Superconductors


history
In 1911, two years after having made a success of the liquefaction of Helium - then reaching more the known low-temperature: 4,2 Kelvin (K), that is to say -269° C - the physicist Heike Kamerlingh Onnes proposed with his pupil Gilles Holst to measure the resistivity of a mercury bar. They discovered that this one was cancelled in lower part of 4,15 K. Holst made and remade the experiments, checked the measuring instruments but the doubt was not allowed any more : the behaviour was confirmed.

Heike Kamerling Onnes
The following year, Onnes discovered that tin and the lead (which is a very bad conductor with ambient temperature), lost their resistance respectively to 3,7 K and 6 K. the absence of resistivity of material, that is to say this one is neither weak nor very weak but completely goes away, is the first spectacular phenomenon of supraconductivity.
In 1933, W. Meissner and R. Oschenfeld discovered the property of a pure metal bar to being impermeable with the magnetic fields: the perfect diamagnetism, which one calls since Meissner effect. This diamagnetism, which is the second surprising effect of the supraconductivity, results in the capacity of a superconductor to emit a magnetic field opposed to another field which would be applied to him, and this, whatever the polarity of the field applied.
Without any theory on which to rest, the researchers had to be satisfied to test, almost randomly, various alloys containing titanium, of strontium, germanium and especially of niobium, which gave the best results. Search was even undertaken in the field of the organic molecules, primarily with fullerenes or footballenes, temperatures of about 33 K.
The question of the origin of supraconductivity haunted the scientists : From which this phenomenon comes which no theory had suspected before. In 1935, of the incomplete theories although powerful were proposed by the London brothers then in 1950 per V. Ginzburg and L. Pram.

John Bardeen, Leon Neil Cooper, John Robert Schrieffer
But in 1957, theory B.C.S appeared, of the name of its discoverers John Bardeen, Leon Cooper and John Schiffer, who described the basic principle of supraconductivity partly. This theory, that we will clarify in the continuation of this project, stipulates that with low-temperature, the electrons move per pairs, called pairs of Cooper, in the form of phonons. It is thanks to this basic theory that the researchers could progress in their search for superconductors to "high temperature". The alloys containing of Niobium appearing most effective, one thus used it in the majority of the superconductive compounds. Unfortunately, all the alloys tested did not exceed an higher temperature to 23 K and theory B.C.S seemed to have found its limits.
Parallel to this search, Brian Josephson predicted into 1962 the quantum effects which bear its name and are used in supersensitive detection of the magnetic fields. The discovery of Josephson, which left to pantois all the researchers, explains why an electric current not no one could circulate of a superconductive block to another superconductive block separated from the first by a thin insulating layer, in the absence even of potential difference between the two blocks. This theory was checked in experiments a few years later, which was worth in Josephson and Giaver to receive the Nobel Prize in 1974.

Brian josephson
It is in 1986, pivotal year in the history of the superconductors, that theory B.C.S was called into question, with the discovery, by engineers of IBM Zurich (Swiss), of a superconductor to 34 K then, nine months later, with 92 K.Ainsi transfer the day of new generations of composed such as Ba-The-Cu-O, There-Ba-Cu-O and Ti-Sr-Ca-Cu-O, the two last making it possible to exceed the temperature of nitrogen liquidates (77 K, that is to say -196° C), costing ten times less expensive than liquid helium and cooling twenty times better. Thus Ti-Sr-Ca-Cu-O reached a critical temperature of 125 K, leaving far behind the psychological barrier of liquefaction liquid nitrogen.
This discovery caused the passion of all the scientific community, which set out again in the race with the superconductors with Critical High temperature (H Tc), then hoping to discover a superconductor with ambient temperature. But the more this temperature increased and the more the performances of materials decreased, the current which can be transported without losses becoming weak. The record of critical temperature reproducible was reached in 1995 with a temperature of 164 K but requiring high pressures.
The race with Tc blowing fault of comprehension at the atomic level, search was thus directed towards the comprehension of the physical phenomena governing supraconductivity. This new search leads to theories which it is currently difficult to confirm or cancel. However, the scientists do not despair to find superconductors with ambient temperature.
Summary of the history of supraconductivity :
1877: Cailletet and Pictet liquefy air at -196.15° C (77 K).
1899: Dewar liquefied hydrogen at -253.15° C (19.85 K).
1908: Liquefaction of helium (4,2 K is -269° C) by Kamerlingh ONNES.
1911: Kamerlingh ONNES discovers supraconductivity while proposing with G. HOLST to very measure the resistivity of mercury to low-temperature.
1913: Failure of the first superconductive magnet.
1933: Highlighting of the diamagnetism of superconductors (MEISSNER and OCHSENFELD).
1954: First superconductive magnet (Nb) which functioned (0,71 Tesla (T) to 4,2 K).
1957: Microscopic theory BCS of supraconductivity (BARDEEN, COOPER and SCHIEFFER). Theory of A. ABRIKOSOV of the superconductors of the type II (network of vortex).
1958: Tuning of conductors of the type II in NbTi and Nb 3 Sn.
1960: Discovered strong densities of current under high induction (Nb 3 Sn).
1962: B.JOSEPHSON predicts the quantum effects which bear its name and which are used in supersensitive detection of magnetic field (Josephson junctions and SQUID).
1964: First significant application of the superconductors: bubble chamber of Argon (2,5 T in several m3).
1965: First cryoalternateurs.
1968: Definition of the filament strand multi by the laboratory Rutherford.
1974: Startup of the most powerful bubble chamber with CERN (830 MJ).
1982: First images IRM, they will ensure supraconductivity its first industrial and commercial application.
1983: Tuning of the alternate strands multifilamentaires. First superconductive accelerator.
1986: BEDNORZet MÜLLER discover supraconductivity in new oxides.
1987: Flight of the critical temperatures.
Starting of CORE SUPRA, superconductive tokamak cooled to 1,8 K and installed in Cadarache (France).
1995: Reproducible record to 164 K (-109° C).

The theory B.C.S and phonons

The comprehension of supraconductivity took a step ahead in 1957 with three American physicists - John Bardeen, Leon Cooper, and John Schrieffer - by their theory on the supraconductivity, known under the name of theory BCS. Theory BCS describes supraconductivity at temperatures close to the absolute zero.
Cooper realized that the vibrations of the atomic grid were directly responsible for the unification of all the current. These low temperatures would force the electrons indeed to unify in pairs which could exceed the obstacles responsible for the resistance of the driver. These pairs of electrons are known under the name of pairs of Cooper.
Cooper and its colleagues knew that the electrons, which are pushed back normally mutually, were to test a kind of forced attraction. The answer to this problem proved to be what one names the phonons: gatherings of sound waves present when the atomic grid, constitutive of the matter, vibrates. Although this vibration of the matter is not audible, its role as moderating is essential.

The diagram above illustrates how two electrons, in pair of Cooper, become attached together.
According to the theory, when an electron negatively charged passes through positively charged ions, the molecular grid undulates. This causes to emit phonons which form basins of positive ions around the electrons. Before the electron entirely crossed the basin and that the grid found its normal position, a second electron is attracted towards this basin. It is by this process that two electrons, which should be normally pushed back, attract each other and bind.
The forces exerted by the phonons overcome the normal repulsion of the electrons. The pairs of electron are in cohesion between them because they cross the driver in unison, that is to say the second electron of a first pair becomes the first electron of the pair which follows. The electrons are separated by a certain distance. When an electron which constitutes a pair of Cooper passes near to an ion in the grid of the crystalline structure of material, attraction between the ion and the electron causes a vibration which is transmitted ion in ion until an electron absorbs this vibration.
The true effect is that the electron emitted a phonon and the following electron absorbed it. It is this exchange which makes it possible to preserve the pairs of Cooper. It is important to understand, however, that the pairs break and are reformed constantly. Since the electrons are indistinguishable particles, it is easier to imagine them constantly connected.
By this arrangement in pairs, the electrons cross the superconductor more easily, with less collisions. The electron can be regarded as a car which would run on L `highway. When it advances, the car splits the air on its passage. It is formed behind it a vacuum which is quickly filled by the air which is engulfed. A car which would follow the first would be aspired by this flow of air.
The car of behind is indeed attracted towards that which precedes it by engouffrement by air. It occurs the same effect with the electrons: when the electron passes between the positive ions, it attracts them towards him. When the grid, thus deformed, will find its initial position, the following electron is attracted towards the first like the two cars with flow of air because of an abrupt increase of positive polarization.
Electrons in superconductive state are as lines of cars which would move quickly. The areas of vacuum between the cars bind them all in ordered lines and it occurs the same phenomenon with the electrons. Gusts of wind impromptues can be planned to induce collisions, just like the phonons thermically excited break pairs.
The theory of B.C.S proves successfully that electrons can be attracted the ones the others by interactions with the crystalline grid. This occurs in spite of the fact that the electrons have the same load. When the atoms of the grid oscillate between positive and negative areas, the pair of electron alternatively is attracted and pushed back without collision. Appareillement of electron in pairs of Cooper is favorable because it for the purpose of putting material in a less state of energy. When electrons are dependant in pairs, they move in the superconductor in an ordered way.

Critical parameters of the superconductors

As a long time as the superconductor is cooled at very low temperatures, the pairs of Cooper remain intact, because of the reduced molecular movement. While the superconductor is heated, the vibrations in the atomic structure become more violent and break the pairs. When these pairs are broken, supraconductivity decreases. The superconductive metals and alloys have temperatures characteristic of transition, normal state at the superconductive state, called Critical Températures.
Below the temperature of superconductive transition, the resistivity of material is absolutely null. Superconductors made starting from various materials have Tc different. Among the ceramics superconductors, the critical temperature of YBa2Cu3O7 is of 90 K whereas that of HgBa2Ca2Cu308+x is of 133 K

The diagram represents the resistance of YBa2Cu3O7 according to the temperature.
Since there is no loss of electrical energy when the superconductors carry electric current, wire relatively narrow facts of superconductors can be employed to carry enormous currents. However, these materials are made to transport a certain maximum current, because above they cease being superconductors. So too much current crosses the superconductor, it will turn over at the normal state even if it is below its temperature of transition. The value of the density of current critical is a function of the temperature: the more the superconductor is cooled, the more it can transport of current.

The diagram is a graph representing the tension according to the current for a superconductive wire.
An electric current crossing a wire creates a magnetic field around this one. The force of the magnetic field increases as the current in the wire increases. Since the superconductors can carry large currents without loss of energy, they are adapted to create strong magnetic fields.
When a superconductor is cooled below its temperature of transition and that one applies a magnetic field to this one, this magnetic field remains around the superconductor without penetrating it. The physicists employ the capital letter H like symbol for the magnetic field. If the magnetic field is increased up to a given point, the superconductor will return in its normal state of resistivity.

The diagram shows me relationship between temperature and the field magnétiqe
The maximum value for the magnetic field at a temperature indicated is known like critical magnetic field and is noted Hc. There exists for each superconductor a zone of temperature and magnetic field for which the material is superconductive. Apart from this area, the material is in its normal state.

Types of different superconductors

Two types of the superconductors to date are known: type I and type II. The very pure tin and mercury, lead samples, are of the examples of superconductors of the type I. the ceramics superconductors at high temperatures such as YBa2Cu3O7 (Yttrium, Baryllum, Cuivre, Oxyde) and Bi2CaSr2Cu2O9 (Bismuth, Calcium, Strontium, Cuivre, Oxyde) are of the examples of the superconductors of the type II.

Diagram 9 shows that when an external magnetic field is applied to a superconductor of the type I
the induced magnetic field pushes back perfectly this magnetic field applied
until the accused material brutally passes from the superconductive state in a normal state.
The superconductors of the type I are very pure metals which too low have critical fields for the use in magnets. The force of magnetic field is measured in Gauss (G). The magnetic field of the ground is roughly of 0,5 Gauss. The force of the field on the surface of a magnet of a neodymium-iron-boron alloy is roughly of 16 kilogauss. The superconductor of the type I most extremely, pure lead, have a critical field of approximately 800 Gauss. Gauss is a very small unit. A unit much larger of field strength is the Tesla (T). Ten kilogauss (1 X 104 Gauss) is equal to 1 Tesla.
When this occurs, the material is known as in a mixed state, with part of the material in the normal state and the other part which is always in the superconductive state. The superconductors of the type I have too low Hc to be very useful. However, the superconductors of the type II have values much larger of Hc2. YBa2Cu3O7 for example, has values of field criticises maximum close relations of 100 Tesla.

The diagram is a graph which represents the induced magnetic field
according to the external magnetic field applied to a superconductor of the type II
This diagram shows a superconductor of the type II in an increasing magnetic field
It will be noted that this graph has Hc1 and Hc2. Below Hc1
the superconductor excludes all the lines from magnetic field
When the field lies between Hc1 and Hc2, it starts to penetrate material.
The behaviour of the superconductors of the type II also helps to explain the Meissner effect. During the levitation of a magnet to the top of a superconductor of the type I, it is necessary to use a basin in order to prevent the magnet from slipping outside the superconductor. The magnet is in a state of forces balanced while floating on the surface of the lines of field. Since the field on the surface of a samarium-cobalt magnet is approximately of 600 G and that Hc1 for a superconductor containing YBCO is lower than 200 G, the material is in a mixed state when one carries out the demonstration of the Meissner effect. Some of the lines of field of the magnet penetrated the sample and are imprisoned in the defects of grain of the crystals of material. This is known under the name of fixer of flow and blocks the magnet in an area above the superconductive plate.
The superconductive state is thus defined by three very important factors: the critical temperature, the critical field and density of current critical. Each one of these parameters depends much on the two other properties. The maintenance of the superconductive state requires that the magnetic field and the density of current, as well as the temperature, remain below the breaking values, which depend on material.
The delimited surface when one takes into account the 3 parameters is called critical Surface. On the basis of this surface towards the origin, the material is superconductive. When one is in the areas apart from this surface, the material is in a normal or mixed state. When the electrons form pairs of Cooper, they can share the same state of energy.
This has like consequence a lower state of energy for the superconductor. The critical temperature and the field criticise are values favorable to the breaking of the pairs of electrons.
A density of current higher than the critical density is forced to cross the superconductor. This flow passing through the normal part of material in mixed state is directly related to the movement of the lines of field. For the majority of the practical applications, the superconductors must be able to carry the high currents and to resist the magnetic field raised without turning over in their normal state.
Higher values of field criticises and on critical current depend on two important parameters which influence the minimisation of energy, the penetration depth of London and the length of coherency. The penetration depth is the length characteristic of the penetration of a magnetic field due to the presence of currents of surface. The length of coherency is the minimal length between which supraconductivity can be established.
The relationship between the penetration depth and the length of agreement is called parameter of Ginzburg-Pram. If this value is larger than 0.7, the complete exclusion of the magnetic flux is more favorable and the magnetic field can penetrate the superconductor in points called swirls. The currents, whirling around the normal points, produce magnetic fields parallel with the field applied. These weak magnetic fields are pushed back and move to be arranged in known lines ordered under the name of grid of flow.
This mixed phase helps to preserve the supraconductivity of Hc1 with Hc2. It is very important that these swirls do not move in answer to the magnetic fields if the superconductors must carry large currents. Movements of swirls involve the appearance of a resistance. The movement of swirl can be indeed associated with sites with atomic defects, such as inclusions, or impurities. The sites of fixing can be intentionally introduced into a superconductor while adding to it of the impurities or by subjecting it to radiations.

The tunnel effect

An example of the microscopic properties is the phenomenon of the tunnel effect in the superconductors. The tunnel effect is a process resulting from undulatory nature of the electron. This effect occurs when electrons cross spaces which are prohibited to them in traditional physics, because of barriers of potential. The tunnel effect on a pair of electrons, between 2 superconductors separated by an insulating barrier, was discovered the first time by Brian Josephson in 1962.
Josephson discovered that if two superconductive metals were separated by a thin insulating barrier, like a thick oxide coating from 10 to 20 Angströms, it is possible that the pairs of electrons pass by the barrier without resistance. This is known like D.C. current effect of Josephson and is contrary with what occurs in ordinary materials, where a potential difference must exist so that power is on. The current which crosses a junction D.C. current of Josephson has a density of current critical which depends on material of the junction as well as its geometry.
A junction of Josephson is composed of two superconductors separated by a thin insulating barrier. The pairs of superconductive electrons will bore a tunnel in this barrier. As a long time as the current is below the critical current for the junction, there will be null resistance and one will observe no voltage drop through the junction.
If it is placed beside a wire in which a power is on, the magnetic field generated by this wire will lower the density of current criticises Jc of the junction. The current which passes then by the junction does not change but becomes larger than the critical current, which was lowered.
The junction then develops a resistance which primarily involves a dissipation of the current by Joule effect.
The junction of Josephson is component of high-speed commutation. The junctions of Josephson can carry out switching functions such as commutations of voltages approximately ten times faster than the ordinary semiconductor circuits. It is a notable advantage for the computers, which depend on the commutation rate one-off. Since the speed of a computer depends on necessary time to transmit impulses of signal, the exceptional speed of the commutation of the devices of junction makes them ideal for the use in fast computers and much smaller.

The transport of the current

The transport of the current between the powerplants and the dwellings or industries passes today only by aluminium or copper cables. The disadvantage of these two metals is to have a resistance, which, although it is weak compared to that of other materials, involves a very large loss of energy during transport, primarily in the form of heat. Moreover, copper being very heavy, one gradually replaces it by L rsquo; aluminium, lighter but more resistive, which increases the losses of current and obliges to make pass from the more important tensions.
The application of the superconductors in the transport of energy then here is justified perfectly. Indeed, because of their null resistivity, the superconductors avoids the loss of current per Joule effect. They make it possible moreover to run much more current than a traditional line, and this in a cable of section lower than that of the conventional cables.
Thus, 8400 kg of copper cable could be replaced by only 110 kg of superconductive cable, which would largely facilitate work of hiding of the phone lines at the time of the completion of existing installations or the establishment of new lines.

A null resistance

The superconductors have the capacity to conduct electricity without loss of energy. When the current enters an ordinary driver, for example a copper wire, a certain energy is lost. In a bulb or an electric stove, electrical resistance creates the light and heat.
In metals such as copper and aluminium, electricity is conducted as much as the electrons migrate individually of one atom to the other. These atoms form a vibrating grid in the metal driver; the hotter metal is, the more it vibrates. While the electrons start to move in this labyrinth, they run up against the tiny impurities or imperfections in this grid.
When the electrons run up against these obstacles, they are propagated in all the directions and lose energy in the form of heat.
The diagram shows atoms laid out in a crystalline grid, and mobile electrons rebounding against the atoms which are on their way.
Inside a superconductor, the behaviour of the electrons is very different. The impurities and the atomic grid are always there, but the movement of the superconductive electrons through the obstacles is completely different. When the electrons cross the superconductor, they pass freely through the atomic grid. Since they do not strike anything and do not create any friction, they can thus transmit electricity without loss of current nor of energy.
The capacity of the electrons to cross a superconductor without resistance embarrassed the scientists during many years. The hotter one material is, plus its atomic vibrations are important. Reciprocally, more one substance is cold and less it vibrates. The first researchers proposed the theory that a lower number of atomic vibrations make it possible the electrons to more easily cross a driver. By corollary, this predicted a slow reduction in the resistivity with the temperature. It quickly became obvious that these simple ideas could not explain supraconductivity.

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