Scientific community celebrated the 100th anniversary of discovery of superconductivity--one of the most exciting, unconventional and mysterious phenomena of the solid-state physics in 2011. What is the essence of this phenomenon, how was the knowledge in this field accumulating, what is the scale of its effect produced on contemporary physics and technology and are there any pending questions related to it? Yevgeny Maximov, RAS Corresponding Member and Head of the Sector at the Institute of Physics named after P. Lebedev (FIAN), answered all these questions for the Agency of Scientific Information FIAN-Inform.

History of superconductivity--a chain of discoveries of more and more complex structures--a kind of "chemical evolution" began in 1911. That time the Dutch experimenter Heike Kamerlingh Onnes who studied properties of different substances under helium temperatures at Leiden University (the Netherlands) noted a curious effect in the course of experiments with pure mercury submerged in liquid He: first, resistance of mercury decreased gradually and then, at the temperature of 4.15 K (about-269ºC), dropped almost to zero. This was in conflict with the classical electron theory of metals at those times. The scientist carried out the experiment several times but the results were the same: under superlow temperatures electrons did not practically meet resistance of atoms of the crystal lattice. 'All doubts dispelled..," he wrote in 1913, speaking about that period, "we discovered a new state of mercury characterized by absence of physical resistance". He continued: "Mercury passed into a new state and, taking into account unique electrical characteristics of this new state, it could be called 'superconductive'"*. It was a sensation: the current in such ring could circulate eternally! On the short notice the Dutchman did not realize the fundamental character of his discovery. But the world scientific public recognized it, and as early as in 1913 Kamerlingh Onnes was awarded the Nobel Prize in physics.

It took little time to establish: many other metals, for example, aluminum, lead, indium and alloys become superconductive under low temperatures. Prospects of use of these newly discovered properties seemed unlimited: power transmission lines with no energy losses, superpowerful magnets, electric motors and transformers of new types. But it took decades to gain an insight into the nature of this exciting phenomenon, develop a sustainable theory and begin its practical application.

"Basically, you can explain behavior of a regular non-superconductive metal in the language of classical mechanics," Yevgeny Maximov commented.--"Electrons, like gas, move and collide, you turn them to one direction, and they, after colliding with other electrons, turn back; that is the source of resistance occurring in the current. Superconductivity is a state of matter when particles do not rub themselves or turn back, in contrast to classical mechanics... It is a quantum state, when the concept of rubbing disappears, all particles are as if bound together and move in strict lines not allowing anyone to jump out."

* See : V. Sytnikov, V. Vysotsky. "Superconducting Technologies in Power Engineering", Science in Russia, No. 2. 2010.--Ed.

The discovery made in 1933 by the German physicist Walther Meipner and his colleague Robert Ochsenfeld--the effect of ousting of the constant magnetic field from the solid conductor when the latter becomes superconductive-played a prominent role in development of the theory under consideration. Today it is called the levitation effect (from Latin levitas--alleviation), when an object levitates in space without visible support not touching ground or water. It became clear that this phenomenon is of quantum-mechanical nature.

A serious step towards comprehension of remarkable properties of materials under low temperatures was made in 1950 by Soviet physicists, future laureates of Nobel Prize academicians Lev Landau (1962) and Vitaly Ginzburg (2003). That time The Magazine of Experimental and Theoretical Physics published their phenomenological theory describing superconductivity by means of the so-called "order parameter", taking into account quantum effects, in particular, wave function characterizing behavior of electrons in a solid body. Scientists assumed: under given parameters, these particles get coherent properties and become indistinguishable.

Thus, Maximov explained, the Ginzburg-Landau equation enabled scientists to describe behavior of a superconductive material in magnetic fields, but did not give answer to the question "how did it become such?". This problem was solved in 1957 by three American physicists--John Bardeen (Foreign Member of the USSR AS from 1982), Leon Cooper and John Schrieffer (Foreign Member of the USSR AS from 1988), who developed a microscopic theory that explains the mysterious phenomenon through electron pairing (i.e. formation of the so-called Cooper pairs) by means of interchange of vibrations of phonons in a crystal cell; in 1972 this theory was awarded Nobel Prize.

In 1957, when checking the Ginzburg-Landau theory, our compatriot Academician Alexei Abrikosov (who today lives in the USA and works at the Argonne National Laboratory) discovered a new class of materials--superconductors of the second type. Unlike type 1 superconductors, they preserve their properties even in a strong magnetic field (up to 25 Tl). Developing the reasoning proposed by Vitaly Ginzburg, the scientist explained these characteristics by formation of a regular grid of magnetic lines closed in ring currents. Such structures were called "Abrikosov vortices". However, theoretical studies carried out by the scientist long before experiments enabled him to forecast a number of phenomena typical for the given superconductive materials. By the way, it is these materials that are mainly used in colliders--charged particle acceler-

Science in Russia, No. 3, 2011

Permanent magnet levitating 1cm above the bottom of a superconductive cup (the Meissner effect).

Metals, their superconducting transition temperatures (T_S,K), the publication year of discovery of superconductivity.

ators based on counter beams*, tomographs and other powerful technical devices. In 2003, Abrikosov together with Vitaly Ginzburg and their American colleague Anthony James Leggett were awarded Nobel Prize for these works.

Next, following the chronology, Maximov marked out the year of 1964, when Ginzburg and Jason Little (USA), independently of each other, proposed an idea on the possibility of an increase of superconducting transition temperature using another non-phonon mechanism. In particular, they calculated: replacement of phonons with excitons (hydrogen-like quasi-particles) would significantly (up to 50-500 K) increase t (note, that time the

* See: L. Smirnova, "Start of the Large Hadron Collider", Science in Russia, No. 5, 2010.--Ed.

temperature limit fluctuated at the level of 25 K). However, practical search for such materials was ineffective, and studies gradually came to nought...

But in 1986, a sensational publication by Swiss Carl Muller (RAS Foreign Member from 1994) and German Johannes Georg Bednorz, employees of the laboratory of a famous IT company IBM, contained a statement about the capacity of copper, lanthanum and barium oxide ceramics to pass into superconductive state at 30 K (!). This fact demonstrates a discovery of a new class of materials-high-temperature superconductors. In 1987, this work was awarded Nobel Prize and was followed by an avalanche of studies in this field. Six months later the American physicist Paul Chu managed to find a junction temperature of 93 K. The highest temperature-138 K-was

achieved for a complex substance HgBa₂Ca₂Cu₃O_x in 1993. And we believe that it is not the limit.

We cannot forget another important thing: if scientists had to cool down "old" materials with helium to work with them, today they can switch to a cheaper and more widespread coolant--liquid nitrogen (boiling point 77 K).

However, Maximov emphasized, with the discovery of a new class of materials, a lot of new questions arose. Are these materials the same metals, even with some distinctive features, or are they absolutely new, not known before? Will Bardeen-Cooper-Schrieffer microscopic theory suit to explain the nature of superconductivity in these materials? Is it possible to achieve such parameters of superconducting transition that will make true the dream of mankind on power transmission lines carrying the current under the room temperature, which would save at least one third of produced energy that is lost today when transmitted to long distances? According to the head of the FIAN Sector, even after 100 years from discovery of the phenomenon, these questions are still pending.

Nevertheless, we should acknowledge success of practical use of superconductivity. This phenomenon is becoming more and more common in modern electronics, power engineering, industry and medicine.

"Superconductors are an ideal base material to produce electromagnets," Maximov commented. They are essential to solve many tasks of physics. For example, a series of Tokamak plants with superconductive winding was designed at the Institute of Nuclear Energy named after I. Kurchatov (today the National Research Center "Kurchatov Institute") that has been studying the controlled thermonuclear fusion* since 1956. The winding is made in the form of tores (or, more simply, "rings") with the magnetic field able to hold dense high-temperature plasma."

Properties of superconductors served as a basis for the Large Hardron Collider**, the largest experimental plant in the world constructed on the border between Switzerland and France. They also serve as a basis for creation of an International Thermonuclear Experimental Reactor (ITER), being constructed in Cadarache (France)***.

Sensitive electronic devices are another field of practical application of superconductivity. Magnetometers capable of measuring fields of about 10^-9 Gs are widely used to study magnetic materials and in medical cardiographs. Highly sensitive detectors are in high demand in geophysics.

Superconductive devices are becoming more and more popular in metrology: they are used in current comparators (plants to compare observed values against the benchmark), in fundamental research works to measure

* See: V. Strelkov, "No Royal Ride in Thermonuclear Research", Science in Russia, No. 1, 2009.--Ed.

** See: L. Smirnova, "The 21st Century Megaproject", Science in Russia, No. 5, 2009.--Ed.

*** See: V. Glukhikh et al., "On the Brink of Thermonuclear Era", Science in Russia, No. 3, 2003; L. Golubchikov, "Tokamak--International Challenge", Science in Russia, No. 1, 2004.--Ed.

fractional charges of nuclear particles and verify the relativity theory.

As for computer technologies, superconductors can provide insignificant power losses, if used together with thin-film elements, and apparent density of circuit wiring. Today we can already speak of pilot thin-film contacts, containing hundreds of logical elements, including memory.

Industrial use of superconductors means, first of all, generation, transmission and consumption of electrical energy. For example, a superconductive cable of some inches in diameter can transmit the same volumes of energy as big power transmission nets, with no or insignificant power losses, and the cost of insulation and cooling down of hyperconductor materials will be balanced by efficiency of the process.

One more perspective area for superconductors is current generators and small electromotors. Winding of superconductive materials could produce strong magnetic fields that would greatly increase power of these devices as compared with conventional gear. By the way, we have already designed prototypes of such equipment. When

ceramic superconductors are introduced in industrial production, this equipment could become cost efficient.

And the last thing. Engineers have been studying the possibility of application of unique properties of the given materials (in particular, lévitation) to design a train suspension. With mutual repulsion of a moving magnet and current induced in a guide conductor, a train can move smoothly, without noise and friction, picking up speed. Such high-tech vehicle was first launched in the 1980s in Birmingham, but 11 years later it was taken out of service due to technical problems. Nowadays such trains are operating in China and Japan: over 10 types of trains with magnetic suspension have been designed there. One of them--MLX01--set up an absolute speed record for railway vehicles in 2003, reaching 581 km/h.

Based on materials of the Agency of Scientific Information F IAN-Inform, December 28, 2010