Superconductivity

Superconductivity . Intrinsic ability of certain materials to conduct electric current with resistance and energy loss close to zero under certain conditions. Superconductivity is a phase of certain materials that normally occurs at low temperatures . Although cooling is not enough, it is also necessary not to exceed a critical current or a critical magnetic field to maintain the superconducting state. This property was discovered in 1911 by the Dutch physicist Heike Kamerlingh Onnes , when he observed that the electrical resistance of mercury disappeared when it was cooled to 4 Kelvin (-269 ° C).

Summary

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  • 1 History
  • 2 Description
  • 3 Theory
  • 4 Behaviors
    • 1 Electrical Behavior
    • 2 Magnetic Behavior
  • 5 Application
  • 6 Sources

History

Superconductivity was discovered in 1911 by the Dutch physicist Heike Kamerlingh Onnes, who observed that mercury had no electrical resistance below 4.2 K (-269 ° C). The phenomenon was not understood until, in 1933 , the Germans Karl W. Meissner and R. Ochsenfeld detected a marked diamagnetism in a superconductor. However, the physical principles of superconductivity were not understood until 1957 , when American physicists John Bardeen , Leon N. Cooper, and John R. Schrieffer proposed a theory that is now known as BCS theory by the initials of their surnames, and by which its authors received the Nobel Prize in Physicsin 1972 .

BCS theory describes superconductivity as a quantum phenomenon, in which the conduction electrons move in pairs, showing no electrical resistance. This theory satisfactorily explained superconduction at high temperatures in metals, but not in ceramic materials. In 1962 , British physicist Brian Josephson studied the quantum nature of superconductivity and predicted the existence of oscillations in the electric current that flows through two superconductors separated by a thin insulating layer in an electric or magnetic field. This phenomenon, known as the Josephson effect, was later experimentally confirmed

Description

Superconductivity is a phenomenon that some conductors present that do not offer resistance to the flow of electric current. Superconductors also present a marked diamagnetism , that is, they are repelled by magnetic fields. Superconductivity only manifests below a certain critical temperature Tc and a critical magnetic field Hc, which depend on the material used. Before 1986 , the highest known Tc value was 23.2 K (-249.95 ° C), in certain niobium – germanium compounds . Liquid helium , an expensive and ineffective refrigerant, was used to achieve such low temperatures .

The need for such low temperatures greatly limits the overall efficiency of a machine with superconducting elements, so large-scale operation of these machines was not considered practical. However, in 1986, the discoveries made in various universities and research centers began to radically change the situation. Some lanthanide-containing metal oxide ceramic compounds were found to be superconducting at temperatures high enough to allow liquid nitrogen to be used as a coolant.

Because liquid nitrogen , whose temperature is 77 K (-196 ° C), cools 20 times more efficiently than liquid helium and costs 10 times less, many potential applications began to appear economically viable. In 1987 it was revealed that the formula for one of these superconducting compounds, with a Tc of 94 K (-179 ° C), was (Y0.6Ba0.4) 2CuO4. Since then it has been shown that lanthanides are not an essential component, since in 1988 a copper and thallium – barium – calcium oxide with a Tc of 125 K (-148 ° C) was discovered .

Theory

The most accepted microscopic theory to explain superconductors is called BCS Theory. Superconductivity can be explained as an application of the Bose-Einstein Condensate . The problem is that electrons are fermions, so this theory cannot be applied directly to them. Precisely the idea of ​​the BCS Theory is that the electrons mate, forming a pair of fermions that behaves like a boson. This pair is called Cooper pair and its bond is justified in the interactions of the electrons with each other mediated by the crystal structure of the material.

Behaviors

Electrical Behavior

The appearance of superdiamagnetism is due to the material’s ability to create supercurrents. These are electron currents that do not dissipate energy, so that they can be maintained eternally without obeying the Joule Effect of energy loss due to heat generation. The currents create the intense magnetic field necessary to support the Meissner effect . These same currents allow energy to be transmitted without energy expenditure, which represents the most spectacular effect of this type of material.

Because the amount of superconducting electrons is finite, the amount of current the material can withstand is limited. Therefore, there is a critical current from which the material ceases to be superconducting and begins to dissipate energy. In type II superconductors, the appearance of fluxons causes that, even for currents below critical, a certain dissipation of energy is detected due to the collision of the vortices with the atoms of the network.

Magnetic Behavior

Although the most outstanding property of superconductors is the absence of resistance, the truth is that we cannot say that it is a material with infinite conductivity, since this type of material by itself does not make thermodynamic sense. Actually a superconducting material is a perfect diamagnetic. This prevents it from penetrating the field, which is known as the Meissner effect.

There are two types of superconductors. Type I batteries do not allow an external magnetic field to penetrate at all. This involves a high energy effort, with which the majority of real materials are transformed into the second type. Type IIs are imperfect superconductors, in the sense that the field actually penetrates through small pipes called Abrikosov vortices, or fluxons. These two types of superconductors are in fact two different phases that were predicted by Lev Davidovich Landau and Aleksey Alekséyevich Abrikósov .

When we apply a weak external magnetic field to a superconductor, it repels it perfectly. If we increase it, the system becomes unstable and prefers to introduce vortices to decrease its energy. These are increasing in number by placing themselves in vortex networks that can be observed using appropriate techniques. When the field is high enough, the number of defects is so high that the material is no longer superconducting. This is the critical field that makes a material stop being superconducting and that depends on temperature.

Application

Due to their lack of resistance, superconductors have been used to manufacture electromagnets that generate intense magnetic fields without loss of energy. Superconducting magnets have been used in material studies and in the construction of powerful particle accelerators. Taking advantage of the quantum effects of superconductivity, devices have been developed that measure electric current, voltage and the magnetic field with unprecedented sensitivity.

The discovery of better semiconductor compounds is a significant step towards a broader range of applications, including faster computers with greater memory capacity, nuclear fusion reactors in which plasma is kept confined by magnetic fields, magnetic levitation trains of high speed and, perhaps most importantly, more efficient generation and transmission of electrical energy . The 1987 Nobel Prize in Physics was awarded to the German physicist J. Georg Bednorz and the Swiss physicist K. Alex Mueller for their work on superconductivity at high temperatures.

 

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