Superconductivity theory

Superconductivity, as a phenomenon, arises as a result of the formation of Cooper pairs of electrons that behave like a single particle.

Superconductivity is a strange thing and, to some extent, even contrary to common sense. When an electric current flows through an ordinary wire, then, as a result of the presence of electrical resistance at the wire, the current does some work aimed at overcoming this resistance from the side of the atoms, as a result of which heat is released. In this case, each collision of an electron – a current carrier – with an atom slows down the electron, and the brake atom itself heats up – that’s why the spiral of an electric hotplate becomes so red and hot. The thing is that the spiral has an electrical resistance, and, as a result, when an electric current flows through it, it releases thermal energy (cm. Ohm’s law).

In 1911, the Dutch experimental physicist Heike Kammerlingh Onnes (1853-1926) made an amazing discovery. By immersing the wire in liquid helium, the temperature of which was no more than 4 ° above absolute zero (which, recall, is -273 ° C on the Celsius scale or -460 ° F on the Fahrenheit scale), he found that at ultra-low temperatures, electrical resistance practically drops to zero. Why this is happening, he, in fact, could not even guess, but the fact turned out to be obvious. At ultralow temperatures, electrons practically did not experience resistance from the atoms of the crystal lattice of the metal and provided superconductivity.

But why is this happening? This remained a mystery until 1957, when three more experimental physicists – John Bardeen (1908-1991), Leon Cooper (b. 1930) and John Robert Schrieffer (b. 1931) invented explanation for this effect. The theory of superconductivity is now called in their honor the “BCS theory” – after the first letters of the names of these physicists.

And its essence lies in the fact that at ultralow temperatures, heavy metal atoms practically do not vibrate due to their low thermal motion, and they can be considered actually stationary. Since any metal only because of the electrically conductive properties inherent in the metal, it releases the electrons of the outer layer into “free floating” (cm. Chemical bonds), we have what we have: ionized, positively charged nuclei of the crystal lattice and negatively charged electrons, freely “floating” between them. And here the conductor comes under the influence of the electrical potential difference. Electrons – willingly or unwillingly – move, being free, between positively charged nuclei. Each time, however, they sluggishly interact with the nuclei (and with each other), but immediately “run away”. However, at the same time, while the electrons “slip” between two positively charged nuclei, they seem to “distract” them to themselves. As a result, after an electron has “slipped” between the two nuclei, they come closer for a short time. Then the two nuclei, of course, smoothly diverge, but the job is done – a positive potential has arisen, and more and more negatively charged electrons are attracted to it. The most important thing here is to understand: due to the fact that one electron “slips” between the atoms, it, thereby, creates favorable energy conditions for the advancement of one more electron. As a result, electrons move inside the atomic-crystal structure in pairs – they simply cannot do otherwise, since this is not energetically beneficial for them. To better understand this effect, an analogy from the world of sports can be drawn. Cyclists on the track often use the “drafting” tactics (namely, “hang on the tail” of the opponent) and, thereby, reduce air resistance. The electrons do the same, forming Cooper pairs

It is important to understand here that at ultra-low temperatures everything electrons form Cooper pairs. Now imagine that each such pair is a vermicelli-like bundle, at each end of which there is a charge-electron. Now imagine that in front of you is a whole bowl of such “noodles”: it all consists of intertwined Cooper pairs. In other words, electrons in a superconducting metal interact in pairs with each other, and this consumes all their energy. Accordingly, the electrons simply do not have energy left to interact with the nuclei of the atoms of the crystal lattice. As a result, it comes to the point that the electrons slow down so that they have nothing to lose (energetically), and the nuclei surrounding them “cool down” so much that they are no longer able to “slow down” free electrons. As a result, electrons begin to move between metal atoms, practically without losing energy as a result of collisions with atoms, and the electrical resistance of the superconductor tends to zero. For the discovery and explanation of the effect of superconductivity, Bardeen, Cooper and Schrieffer received the Nobel Prize in 1972.

Many years have passed since then, and superconductivity from the category of unique and laboratory-curious phenomena has turned into a generally recognized fact and a source of multibillion-dollar income for enterprises in the electronic industry. And the thing is that any electric current excites a magnetic field around itself (cm. Faraday’s law of electromagnetic induction). Since superconductors conduct current for a long time with virtually no loss, if kept at ultra-low temperatures, they are ideal materials for making electromagnets. And, if you have ever undergone a medical diagnostic procedure called electron tomography and is performed on a scanner that uses the principle of nuclear magnetic resonance (NMR), then you yourself, perhaps without knowing it, were a few centimeters away from superconducting electromagnets … It is they who create the field that allows doctors to obtain high-precision sectional images of human body tissue without the need to resort to a scalpel.

Modern superconductors retain their unique properties when heated up to temperatures of the order of 20K (twenty degrees above absolute zero). For a long time, this was considered the temperature limit of superconductivity. However, in 1986, Georg Bednorz (b. 1950) and Alexander Müller (b. 1927), employees of the Swiss laboratory of the IBM computer company, discovered an alloy whose superconducting properties are retained even at 30K. Today, science knows materials that remain superconductors even at 160K (that is, just below –100 ° C). With this generally accepted theory that would explain this class high temperature superconductivity, has not yet been created, but it is quite clear that it cannot be explained within the BCS theory. High-temperature superconductors do not find practical application today due to their extremely high cost and fragility, however, developments in this direction continue.

John BARDIN
John BARDIN
John Bardeen, 1908–91

American physicist, one of the few twice Nobel laureates. Born in Madison, Wisconsin, the son of a professor of pathology. Educated at Madison and Princeton Universities. Between his first and second studies, he worked for the oil company Gulf Oil for several years as a seismologist-prospecting for oil deposits. During the Second World War, he served in the navigation laboratory of the US Navy in Washington, after the war he worked in the radio laboratory of the Bell telephone company, where he co-authored the invention of the transistor, for which he was awarded his first Nobel Prize in physics in 1956. After that, Bardeen became a professor at the University of Illinois, where he began to develop the BCS theory, for which, together with co-authors, in 1972 he received the Nobel Prize for the second time.

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