The supraconductivity is a phenomenon occurring in certain materials known as superconductive. It is characterized by the absence of Electrical resistance and the cancellation of the Magnetic field inside the material (Effet Meissner). Conventional supraconductivity appears at very low temperatures, close to the Absolute zero (- 273.15 °C).
The phenomenon was discovered in 1911 by a student in physics, Gilles Holst, under the direction of the Dutch physicist Kamerlingh Onnes (this last being thereafter adapted this discovery). It showed that the electrical resistance of the mercury becomes nonmeasurable in lower part of a certain temperature called temperature criticizes Tc. In the conventional superconductors, complex interactions occur between the Atome S and the free electrons and lead to the appearance of dependant pairs of electron S, called even of Cooper. These pairs behave like Boson S, of Spin equal to 0, and “are condensed” in only one quantum state. A characteristic of this state is a flow without any resistance (Superfluidité).
There also exists of other material classes, collectively called “nonconventional superconductors”, whose properties are not explained by the conventional theory. In particular, the class of the Cuprate S (or “superconductors at high temperature criticizes”), discovered in 1986, present of the superconductive properties at temperatures much higher than the conventional superconductors. However, which the physicists name “high temperature” remains extremely low (the maximum is 138 K, that is to say -135°C).
Although this subject is, since nearly two decades, regarded as the most important subject of the solid state physics, no theory is currently satisfactory to describe it.
The temperature of the Nitrogen liquid -196 °C industrially easy to reach is generally taken in reference like temperature in lower part of which one enters the very low temperatures. Another definition calls upon concepts of magnetic phase shift.
Elementary properties
A superconductor is a material which, when it is cooled in lower part of a temperature criticizes presents two index properties: null resistance and perfect Diamagnetism. The existence of these common characteristics making it possible to define supraconductivity shows that it is about real a thermodynamic Phase. The study of the variations of the physical properties of the superconductors when they pass in the superconductive state confirms this and establishes that the superconductive transition is true a Transition from phase.
Null resistivity
The complete absence of Electrical resistance of a superconductor traversed by a limited current is obviously their most known property. It is besides this one which gave its name to the phenomenon.
Meissner effect
See also: Effect Meissner
The Meissner effect, named according to Walther Meissner which discovered it in company of Robert Ochsenfeld in 1933, is the fact that a sample subjected to a external Magnetic field expels this one when it is cooled in lower part of his critical temperature, and this whatever its former state.
According to the Maxwell's equations, in any material whose resistance is null, the magnetic field must remain constant during time. However, the existence of the Meissner effect, watch which supraconductivity does not summarize with the existence of an infinite conductivity.
In experiments, one shows the Meissner effect by cooling a superconductive sample in lower part of his critical temperature in the presence of a magnetic field. It is then possible to show that the magnetic field inside the sample is null, whereas for a hypothetical perfect driver, it should be equal to the magnetic field applied.
(Note: certain superconductors, known as of standard II , present the Meissner effect only for low values of the magnetic field, while remaining superconductive with higher values. cf will infra )
Theories
Theory of Ginzburg-Pram
See also: Theory of Ginzburg-Pram
The theory developed by Ginzburg and Landau in 1950 introduces a complex parameter of order ψ ( R ) characterizing supraconductivity within the general framework of the theory of Pram of the transitions from phase of the second order. The physical significance of this parameter is that is proportional to the density superconductive electrons ( i.e. of electrons constituting of the pairs of Cooper). The starting postulate of the theory is that the density of free energy fs can be developed in a series of the parameter of order close to the superconductive transition in the following form:
where fn0 is the density of free energy in the normal state in null field, has is the vector potential and B is the local intensity of magnetic induction.
The second and third terms are the development with the second order in |ψ|² , the third can be seen like the invariant expression of gauge of the kinetic energy associated with the “superconductive load and mass, charge carriers” m* q* while the fourth is simply the density of energy magnetic.
In the superconductive state, in the absence of field and of Gradient S, the preceding equation becomes:
Let us note that β is necessarily positive because if not, it there would not have total minimum for the free energy, and thus not of state of balance. If α > 0 , the minimum takes place for ψ = 0 : the material is in the normal state. The case interesting is thus that where α < 0 . One has then, with balance, , from where:
to supplement
Vortex and superconductors of the type II
Theory BCS
See also: Theory BCS
This theory is based on the coupling of the electrons of a metal in pair: even of Cooper. They form a state single, coherent, of energy lower than that of normal metal, with not paired electrons.
The problem is to explain this pairing taking into account the Coulomb Répulsion. In a metal, the electrons interact with the Crystal lattice formed of Ion S positive. Those attract the electrons and move slightly (the positive ions have large a Inertie). The physicists gave the name of Phonon S to these natural atomic vibrations. This interaction between the electrons and the phonons is at the origin of the Résistivité and supraconductivity: attracted by the very fast passage of an electron (106 m/s), the ions move and create a local zone electrically positive. Taking into account inertia, this zone persists whereas the electron passed, and can attract another electron which is thus, via a phonon, paired with the precedent. And this in spite of the Coulomb repulsion. Thermal agitation ends up destroying this fragile balance from where the harmful effect of the temperature.
A characteristic of the pairs of Cooper is that their intrinsic magnetic moment (also called " spin") is null. Indeed, the two paired electrons have the same spin (1/2, spin characteristic of the Fermion S), but of opposite sign. It is the condition so that the energy of the pair is lower than the sum of energies of the two electrons. They then form a unit which behaves like a Boson (particle of whole spin obeying statistics of Bump-Einstein): the pairs move without meeting least resistance. From where supraconductivity.
The difference in energy between the superconductive state and the normal state is called gap energy. It is energy necessary to pass from the superconductive state in a normal state by breaking the pairs of Cooper. This energy tends towards zero when the temperature tends towards the critical temperature.
The electron-phonon interaction thus plays a crucial role for the pairing of the electrons for supraconductivity
This theory was created before the discovery of superconductors at high critical temperatures. A question arises then: do the superconductors with high Tc contradict theory BCS? The theorists are not agreement on this subject. Some think that the coupling between the electrons is not due any more to the network (thus with the phonons), but with other interactions (electronic, magnetic, both,…). Others propose entirely new models. The subject remains still open…
Classes of superconductors
Conventional superconductors
The conventional superconductors are those which are well described by theory BCS.
Nonconventional superconductors
The nonconventional superconductors are the materials which have properties of supraconductivity but which do not conform to the Théorie BCS or its extensions.
The first nonconventional superconductor was discovered by Johannes Georg Bednorz and Karl Alexander Müller in 1985. It is about a ceramics made up of Oxyde S mixed of Baryum, Lanthane and Cuivre whose critical temperature is approximately 35 K (- 238 °). This temperature was quite higher than the highest known critical temperatures at that time (23K); this new family of material was called superconductive at high temperature . Bednorz and Müller accepted in 1987 the Nobel Prize of physics for their discovery.
Since then, of many other superconductors at high temperature were synthesized.
Since 1987, one reached supraconductivity with the top of 77K, the boiling point of the Azote,
what is very important for the technological applications because the liquid nitrogen is much cheaper than the Hélium liquid which was to be used hitherto. YBa2Cu3O7 example, Tc = 95 K.
The temperature criticizes record is of approximately 133K (- 140°) to the normal pressure and of the temperatures slightly higher can be reached with higher pressures.
Nevertheless, it is regarded as not very probable that a material containing cuprate can reach supraconductivity with room temperature.
However, these last years, other nonconventional superconductors were discovered. Among those, some are not superconductive at high temperature but are nonconventional according to other criteria (for example, the origin of the force at the origin of the formation of the pairs of Cooper can be different that postulated by theory BCS); but of others, having critical temperatures unusually raised but not being containing cuprate were also discovered. Some of the latter could be examples of conventional superconductors extreme (one suspect that it is the case of the diboride of Magnésium MgB2, Tc=39K); others have less conventional characteristics.
Exotic superconductors
August 1st
History
The phenomenon of supraconductivity was discovered in 1911 by a student in physics, Gilles Holst, under the direction of the Dutch physicist Kamerlingh Onnes (this last being thereafter adapted this discovery), lasting an experiment on the conductivity of the mercury in a solid state. He realized that the resistance of this metal was cancelled to 4,15 K. For this discovery, he received the Nobel Prize of physics in 1913. Experiments with many other elements showed that some had faculties of supraconductivity, but others not:
Let us quote in 1913, the Plomb to 7 K and in 1941, the nitride of Niobium to 16 K.
In 1933, Meissner and Ochsenfeld discover that the superconductors push back the magnetic field, a phenomenon known by the name of Effet Meissner. In 1935, the Fritz brothers and Heinz London showed that the Meissner effect is a consequence of the minimization of the free energy transported by the superconductive current.
In 1950, a phenomenologic theory known as of Ginzburg-Pram was worked out by Landau and Ginzburg. This theory was a success to explain the macroscopic properties of the superconductors by using the equation of Schrödinger. In particular, Abrikosov showed that with this theory one can provide that there exist two categories of superconductors (called type I or type II). Abrikosov and Ginzburg received the Nobel Prize 2003 for this work (Pram being deceased in 1968).
It is in 1950 that one notes that the critical temperature depends on the isotopic mass.
A complete theory of supraconductivity was proposed in 1957 by Bardeen, Cooper and Schrieffer. Known by the name of their initial Theory BCS , she explains supraconductivity by the formation of pairs of electrons (even of Cooper) forming then Boson S interacting with Phonon S. For their work, the authors had the Nobel Prize of physics in 1972.
In 1959, Gorkov showed that theory BCS is reduced to the theory of Ginzburg-Pram in the vicinity of the critical temperature of appearance of supraconductivity.
In 1962, the first superconductive wire (a niobium-titanium alloy) are marketed by Westinghouse. The same year, Josephson provides theoretically that a current can circulate through a thin insulator separating two superconductors; this phenomenon which bears its name: the Effect Josephson, is used in SQUID S. These devices are used to make very precise measurements of h/e , and combined with the quantum Hall effect), with the measurement of the Constante of Planck H . Josephson received the Nobel Prize 1973.
In 1986, Bednorz and Müller discovered a supraconductivity at a temperature of 35 K in structural materials copper perovskite containing Lanthane (Nobel Prize of physics, 1987).
Very quickly while replacing lanthanum by Yttrium, i.e while producing of YBa2Cu3O7, the critical temperature is assembled to 92 K, exceeding the temperature of the liquid nitrogen which is of 77 K. That is very important because the liquid nitrogen is produced industrially and at low prices and can even be produced locally. Many superconductive cuprates were produced thereafter and the comprehension of the mechanisms of this supraconductivity is still to discover. Unfortunately these materials are Céramique S and cannot be worked easily, moreover, they easily lose their supraconductivity with strong field and thus the applications are made wait. Research continues, to decrease the sensitivity to the fields, and to increase the critical temperature. After the temperature of nitrogen liquidates, attack, the threshold psychological and economic (- 80 °C) is the carbonic ice.
May 31st, 2007, a Franco-Canadian team of physicists published in the nature magazine a study which, CNRS, would make it possible to advance appreciably in the comprehension of these materials.
Applications
Magnetic Canon
Electromagnets
The realization of electromagnet S superconductors constitutes certainly the most current application of supraconductivity. One finds them in the fields:
- of the medical imagery for which a magnetic field of several Teslas is produced by a superconductive Solénoïde. They also make it possible to produce a very homogeneous magnetic field, which makes it possible to obtain an image of great quality;
- of the particle accelerator : project LHC (Broad Collider High-energy particle) of CERN: 1 700 tons of superconductor (1);
- of the magnetic Levitation, with in particular the trains with electromagnetic lift (the Japanese maglev, to see electromagnetic Lift) and accumulating electromechanics with Wheel of inertia.
Storage of energy
SMES: Superconducting Magnet Energy Storage
A winds superconductive is connected to the network via a reversible alternate-continuous converter. The reel is fed by the Redresseur which makes it possible to store energy in the form ½ L I ². Where necessary (defect of the line) the energy stored in the superconductive reel is retransférée with the installation via the inverter.
The property of levitation of the superconductors can also be made profitable to make storage of energy. It is the case of the accumulators of rotary kinetic energy (in English). In these applications, a magnetized wheel is placed in levitation with the top of a superconductor. The wheel is put in rotation (ideally in the vacuum to reduce to the maximum frictions) by means of an engine (phase of load). Once the wheel " chargée" , it preserves energy in the form of energy kinetic of rotation, with little loss, since there is almost no friction. Energy can be recovered by slowing down the wheel.
SMES and Flywheel are thus two technological solutions which could replace a traditional battery, although the maintenance of the cryogenic temperatures is énergivore.
Electromagnetic containment
With an aim of carrying out controlled thermonuclear fusion: the Tokamak S or the Stellarator S are toric enclosures inside which one confines of the plasma S under pressures and at considerable temperatures (1). See also the Project ITER.
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