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The quantum mechanics of superconductivity

The quantum mechanics of superconductivity

In 1908, the Dutch physicist Heike Onnes discovered how to transform gaseous helium into liquefied helium for the first time. will levitate.

In 1933, Walther Meissner and Robert Ochsenfeld discovered that when a metal is cooled in a small magnetic field, flux is spontaneously excluded when the metal becomes superconducting. This is now known as the Meissner effect. The superconducting material expels the magnetic flux fields. Since magnetic fields cannot pass through it, a magnet above a superconducting magnet lifts up so the flux flows to the opposite pole. This is what causes the levitation.

In 1957, John Bardeen, Leon Cooper, and John Robert Schrieffer proposed what is now known as the BCS theory.
Understand how electrons can flow without resistance in superconductors. We must first understand what causes the resistance in the first place. Inside a metal, the outermost electrons of the valence shell, being furthest from the nucleus, are so free to move that a sample of metal can be treated as a group of atoms surrounded by ‘a sea of ​​electrons.

But as the electrons pass through the material, the atoms, which now have slightly positive charges because they gave up an electron in their outer shell, keep them from colliding with atoms that are likely to vibrate. This causes the electron to scatter and ends up giving up some of its energy to the atom, causing it to vibrate a bit more. This additional vibration makes the whole network vibrate a little more. This higher vibration causes the metal to heat up and explains the loss of energy due to resistance.

To understand how resistance disappears, I have to tell you about fermions and bosons. A particle with an entire half-spin is called a Fermion, named after Italian scientist Enrico Fermi. A particle that has an integer spin is called a boson, named after Indian scientist Satyendra Bose.

An electron can have a spin of +1/2 or -1/2, so it is a fermion. A photon can have a spin of +1 or -1, so it is a boson. It turns out that while any number of identical bosons can occupy the lowest energy level in a quantum system, two or more identical fermions cannot occupy the same energy level in a quantum system. This is called the Pauli exclusion principle. This is why you can only have 2 electrons in a given orbital of an atom.

However, bosons are different. They are not subject to this restriction and can be packed together when there are several, especially at low temperatures.

When an electron moves through a conductor, it is repelled from other electrons due to their mutual negative charge, but it also attracts the positive ions that make up the rigid lattice of the metal. This attraction distorts the ionic lattice, moving the ions slightly towards the electron, increasing the positive charge density of the nearby lattice. This positive charge density attracts other electrons. Over long distances, this attraction between electrons due to the displaced ions can overcome the repulsion of the electrons and cause them to join together. These two electrons form a combination called a Cooper pair.

When two electrons meet in this way, their half-spins interact in such a way that together they form an entire spin. They start to behave like bosons instead of fermions. They are no longer subject to the Pauli exclusion principle, so now they occupy the lowest possible energy level.

The collection of Cooper pairs begins to act as an entity or a unit – this is called a Bose-Einstein condensate. So, all Cooper pairs act together on a giant electron. There is no resistance because the condensate cannot go to a lower energy state, so no interaction with the atoms in the lattice occurs.
# superconductivity #cooperpairs
The interaction of electrons in a Cooper pairing is very weak, so it usually only occurs at very low temperatures. When the temperature exceeds the critical temperature, the Cooper pairs are disturbed because there is enough energy to break them, and thus the superconductivity is lost. It is the formation of Cooper pairs by their interaction with the phonons of the material that are at the origin of superconductivity.

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