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Superconductivity: an old mystery for new physics
Superconductivity: an old mystery for new physics

Superconductivity is a quantum phenomenon that has shaken up the 20th century scientific community and has won scientists Nobel Prizes on seven occasions. Superconductivity is the property of certain substances to exhibit zero electrical resistance below a critical temperature (usually well below -100ºС). As a result, electrons in the crystal lattice of a substance can move through it without energy loss. The flow of electric current without attenuation in a closed superconductor was demonstrated during an experiment which lasted two and a half years and was interrupted only because workers delivering cryogenic liquids went on strike.

The history of superconductivity is remarkable both in its accomplishments and in its curiosities. It begins back in 1911. The Dutch physicist Heike Kamerlingh Onnes was the first to produce liquid helium and thus paved the way for systematic studies of the properties of materials at temperatures close to absolute zero (for which Kamerlingh Onnes was jokingly nicknamed ‘Mr. Absolute Zero’ in scientific circles). He discovered that mercury completely loses electrical resistance when cooled to 4 K (-269°C). This fact disproved Kelvin’s theory that resistance would increase as temperature decreased, and on the whole came as a big surprise to physicists at the time.

Zero electrical resistance is not the only distinguishing feature of superconductors. In 1933, German scientists Walther Meissner and Robert Ochsenfeld discovered that magnets levitate over superconductors at low temperatures.


A magnet 'levitates' over a superconductor


The fact is that when ordinary substances reach a magnetic field, they 'let it flow' through themselves. Superconductors do not allow the magnetic field to penetrate them, and create their own 'retaliatory' magnetic field, which can compensate for the gravitational force of the magnet placed above the superconductor. Repelling from a stationary superconductor, a magnet levitates, or simply put, 'hovers' above it. This discovery was named the 'Meissner–Ochsenfeld effect'.

Superconductivity: an old mystery for new physics

Meissner–Ochsenfeld effect

At the very beginning of the 1950s, the Russian scientist Vitaly Ginzburg, working together with Lev Landau, was inspired with his theory of phase transitions and the challenge of understanding superconductivity.  Together with Landau, he formulated a phenomenological (that is, descriptive and without revealing the mechanisms of a physical phenomenon) theory, which is now called the Ginzburg-Landau theory.  At about the same time, the Russian scientist Alexey Abrikosov presented a theoretical explanation for the existence of type I and type II superconductors (see below).

In 1957, American physicists John Bardeen, Leon Cooper, and John Schrieffer proposed a microscopic theory, essentially based on the laws of quantum mechanics.  This theory explained some properties of superconductors and subsequently received the name 'BCS theory', formed from the initials of the authors' names.

The BCS theory is based on the assertion that electrons in a superconductor move in pairs (Cooper pair).  The theory goes something like this: the first electron, flying between the positive ions of the crystal lattice, attracts them and, as a result, the ions come close to one another.  In the space between them, an area of excess positive charge is formed.  The second electron of the Cooper pair, following the first, is attracted to this region, and then to the first electron.

Superconductivity: an old mystery for new physics

Cooper pair of electrons

In this movement, the electrons move easily along the conductor without losing energy. This theory is consistent with experimental data and brings scientists closer to uncovering the secrets of this amazing phenomenon, although it is still far from a complete understanding and explanation of superconductivity.

In the first half of the 20th century, parameters such as critical temperature and critical magnetic field (Tc and Bc, respectively) were measured for a large number of superconductors.  Researchers found that if the critical values of temperature or field are exceeded, the material loses its superconductive properties.  Additionally, scientists have identified not one, but two values of the critical magnetic field in some materials (as Abrikosov had previously foreseen).  Thus, conventionally, superconductors were divided into two groups: type I and type II.  Type I superconductors are substances that exist in a superconducting or normal state (Fig. 4, top chart).  The second class superconductors are substances that can be in one of three states: superconducting, mixed, or normal (see Fig 4, lower chart).

Superconductivity: an old mystery for new physics

Phase state diagram of type I and type II superconductors

The mixed state implies the coexistence of superconductivity and non-superconducting thin threads (or the so-called 'Abrikosov vortices') penetrating the substance. This feature of type II superconductors allows them to support greater superconducting currents and magnetic fields compared with type I superconductors, and their application therefore is more promising. The classification of substances according to their superconducting properties is not absolute. Any type I superconductor can be turned into a type II superconductor if defects are created in its crystal lattice.

The critical temperature for all known superconductors up to the mid-1980s did not exceed a few Kelvin.  This condition severely limited industrial use of superconducting materials, since liquid helium, which has a boiling point of 4.1 K, was required for cooling, and its liquefaction is a laborious and expensive process.  The search for superconducting materials with a higher critical temperature has become an important task for researchers.  The first successes in this field were made in 1986.  German and Swiss scientists Bednorz and Müller discovered that a barium-doped compound of lanthanum and copper oxide superconducts at 35 K.  Replacing lanthanum with yttrium made it possible to raise the critical temperature to around 100 K (-173°C).  Thanks to these discoveries, the search for superconducting substances has fundamentally changed, as previously the search was mainly limited to metals and their alloys.  Later, superconducting properties were found in substances that normally do not conduct electric current, in other words, insulators.  This was the beginning of a whole class of so-called high-temperature superconductors: YBCO or 'ceramics'.  Finally, the 'helium barrier' was broken: it has now become possible to use cheaper and more accessible liquid nitrogen with a boiling point of 77 K (-196°С) to achieve superconductivity.

All these events have pushed scientists to search further for high-temperature superconductors, and materials with a superconducting transition temperature of 165 K (-108°С) are now known.  Hydrogen sulphide was previously considered a record for critical temperature: at high pressure, its critical temperature is 203 K (a 'mere' -70°С).  More recently, in December 2018, an international team of scientists declared superconductivity of lanthanum hydride at a temperature of 250-252 K (-21°С) and enormous pressure - about 170 GPa.  According to theoretical calculations, metallised hydrogen will have a critical temperature of up to 400 K (127°С).  However, today there is no practical prospect of creating such high pressures in order to conduct such an experiment.

Since the discovery of superconductivity and up to now, huge experimental 'baggage' has been packed: a large number of 'pure' superconductors and their alloys have been discovered.  High-temperature superconductivity has been found in ferropnicides (compounds of iron with arsenic and other non-metals), in carbon nanotubes, and in many other materials.

As for the practical applications of superconducting materials, they are increasingly being used to construct turbo-generators, electric motors, rigid and flexible cables, switching and current-limiting devices, magnetic separators, and transport systems.  For example, high-temperature YBCO superconductors are used in the Large Hadron Collider (LHC) and the planned international thermonuclear reactor (ITER) to create extremely powerful magnetic fields, and the combination of semiconductors and superconductors opens up new possibilities in computer design, the 'quantum computer'.  Recently invented MRI scanners use superconducting magnets to help diagnose various diseases.  Today, around the world (including in Russia), active work is already underway on producing and laying high-temperature superconducting power cables, capable of transmitting electricity with virtually no energy loss.

Thus, the contribution of superconductivity to technological progress and in everyday human life cannot be overestimated.  The discovery of superconductors with a critical temperature close to normal conditions will further change our surroundings and many things we are accustomed to.  However, a unified theory explaining the nature of superconductivity has not yet been formulated.  Scientists around the world are still devising one.