The Meissner effect is the expulsion of a magnetic field from a superconductor during its transition to the superconducting state. The German physicists Walther Meissner and Robert Ochsenfeld discovered this phenomenon in 1933 by measuring the magnetic field distribution outside superconducting tin and lead samples; the samples, in the presence of an applied magnetic field, were cooled below their superconducting transition temperature, whereupon the samples cancelled nearly all interior magnetic fields. They detected this effect only indirectly because the magnetic flux is conserved by a superconductor: when the interior field decreases, the exterior field increases; the experiment demonstrated for the first time that superconductors were more than just perfect conductors and provided a uniquely defining property of the superconductor state. The ability for the expulsion effect is determined by the nature of equilibrium formed by the neutralization within the unit cell of a superconductor.
A superconductor with little or no magnetic field within it is said to be in the Meissner state. The Meissner state breaks down. Superconductors can be divided into two classes according to. In type-I superconductors, superconductivity is abruptly destroyed when the strength of the applied field rises above a critical value Hc. Depending on the geometry of the sample, one may obtain an intermediate state consisting of a baroque pattern of regions of normal material carrying a magnetic field mixed with regions of superconducting material containing no field. In type-II superconductors, raising the applied field past a critical value Hc1 leads to a mixed state in which an increasing amount of magnetic flux penetrates the material, but there remains no resistance to the electric current as long as the current is not too large. At a second critical field strength Hc2, superconductivity is destroyed; the mixed state is caused by vortices in the electronic superfluid, sometimes called fluxons because the flux carried by these vortices is quantized.
Most pure elemental superconductors, except niobium and carbon nanotubes, are type I, while all impure and compound superconductors are type II. The Meissner effect was given a phenomenological explanation by the brothers Fritz and Heinz London, who showed that the electromagnetic free energy in a superconductor is minimized provided ∇ 2 H = λ − 2 H where H is the magnetic field and λ is the London penetration depth; this equation, known as the London equation, predicts that the magnetic field in a superconductor decays exponentially from whatever value it possesses at the surface. In a weak applied field, a superconductor "expels" nearly all magnetic flux, it does this by setting up electric currents near its surface. The magnetic field of these surface currents cancels the applied magnetic field within the bulk of the superconductor; as the field expulsion, or cancellation, does not change with time, the currents producing this effect do not decay with time. Near the surface, within the London penetration depth, the magnetic field is not cancelled.
Each superconducting material has its own characteristic penetration depth. Any perfect conductor will prevent any change to magnetic flux passing through its surface due to ordinary electromagnetic induction at zero resistance; the Meissner effect is distinct from this: when an ordinary conductor is cooled so that it makes the transition to a superconducting state in the presence of a constant applied magnetic field, the magnetic flux is expelled during the transition. This effect cannot be explained by infinite conductivity; the placement and subsequent levitation of a magnet above an superconducting material does not demonstrate the Meissner effect, while an stationary magnet being repelled by a superconductor as it is cooled through its critical temperature does. Superconductors in the Meissner state exhibit perfect diamagnetism, or superdiamagnetism, meaning that the total magnetic field is close to zero deep inside them; this means that their magnetic susceptibility, χ v = −1. Diamagnetics are defined by the generation of a spontaneous magnetization of a material which directly opposes the direction of an applied field.
However, the fundamental origins of diamagnetism in superconductors and normal materials are different. In normal materials diamagnetism arises as a direct result of the orbital spin of electrons about the nuclei of an atom induced electromagnetically by the application of an applied field. In superconductors the illusion of perfect diamagnetism arises from persistent screening currents which flow to oppose the applied field; the discovery of the Meissner effect led to the phenomenological theory of superconductivity by Fritz and Heinz London in 1935. This theory explained resistanceless transport and the Meissner effect, allowed the first theoretical predictions for superconductivity to be made. However, this theory only explained experimental observations—it did not allow the microscopic origins of the superconducting properties to be identified; this was done by the BCS theory in 1957, from which the penetration depth and the Meissner effect result. However, some physicists argue; the Meissner superconductivity effect serves as an important paradigm for the generation mechanism of a mass M (i.e. a r
A Bitter electromagnet or Bitter solenoid is a type of electromagnet invented in 1933 by American physicist Francis Bitter used in scientific research to create strong magnetic fields. Bitter electromagnets have been used to achieve the strongest continuous manmade magnetic fields on earth―up to 45 teslas, As of 2011. Bitter electromagnets are used where strong fields are required; the iron cores used in conventional electromagnets saturate, are limited to fields of about 2 teslas. Superconducting electromagnets can produce stronger magnetic fields but are limited to fields of 10 to 20 teslas, due to flux creep, though theoretical limits are higher. For stronger fields resistive solenoid electromagnets of the Bitter design are used, their disadvantage is that they require high drive currents, dissipate large quantities of heat. Bitter magnets are constructed of circular conducting metal plates and insulating spacers stacked in a helical configuration, rather than coils of wire; the current flows in a helical path through the plates.
This design was invented in 1933 by Francis Bitter. In his honor, the plates are known as Bitter plates; the purpose of the stacked plate design is to withstand the enormous outward mechanical pressure produced by Lorentz forces due to the magnetic field acting on the moving electric charges in the plate, which increase with the square of the magnetic field strength. Additionally, water circulates through holes in the plates as a coolant, to carry away the enormous heat created in the plates due to resistive heating by the large currents flowing through them; the heat dissipation increases with the square of the magnetic field strength. In the mid-1990s researchers at the National High Magnetic Field Laboratory at Florida State University in Tallahassee improved on this basic design and created what they refer to as the Florida Bitter. By elongating the mounting and cooling holes, there is a substantial drop in the stresses developed in the system and an improvement in cooling efficiency; as the stresses increased in the original bitter plates, they would flex causing the small circular cooling holes to move out of alignment reducing the efficacy of the cooling system.
The Florida Bitter plates will flex less due to the reduced stresses, the elongated cooling holes will always be in partial alignment despite any flexure the discs experience. This new design allowed for a 40% increase in efficiency and has become the design of choice for bitter plate based resistive magnets. Unlike a copper wire, the current density of a current carrying disc is not uniform across its cross-sectional area, but is instead a function of the ratio of the inner diameter of the disc to an arbitrary radius within the disc; the implications of this relationship is that the current density decreases with an increase in radius. As such, the bulk of the current is flowing closer to the inner radius of the disc. Large discs will have a larger discrepancy in the current density between the inner and outer portions of the disc; this will reduce the efficiency and cause additional complications in the system because there will be a more substantial temperature and stress gradient along the disc.
As such, a series of nested coils is used as it will more evenly distribute the current across a large combined area as opposed to a single coil with large discs. The non-uniform current density must be considered when calculating the magnetic flux density. Ampère's Law for a basic current carrying loop of wire gives that the on-axis magnetic flux is proportional to the current running through the wire and is related to the basic geometry of the loop, but is not concerned with the geometry of the cross section of the wire; the current density is uniform across the cross-sectional area of a wire. This is not the case for a Bitter disc; as such, the current term must be replaced with terms discussing the cross-sectional area of the disc and the current density. The equation for the on-axis magnetic flux density of a Bitter disc becomes much more complex as a result; the differential flux density is related to the differential area. The introduction of a space factor must be included to compensate for variations in the disc related to cooling and mounting holes.
The strongest continuous magnetic fields on Earth have been produced by Bitter magnets. As of 31 March 2014 the strongest continuous field achieved by a room temperature magnet is 37.5 T produced by a Bitter electromagnet at the Radboud University High Field Magnet Laboratory in Nijmegen, Netherlands. The strongest continuous manmade magnetic field, 45 T, was produced by a hybrid device, consisting of a Bitter magnet inside a superconducting magnet; the resistive magnet produces 33.5 T and the superconducting coil produces the remaining 11.5 T. This magnet requires 30 MW of power; this magnet must be kept at 1.8 K using liquid helium. The magnet takes 6 weeks to cool to temperature and thus once cooled the cooling system is run continuously, it costs $1452 per hour to run at full field. Superconducting magnet National High Magnetic Field Laboratory Magnet Projects Page at Florida State University Magnets at Nijmegen High Field Magnet Laboratory The Frog That Learned to Fly and a ball of water inside a Bitter solenoid at the High Field Magnet Laboratory Diagrams and description of the Bitter solenoid used in the frog levitation demonstration Bitter magnet designs: NHMFL Bitter Magnet and Radbound University Bitter Solenoid
Paramagnetism is a form of magnetism whereby certain materials are weakly attracted by an externally applied magnetic field, form internal, induced magnetic fields in the direction of the applied magnetic field. In contrast with this behavior, diamagnetic materials are repelled by magnetic fields and form induced magnetic fields in the direction opposite to that of the applied magnetic field. Paramagnetic materials include some compounds; the magnetic moment induced by the applied field is rather weak. It requires a sensitive analytical balance to detect the effect and modern measurements on paramagnetic materials are conducted with a SQUID magnetometer. Paramagnetism is due to the presence of unpaired electrons in the material, so all atoms with incompletely filled atomic orbitals are paramagnetic. Due to their spin, unpaired electrons have a magnetic dipole act like tiny magnets. An external magnetic field causes the electrons' spins to align parallel to the field, causing a net attraction.
Paramagnetic materials include aluminium, oxygen and iron oxide. Unlike ferromagnets, paramagnets do not retain any magnetization in the absence of an externally applied magnetic field because thermal motion randomizes the spin orientations, thus the total magnetization drops to zero. In the presence of the field there is only a small induced magnetization because only a small fraction of the spins will be oriented by the field; this fraction is proportional to the field strength and this explains the linear dependency. The attraction experienced by ferromagnetic materials is non-linear and much stronger, so that it is observed, for instance, in the attraction between a refrigerator magnet and the iron of the refrigerator itself. Constituent atoms or molecules of paramagnetic materials have permanent magnetic moments in the absence of an applied field; the permanent moment is due to the spin of unpaired electrons in atomic or molecular electron orbitals. In pure paramagnetism, the dipoles do not interact with one another and are randomly oriented in the absence of an external field due to thermal agitation, resulting in zero net magnetic moment.
When a magnetic field is applied, the dipoles will tend to align with the applied field, resulting in a net magnetic moment in the direction of the applied field. In the classical description, this alignment can be understood to occur due to a torque being provided on the magnetic moments by an applied field, which tries to align the dipoles parallel to the applied field. However, the true origins of the alignment can only be understood via the quantum-mechanical properties of spin and angular momentum. If there is sufficient energy exchange between neighbouring dipoles, they will interact, may spontaneously align or anti-align and form magnetic domains, resulting in ferromagnetism or antiferromagnetism, respectively. Paramagnetic behavior can be observed in ferromagnetic materials that are above their Curie temperature, in antiferromagnets above their Néel temperature. At these temperatures, the available thermal energy overcomes the interaction energy between the spins. In general, paramagnetic effects are quite small: the magnetic susceptibility is of the order of 10−3 to 10−5 for most paramagnets, but may be as high as 10−1 for synthetic paramagnets such as ferrofluids.
In conductive materials, the electrons are delocalized, that is, they travel through the solid more or less as free electrons. Conductivity can be understood in a band structure picture as arising from the incomplete filling of energy bands. In an ordinary nonmagnetic conductor the conduction band is identical for both spin-up and spin-down electrons; when a magnetic field is applied, the conduction band splits apart into a spin-up and a spin-down band due to the difference in magnetic potential energy for spin-up and spin-down electrons. Since the Fermi level must be identical for both bands, this means that there will be a small surplus of the type of spin in the band that moved downwards; this effect is a weak form of paramagnetism known as Pauli paramagnetism. The effect always competes with a diamagnetic response of opposite sign due to all the core electrons of the atoms. Stronger forms of magnetism require localized rather than itinerant electrons. However, in some cases a band structure can result in which there are two delocalized sub-bands with states of opposite spins that have different energies.
If one subband is preferentially filled over the other, one can have itinerant ferromagnetic order. This situation only occurs in narrow bands, which are poorly delocalized. Strong delocalization in a solid due to large overlap with neighboring wave functions means that there will be a large Fermi velocity; this is why s- and p-type metals are either Pauli-paramagnetic or as in the case of gold diamagnetic. In the latter case the diamagnetic contribution from the closed shell inner electrons wins over the weak paramagnetic term of the free electrons. Stronger magnetic effects are only observed when d or f electrons are involved; the latter are strongly localized. Moreover, the size of the magnetic
Bismuth is a chemical element with symbol Bi and atomic number 83. It is a pentavalent post-transition metal and one of the pnictogens with chemical properties resembling its lighter homologs arsenic and antimony. Elemental bismuth may occur although its sulfide and oxide form important commercial ores; the free element is 86% as dense as lead. It is a brittle metal with a silvery white color when freshly produced, but surface oxidation can give it a pink tinge. Bismuth is the most diamagnetic element, has one of the lowest values of thermal conductivity among metals. Bismuth was long considered the element with the highest atomic mass, stable, but in 2003 it was discovered to be weakly radioactive: its only primordial isotope, bismuth-209, decays via alpha decay with a half-life more than a billion times the estimated age of the universe; because of its tremendously long half-life, bismuth may still be considered stable for all purposes. Bismuth metal has been known since ancient times, although it was confused with lead and tin, which share some physical properties.
The etymology is uncertain, but comes from Arabic bi ismid, meaning having the properties of antimony or the German words weiße Masse or Wismuth, translated in the mid-sixteenth century to New Latin bisemutum. Bismuth compounds account for about half the production of bismuth, they are used in cosmetics, a few pharmaceuticals, notably bismuth subsalicylate, used to treat diarrhea. Bismuth's unusual propensity to expand as it solidifies is responsible for some of its uses, such as in casting of printing type. Bismuth has unusually low toxicity for a heavy metal; as the toxicity of lead has become more apparent in recent years, there is an increasing use of bismuth alloys as a replacement for lead. The name bismuth dates from around the 1660s, is of uncertain etymology, it is one of the first 10 metals to have been discovered. Bismuth appears in the 1660s, from obsolete German Bismuth, Wissmuth; the New Latin bisemutum is from the German Wismuth from weiße Masse, "white mass". The element was confused in early times with tin and lead because of its resemblance to those elements.
Bismuth has been known since ancient times, so no one person is credited with its discovery. Agricola, in De Natura Fossilium states that bismuth is a distinct metal in a family of metals including tin and lead; this was based on observation of their physical properties. Miners in the age of alchemy gave bismuth the name tectum argenti, or "silver being made," in the sense of silver still in the process of being formed within the Earth. Beginning with Johann Heinrich Pott in 1738, Carl Wilhelm Scheele and Torbern Olof Bergman, the distinctness of lead and bismuth became clear, Claude François Geoffroy demonstrated in 1753 that this metal is distinct from lead and tin. Bismuth was known to the Incas and used in a special bronze alloy for knives. Bismuth is a brittle metal with a white, silver-pink hue with an iridescent oxide tarnish showing many colors from yellow to blue; the spiral, stair-stepped structure of bismuth crystals is the result of a higher growth rate around the outside edges than on the inside edges.
The variations in the thickness of the oxide layer that forms on the surface of the crystal cause different wavelengths of light to interfere upon reflection, thus displaying a rainbow of colors. When burned in oxygen, bismuth burns with a blue flame and its oxide forms yellow fumes, its toxicity is much lower than that of its neighbors in the periodic table, such as lead and polonium. No other metal is verified to be more diamagnetic than bismuth. Of any metal, it has one of the lowest values of thermal conductivity and the highest Hall coefficient, it has a high electrical resistivity. When deposited in sufficiently thin layers on a substrate, bismuth is a semiconductor, despite being a post-transition metal. Elemental bismuth is denser in the liquid phase than the solid, a characteristic it shares with germanium, silicon and water. Bismuth expands 3.32% on solidification. Though unseen in nature, high-purity bismuth can form distinctive, colorful hopper crystals, it is nontoxic and has a low melting point just above 271 °C, so crystals may be grown using a household stove, although the resulting crystals will tend to be lower quality than lab-grown crystals.
At ambient conditions bismuth shares the same layered structure as the metallic forms of arsenic and antimony, crystallizing in the rhombohedral lattice, classed into trigonal or hexagonal crystal systems. When compressed at room temperature, this Bi-I structure changes first to the monoclinic Bi-II at 2.55 GPa to the tetragonal Bi-III at 2.7 GPa, to the body-centered cubic Bi-IV at 7.7 GPa. The corresponding transitions can be monitored via changes in electrical conductivity. Bismuth is stable to both moist air at ordinary temperatures; when red-hot, it reacts with water to make bismuth oxide. 2 Bi + 3 H2O → Bi2O3 + 3 H2It reacts with fluorine to
Mercury is a chemical element with symbol Hg and atomic number 80. It is known as quicksilver and was named hydrargyrum. A heavy, silvery d-block element, mercury is the only metallic element, liquid at standard conditions for temperature and pressure. Mercury occurs in deposits throughout the world as cinnabar; the red pigment vermilion is obtained by synthetic mercuric sulfide. Mercury is used in thermometers, manometers, sphygmomanometers, float valves, mercury switches, mercury relays, fluorescent lamps and other devices, though concerns about the element's toxicity have led to mercury thermometers and sphygmomanometers being phased out in clinical environments in favor of alternatives such as alcohol- or galinstan-filled glass thermometers and thermistor- or infrared-based electronic instruments. Mechanical pressure gauges and electronic strain gauge sensors have replaced mercury sphygmomanometers. Mercury remains in use in scientific research applications and in amalgam for dental restoration in some locales.
It is used in fluorescent lighting. Electricity passed through mercury vapor in a fluorescent lamp produces short-wave ultraviolet light, which causes the phosphor in the tube to fluoresce, making visible light. Mercury poisoning can result from exposure to water-soluble forms of mercury, by inhalation of mercury vapor, or by ingesting any form of mercury. Mercury is a silvery-white liquid metal. Compared to other metals, it is a fair conductor of electricity, it has a freezing point of −38.83 °C and a boiling point of 356.73 °C, both the lowest of any stable metal, although preliminary experiments on copernicium and flerovium have indicated that they have lower boiling points. Upon freezing, the volume of mercury decreases by 3.59% and its density changes from 13.69 g/cm3 when liquid to 14.184 g/cm3 when solid. The coefficient of volume expansion is 181.59 × 10−6 at 0 °C, 181.71 × 10−6 at 20 °C and 182.50 × 10−6 at 100 °C. Solid mercury can be cut with a knife. A complete explanation of mercury's extreme volatility delves deep into the realm of quantum physics, but it can be summarized as follows: mercury has a unique electron configuration where electrons fill up all the available 1s, 2s, 2p, 3s, 3p, 3d, 4s, 4p, 4d, 4f, 5s, 5p, 5d, 6s subshells.
Because this configuration resists removal of an electron, mercury behaves to noble gases, which form weak bonds and hence melt at low temperatures. The stability of the 6s shell is due to the presence of a filled 4f shell. An f shell poorly screens the nuclear charge that increases the attractive Coulomb interaction of the 6s shell and the nucleus; the absence of a filled inner f shell is the reason for the somewhat higher melting temperature of cadmium and zinc, although both these metals still melt and, in addition, have unusually low boiling points. Mercury does not react with most acids, such as dilute sulfuric acid, although oxidizing acids such as concentrated sulfuric acid and nitric acid or aqua regia dissolve it to give sulfate and chloride. Like silver, mercury reacts with atmospheric hydrogen sulfide. Mercury reacts with solid sulfur flakes. Mercury dissolves many metals such as silver to form amalgams. Iron is an exception, iron flasks have traditionally been used to trade mercury.
Several other first row transition metals with the exception of manganese and zinc are resistant in forming amalgams. Other elements that do not form amalgams with mercury include platinum. Sodium amalgam is a common reducing agent in organic synthesis, is used in high-pressure sodium lamps. Mercury combines with aluminium to form a mercury-aluminium amalgam when the two pure metals come into contact. Since the amalgam destroys the aluminium oxide layer which protects metallic aluminium from oxidizing in-depth small amounts of mercury can corrode aluminium. For this reason, mercury is not allowed aboard an aircraft under most circumstances because of the risk of it forming an amalgam with exposed aluminium parts in the aircraft. Mercury embrittlement is the most common type of liquid metal embrittlement. There are seven stable isotopes of mercury, with 202Hg being the most abundant; the longest-lived radioisotopes are 194Hg with a half-life of 444 years, 203Hg with a half-life of 46.612 days. Most of the remaining radioisotopes have half-lives.
199Hg and 201Hg are the most studied NMR-active nuclei, having spins of 1⁄2 and 3⁄2 respectively. Hg is the modern chemical symbol for mercury, it comes from hydrargyrum, a Latinized form of the Greek word ὑδράργυρος, a compound word meaning "water-silver" – since it is liquid like water and shiny like silver. The element was named after the Roman god Mercury, known for his mobility, it is associated with the planet Mercury. Mercury is the only metal for which the al
Quantum mechanics, including quantum field theory, is a fundamental theory in physics which describes nature at the smallest scales of energy levels of atoms and subatomic particles. Classical physics, the physics existing before quantum mechanics, describes nature at ordinary scale. Most theories in classical physics can be derived from quantum mechanics as an approximation valid at large scale. Quantum mechanics differs from classical physics in that energy, angular momentum and other quantities of a bound system are restricted to discrete values. Quantum mechanics arose from theories to explain observations which could not be reconciled with classical physics, such as Max Planck's solution in 1900 to the black-body radiation problem, from the correspondence between energy and frequency in Albert Einstein's 1905 paper which explained the photoelectric effect. Early quantum theory was profoundly re-conceived in the mid-1920s by Erwin Schrödinger, Werner Heisenberg, Max Born and others; the modern theory is formulated in various specially developed mathematical formalisms.
In one of them, a mathematical function, the wave function, provides information about the probability amplitude of position and other physical properties of a particle. Important applications of quantum theory include quantum chemistry, quantum optics, quantum computing, superconducting magnets, light-emitting diodes, the laser, the transistor and semiconductors such as the microprocessor and research imaging such as magnetic resonance imaging and electron microscopy. Explanations for many biological and physical phenomena are rooted in the nature of the chemical bond, most notably the macro-molecule DNA. Scientific inquiry into the wave nature of light began in the 17th and 18th centuries, when scientists such as Robert Hooke, Christiaan Huygens and Leonhard Euler proposed a wave theory of light based on experimental observations. In 1803, Thomas Young, an English polymath, performed the famous double-slit experiment that he described in a paper titled On the nature of light and colours.
This experiment played a major role in the general acceptance of the wave theory of light. In 1838, Michael Faraday discovered cathode rays; these studies were followed by the 1859 statement of the black-body radiation problem by Gustav Kirchhoff, the 1877 suggestion by Ludwig Boltzmann that the energy states of a physical system can be discrete, the 1900 quantum hypothesis of Max Planck. Planck's hypothesis that energy is radiated and absorbed in discrete "quanta" matched the observed patterns of black-body radiation. In 1896, Wilhelm Wien empirically determined a distribution law of black-body radiation, known as Wien's law in his honor. Ludwig Boltzmann independently arrived at this result by considerations of Maxwell's equations. However, it underestimated the radiance at low frequencies. Planck corrected this model using Boltzmann's statistical interpretation of thermodynamics and proposed what is now called Planck's law, which led to the development of quantum mechanics. Following Max Planck's solution in 1900 to the black-body radiation problem, Albert Einstein offered a quantum-based theory to explain the photoelectric effect.
Around 1900–1910, the atomic theory and the corpuscular theory of light first came to be accepted as scientific fact. Among the first to study quantum phenomena in nature were Arthur Compton, C. V. Raman, Pieter Zeeman, each of whom has a quantum effect named after him. Robert Andrews Millikan studied the photoelectric effect experimentally, Albert Einstein developed a theory for it. At the same time, Ernest Rutherford experimentally discovered the nuclear model of the atom, for which Niels Bohr developed his theory of the atomic structure, confirmed by the experiments of Henry Moseley. In 1913, Peter Debye extended Niels Bohr's theory of atomic structure, introducing elliptical orbits, a concept introduced by Arnold Sommerfeld; this phase is known as old quantum theory. According to Planck, each energy element is proportional to its frequency: E = h ν, where h is Planck's constant. Planck cautiously insisted that this was an aspect of the processes of absorption and emission of radiation and had nothing to do with the physical reality of the radiation itself.
In fact, he considered his quantum hypothesis a mathematical trick to get the right answer rather than a sizable discovery. However, in 1905 Albert Einstein interpreted Planck's quantum hypothesis realistically and used it to explain the photoelectric effect, in which shining light on certain materials can eject electrons from the material, he won the 1921 Nobel Prize in Physics for this work. Einstein further developed this idea to show that an electromagnetic wave such as light could be described as a particle, with a discrete quantum of energy, dependent on its frequency; the foundations of quantum mechanics were established during the first half of the 20th century by Max Planck, Niels Bohr, Werner Heisenberg, Louis de Broglie, Arthur Compton, Albert Einstein, Erwin Schrödinger, Max Born, John von Neumann, Paul Dirac, Enrico Fermi, Wolfgang Pauli, Max von Laue, Freeman Dyson, David Hilbert, Wi
Superconductivity is a phenomenon of zero electrical resistance and expulsion of magnetic flux fields occurring in certain materials, called superconductors, when cooled below a characteristic critical temperature. It was discovered by Dutch physicist Heike Kamerlingh Onnes on April 1911, in Leiden. Like ferromagnetism and atomic spectral lines, superconductivity is a quantum mechanical phenomenon, it is characterized by the Meissner effect, the complete ejection of magnetic field lines from the interior of the superconductor during its transitions into the superconducting state. The occurrence of the Meissner effect indicates that superconductivity cannot be understood as the idealization of perfect conductivity in classical physics; the electrical resistance of a metallic conductor decreases as temperature is lowered. In ordinary conductors, such as copper or silver, this decrease is limited by impurities and other defects. Near absolute zero, a real sample of a normal conductor shows some resistance.
In a superconductor, the resistance drops abruptly to zero when the material is cooled below its critical temperature. An electric current through a loop of superconducting wire can persist indefinitely with no power source. In 1986, it was discovered that some cuprate-perovskite ceramic materials have a critical temperature above 90 K; such a high transition temperature is theoretically impossible for a conventional superconductor, leading the materials to be termed high-temperature superconductors. The cheaply-available coolant liquid nitrogen boils at 77 K, thus superconduction at higher temperatures than this facilitates many experiments and applications that are less practical at lower temperatures. There are many criteria; the most common are: A superconductor can be Type I, meaning it has a single critical field, above which all superconductivity is lost and below which the magnetic field is expelled from the superconductor. These points are called vortices. Furthermore, in multicomponent superconductors it is possible to have combination of the two behaviours.
In that case the superconductor is of Type-1.5. It is conventional if it can be explained by the BCS theory or its derivatives, or unconventional, otherwise. A superconductor is considered high-temperature if it reaches a superconducting state when cooled using liquid nitrogen – that is, at only Tc > 77 K) – or low-temperature if more aggressive cooling techniques are required to reach its critical temperature. Superconductor material classes include chemical elements, ceramics, superconducting pnictides or organic superconductors. Most of the physical properties of superconductors vary from material to material, such as the heat capacity and the critical temperature, critical field, critical current density at which superconductivity is destroyed. On the other hand, there is a class of properties. For instance, all superconductors have zero resistivity to low applied currents when there is no magnetic field present or if the applied field does not exceed a critical value; the existence of these "universal" properties implies that superconductivity is a thermodynamic phase, thus possesses certain distinguishing properties which are independent of microscopic details.
The simplest method to measure the electrical resistance of a sample of some material is to place it in an electrical circuit in series with a current source I and measure the resulting voltage V across the sample. The resistance of the sample is given by Ohm's law as R = V / I. If the voltage is zero, this means. Superconductors are able to maintain a current with no applied voltage whatsoever, a property exploited in superconducting electromagnets such as those found in MRI machines. Experiments have demonstrated that currents in superconducting coils can persist for years without any measurable degradation. Experimental evidence points to a current lifetime of at least 100,000 years. Theoretical estimates for the lifetime of a persistent current can exceed the estimated lifetime of the universe, depending on the wire geometry and the temperature. In practice, currents injected in superconducting coils have persisted for more than 23 years in superconducting gravimeters. In such instruments, the measurement principle is based on the monitoring of the levitation of a superconducting niobium sphere with a mass of 4 grams.
In a normal conductor, an electric current may be visualized as a fluid of electrons moving across a heavy ionic lattice. The electrons are colliding with the ions in the lattice, during each collision some of the energy carried by the current is absorbed by the lattice and converted into heat, the vibrational kinetic energy of the lattice ions; as a result, the energy carried by the current is being dissipated. This is the phenomenon of electrical Joule heating; the situation is different in a superconductor. In a conventional superconductor, the electronic fluid cannot be resolved into individual electrons. Instead, it consists of bound pairs of electrons known as Cooper pairs; this pairing is caused by an attractive force between electrons from the exchange of phonons. Due to quantum mechanics, the energy spectr