In 1908, Dutch physicist Heiko Onnes first discovered a method of converting helium into liquid helium. This is a remarkable achievement, because helium only liquefies at 4 degrees above absolute zero, which is minus 269 degrees Celsius. Later, he cooled a mercury sample to this temperature and energized him, and to his shock, he found that it had no resistance, which meant no energy loss. This is a very unusual phenomenon because, usually, at least some energy is lost as an electric current passes through the material. Recognizing the importance of this phenomenon, he called this new state of matter a superconductor, for which he won the 1913 Nobel Prize in Physics.
In general, when an electric current passes through a material, there will always be a resistance, because the collision of electrons with atoms will cause some energy loss. But somehow, in this new state of superconductivity, electrons pass directly through the material as if there were no atoms blocking their way. In fact, if you put an electric current in a superconducting coil, the current will flow continuously almost forever without increasing the voltage or energy. Superconductors also have a seemingly magical property, that is, they can discharge magnetic fields. So if you put a magnet on a superconductor, the magnet will levitate.
How can superconducting materials transmit current perfectly without losing energy? To answer this question, we have to go deep into the foundations of the subatomics, which means we have to invoke quantum mechanics. What is superconductivity? Why is it so special, and how does quantum mechanics explain it?
The Meissner effect
At the beginning of the 20th century, the idea that materials reach low resistance at very cold temperatures was widely accepted, but what people did not understand was what happened to the resistance when it was close to absolute zero. Kelvin believed that electrons would stop completely, so the resistance would become infinite. Therefore, when it was first discovered that the resistance of a material could go to zero at very low temperatures, this was unexpected. In 1911, Onnes was the first to discover this in mercury and found it to be superconducting at a temperature of 4.2 Kelvin. Later, it was discovered that other metals and alloys could superconduct at higher temperatures. However, the typical temperature is still very cold, usually below 150 Kelvin.
In 1933, Walter Meissner and Robert Oxenfeld made another major discovery. They found that when a metal cools in a small magnetic field, magnetic flux is spontaneously excluded as the metal becomes superconducting, now known as the Meissner effect. Usually, a substance allows a magnetic field to pass through it. However, one property of superconductivity is that superconducting materials emit a magnetic flux field, in other words, a magnetic field cannot pass through it. Therefore, the magnetic field of the magnet lifts the material so that the magnetic flux can flow smoothly to another magnetic pole, which is the cause of suspension.
Even after this discovery, it is still not known what the exact cause of superconductivity is. It's been 46 years since superconductivity was discovered that we have the first truly microscopic theory to describe what happens with superconductivity. In 1957, John Bardeen, Leon Cooper, and John Schliefer came up with the theory now known as BCS, which won the Nobel Prize in Physics in 1972. What did they find?
The cause of the resistance
In order to understand how electrons flow without resistance in superconductors, we first need to understand what causes resistance. Inside metals, the outermost electrons furthest from the nucleus can move freely, so much so that the metal can be seen as a stack of atoms surrounded by a sea of electrons, and electrons are able to flow in a fluid-like manner. If we energize on one side of the metal, they can easily accept these new electrons and push out some electrons on the other side to make room, which we interpret as an electric current.
But the flow of electrons is not perfect. As electrons move through the material, atoms block their way, and if the atoms are completely stationary, electrons can pass through the material more easily. But this is usually not the case, the atoms vibrate, or there are defects in the lattice, and the electron collides with the atom that may be vibrating. This causes the electrons to scatter, eventually releasing some of their energy to the atoms, causing them to vibrate more violently. This increased vibration causes the entire lattice to vibrate more, and this higher vibration causes the metal to heat up, which is why resistance causes energy loss.
As the temperature increases, the vibrations of the atoms become more intense, which leads to collisions of higher energies and higher resistance. This vibration that causes electron scattering can be reduced by lowering the temperature of the metal. But the vibration of atoms cannot be stopped completely, because Heisenberg's uncertainty principle is limited, so how does the resistance completely disappear?
Fermions and bosons
To understand this, let's first revisit the concepts of fermions and bosons. Particles have a characteristic related to momentum called spin, and spin does not refer to physical rotation, but to the intrinsic properties of particles. These spin values are multiples of Planck's constant: it is either an integer multiple or a half integer multiple. Particles with half-integer spins are called fermions, and particles with integer spins are called bosons. For example, an electron electron can have spins of +1/2 or -1/2, so it is a fermion; a photon can have spins of +1 or -1, so it is a boson.
Bosons and fermions behave differently at the subatomic level. In a quantum system, any number of identical bosons can occupy the same energy level, but this is not the case with fermions, where two or more identical Fermi particles cannot occupy the same energy level, which is known as the Pauli incompatibility principle. Simply put, the same fermions cannot be stacked together, while bosons do not have this limitation, but instead prefer to pile together at low temperatures.
Superconductivity: Cooper pairs
When an electron moves through a conductor, it is repelled by other electrons, but it also attracts the positive ions that make up the rigid lattice of metals. This attraction distorts the ion lattice, causing the ions to move slightly toward the electron, increasing the positive charge density near the lattice. This positive charge density can attract other electrons at long distances, and due to the shifting of ions, this attraction can overcome the repulsion of electrons and cause them to bind in pairs. The two electrons are joined together in this way, called Cooper pairs.
If the temperature of the material is low enough, the Cooper pair will stay together because it doesn't have enough energy to split. We can then treat this combination as individual particles. When two electrons join together in this way, their semi-spins together form an integer spin. In other words, they begin to behave like bosons, and they are no longer limited by the Pauli incompatibility principle.
Now that the situation is that since any number of bosons can enter the same low-energy state, the set of Cooper pairs begins to behave like a single entity. When a beam of bosons is cooled to a low temperature occupying the lowest quantum ground state, it is called a Bose-Einstein condensate. They are like a boson electron, all in the same low-energy state. It is negatively charged because it is made up of negatively charged electrons, so that means it can conduct electricity.
Normally, when an electron collides with an atom and scatters, it loses some energy as a result of the collision. But for Cooper pairs, they don't have lower energies, because they're already in the lowest energy state, so they can't lose any more energy. The lack of interaction between the Cooper pair and the atoms effectively causes electrons to flow without resistance, and the material becomes a superconductor. The interaction of electrons in Cooper's pair is very weak, so superconductivity usually only occurs at very low temperatures. When the temperature exceeds the critical temperature, the Cooper pair is destroyed because there is already enough energy to break them down, so superconductivity is lost.
The mechanism described above is Cooper's general understanding of how to form, but there may be other mechanisms we don't yet understand.
The reproduced content represents the views of the author only
Does not represent the position of the Institute of Physics, Chinese Academy of Sciences
Source: Vientiane Experience
Edit: Paathurnax