.

Diamagnetic —The material with a magnetization vector opposite to the applied magnetic field, which gives rise to a negative magnetic susceptibility.

Josephson junction —A structure with two super-conductors separated by a thin insulating barrier.

Meissner effect —The expulsion of magnetic flux lines from a superconductor when the superconductor is cooled to a temperature below T _c.

Superconducting transition temperature, T _c —The highest temperature at which a given superconducting material remains superconducting.

Thermodynamic critical field, H_c —Minimum value of externally applied magnetic field that will cause breakdown of diamagnetism in a superconductor.

magnetic resonance imaging, magnetic sorting of metals, magnetic levitation trains, and magnetic shielding. For power utility applications, superconductors are promising for magnetic energy storage, electrical power transmission, motors, and generators. They are also useful as the coatings for radio-frequency cavities.

In electronic applications, thin-film interconnections and Josephson junctions are two key elements. Superconductors offer fast switching speeds and reduced wiring delays so that they are applicable for logic devices and memory cells. Superconducting field-effect transistors and Josephson junction integrated circuits have been demonstrated. At the temperature of liquid nitrogen, 77 K, superconductors can be further integrated with semiconductors to form hybrid devices. For sensor operation, superconducting quantum interference devices (SQUIDs), based on the Josephson junction technology, are the most sensitive detector for measuring changes in magnetic field. For example, they can detect very faint signals (on the order of 10^-15 Tesla) produced by the human brain and heart. In addition, SQUID-based gradiometry is a very powerful instrument for non-destructive evaluation of nonliving materials. The increased energy gap in HTSCs allows the fabrication of superconducting electromagnetic radiation detectors used over the spectrum from x ray to the far infrared.

As time goes by, superconductors will find more and more applications. Recently, Y-Ba-Cu-O has been shown to be a good material for the top and bottom electrodes of oxide ferroelectric thin-film capacitors,

which exhibit fatigue resistance superior to that of capacitors with conventional Pt electrodes (used in dynamic random-access computer memories). This suggests that when the microstructures and the properties of HTSC materials can be well controlled and tailored, oxide superconductors are promising for many hybrid designs—designs incorporating both conventional and superconducting materials. Scientists can also expect upcoming hybrid fabrication technologies. Processes for thin films, thick films, wires, and tapes may all be needed for the integration of a single superconductor-based instrument. Future growth in superconductor technology in electronic components (such as fault current controllers), medical sensing, geology, military technology, transportation (such as magnetic levitation trains [maglev trains]), and power transmission and storage is very promising, especially if, as researchers conjecture, transition temperatures and critical current densities can be significantly increased.