What is the phenomenon of superconductivity? Superconductivity is a phenomenon with zero electrical resistance and emission of magnetic flux fields that occur in certain materials, called superconductors, when cooled below a characteristic critical temperature.
The phenomenon was discovered by the Dutch physicist Heike Kamerling-Onnes on April 8, 1911 in Leiden. Like ferromagnetism and atomic spectral lines, superconductivity is a quantum-mechanical phenomenon. It is characterized by the Meissner effect — the complete ejection of magnetic field lines from within the superconductor upon its transition to the superconducting state.
This is the essence of the phenomenon of superconductivity. The occurrence of the Meissner effect indicates that superconductivity cannot be understood simply as the idealization of ideal conductivity in classical physics.
What is the phenomenon of superconductivity
The electrical resistance of a metal conductor gradually decreases with decreasing temperature. In conventional conductors, such as copper or silver, this reduction is limited by impurities and other defects. Even near absolute zero, a real sample of a normal conductor shows some resistance. In a superconductor, resistance drops sharply to zero when the material cools below its critical temperature. Electric current through the loop of the superconducting wire can be stored indefinitely without a power source. This is the answer to the question of what is the phenomenon of superconductivity.
History
In 1911, studying the properties of a substance at very low temperatures, the Dutch physicist Heike Kamerling Onnes and his team found that the electrical resistance of mercury drops to zero below 4.2 K (-269 ° C). This was the very first observation of the phenomenon of superconductivity. Most chemical elements become superconducting at a fairly low temperature.
Below a certain critical temperature, the materials go into a superconducting state, characterized by two main properties: firstly, they do not resist the passage of electric current. When the resistance drops to zero, current can circulate inside the material without dissipating energy.
Secondly, provided that they are rather weak, external magnetic fields do not penetrate the superconductor, but remain on its surface. This phenomenon of field expulsion became known as the Meissner effect after the physicist first observed it in 1933.
Three names, three letters and incomplete theory
Ordinary physics does not provide an adequate explanation of the superconducting state, nor does the elementary quantum theory of the solid state, which considers the behavior of electrons separately from the behavior of ions in the crystal lattice.
Only in 1957 did three American researchers, John Bardin, Leon Cooper, and John Schriffer, create the microscopic theory of superconductivity. According to their BCS theory, electrons are grouped in pairs by interacting with lattice vibrations (the so-called "phonons"), thus forming Cooper pairs that move inside a solid without friction. A solid can be considered as a lattice of positive ions immersed in a cloud of electrons. When an electron passes through this lattice, the ions move slightly, being attracted by the negative charge of the electron. This movement generates an electrically positive region, which, in turn, attracts another electron.
The electron interaction energy is rather weak, and the vapors can be easily broken by thermal energy - therefore, superconductivity usually occurs at a very low temperature. Nevertheless, the BCS theory does not explain the existence of high-temperature superconductors at a temperature of about 80 K (-193 ° C) and higher, for which other electron coupling mechanisms must be used. The application of the phenomenon of superconductivity is based on the above process.
Temperature
In 1986, it was discovered that some cuprate-perovskite ceramic materials have a critical temperature above 90 K (-183 ° C). Such a high transition temperature is theoretically impossible for a conventional superconductor, which leads to the fact that the materials are called high-temperature superconductors. The available cooling liquid nitrogen boils at 77 K, and thus superconductivity at higher temperatures than these facilitates many experiments and applications that are less practical at lower temperatures. This is the answer to the question at what temperature the phenomenon of superconductivity occurs.
Classification
Superconductors can be classified according to several criteria, which depend on our interest in their physical properties, on the understanding that we have about them, on how expensive their cooling is, or on the material from which they are made.
According to its magnetic properties
Type I superconductors: those that have only one critical field, Hc, and abruptly transfer from one state to another when it is reached.
Type II superconductors: having two critical fields, Hc1 and Hc2, being perfect superconductors under the lower critical field (Hc1) and completely emerging from the superconducting state above the upper critical field (Hc2), in a mixed state between critical fields.
By the understanding that we have about them
Conventional superconductors: those that can be fully explained by BCS or related theories.
Unconventional superconductors: those that could not be explained using such theories, for example: heavy fermionic superconductors.
This criterion is important because the BCS theory has been explaining the properties of ordinary superconductors since 1957, but, on the other hand, there was no satisfactory theory for explaining completely unconventional superconductors. In most cases, type I superconductors are common, but there are a few exceptions, such as niobium, which is both ordinary and type II.
By their critical temperature
Low temperature superconductors, or LTS: those whose critical temperature is below 30 K.
High-temperature superconductors, or HTSC: those whose critical temperature is higher than 30 K. Some now use 77 K as a separation to emphasize whether we can cool the sample with liquid nitrogen (whose boiling point is 77 K), which is much more feasible than liquid helium (an alternative to achieving the temperatures necessary to obtain low temperatures superconductors).
Other nuances
The superconductor can be of type I, which means that it has a single critical field, above which all superconductivity is lost, and below which the magnetic field is completely excluded from the superconductor. Type II, meaning that it has two critical fields between which it allows partial penetration of the magnetic field through isolated points. These points are called vortices. In addition, in multicomponent superconductors, a combination of two behaviors is possible. In this case, the superconductor is of type 1.5.
The properties
Most of the physical properties of superconductors vary from material to material, such as specific heat and critical temperature, critical field and critical current density, at which superconductivity is destroyed.
On the other hand, there is a class of properties that are independent of the base material. For example, all superconductors have absolutely zero resistivity at low applied currents when there is no magnetic field or if the applied field does not exceed a critical value.
The presence of these universal properties implies that superconductivity is a thermodynamic phase and, therefore, has certain distinctive properties, which are largely independent of microscopic details.
The situation is different in superconductor. In a conventional superconductor, an electron liquid cannot be separated into individual electrons. Instead, it consists of bound electron pairs known as Cooper pairs. This pairing is caused by the force of attraction between the electrons as a result of the phonon exchange. Due to quantum mechanics, the energy spectrum of this Cooper pair liquid has an energy gap, that is, there is a minimum amount of energy ΔE that must be supplied to excite the liquid.
Therefore, if ΔE is greater than the thermal energy of the lattice given by kT, where k is the Boltzmann constant and T is the temperature, the liquid will not be scattered by the lattice. Thus, the liquid of the Cooper pair is superfluid, which means that it can flow without dissipating energy.
Superconductivity Characteristics
In superconducting materials, superconductivity characteristics appear when the temperature T drops below the critical temperature Tc. The value of this critical temperature varies from material to material. Conventional superconductors typically have critical temperatures ranging from about 20 K to less than 1 K.
For example, solid mercury has a critical temperature of 4.2 K. As of 2015, the highest critical temperature found for a conventional superconductor is 203 K for H2S, although a high pressure of about 90 gigapascals was required. Cuprate superconductors can have much higher critical temperatures: YBa2Cu3O7, one of the first cuprate superconductors discovered, has a critical temperature of 92 K, and mercury-based cuprates with critical temperatures in excess of 130 K have been found. The explanation for these high critical temperatures remains unknown.
Electron pairing due to phonon exchanges explains superconductivity in conventional superconductors, but does not explain superconductivity in newer superconductors, which have a very high critical temperature.
Magnetic fields
Similarly, at a fixed temperature below a critical temperature, superconducting materials cease to superconduct when an external magnetic field is applied that is greater than the critical magnetic field. This is because the Gibbs free energy of the superconducting phase increases quadratically with the magnetic field, while the free energy of the normal phase is approximately independent of the magnetic field.
If the material is superconducting in the absence of a field, then the free energy of the superconducting phase is less than that of the normal phase, and therefore, for some finite value of the magnetic field (proportional to the square root of the difference of free energies at zero), the two free energies will be equal, and a phase transition to normal phase. In a more general sense, a higher temperature and a stronger magnetic field lead to a decrease in the proportion of superconducting electrons and, consequently, to a greater depth of penetration of external magnetic fields and currents into London. The penetration depth becomes infinite during the phase transition.
Physical aspect
The onset of superconductivity is accompanied by sharp changes in various physical properties, which is a hallmark of the phase transition. For example, the electronic heat capacity is proportional to the temperature in the normal (not superconducting) mode. At the superconducting transition, he experiences a jump-like jump and then ceases to be linear. At low temperatures, it changes instead of e − α / T for some constant α. This exponential behavior is one evidence of the existence of an energy gap.
Phase transition
The explanation of the phenomenon of superconductivity is quite obvious. The order of the superconducting phase transition has been discussed for a long time. Experiments show that there is no second-order transition, that is, latent heat. However, there is latent heat in the presence of an external magnetic field because the superconducting phase has lower entropy, lower critical temperature than the normal phase.
The following has been experimentally demonstrated: when the magnetic field increases and goes beyond the critical field, the resulting phase transition leads to a decrease in the temperature of the superconducting material. The phenomenon of superconductivity has been briefly described above, now is the time to tell you something about the nuances of this important effect.
Calculations carried out in the 1970s showed that, in fact, it can be weaker than the first order due to the influence of long-range fluctuations in the electromagnetic field. In the 1980s, it was theoretically shown using disorder field theory, in which the superconductor vortex lines play the main role, that the transition is of the second order in type II mode and the first order (i.e. latent heat) in type I mode, and that two areas are separated by a tricritical point.
The results were strongly confirmed by computer simulations in Monte Carlo. This played a large role in the study of the phenomenon of superconductivity. Work continues at the present time. The essence of the phenomenon of superconductivity is not fully understood and explained from the point of view of modern science.