FERMIONIC CONDENSATE: PROPERTIES, APPLICATIONS AND EXAMPLES - PHYSICAL - 2024

A Fermi condensate is, in the strictest sense, a very dilute gas made up of fermionic atoms that have been subjected to a temperature close to absolute zero. In this way, and under suitable conditions, they pass into a superfluid phase, forming a new state of aggregation of matter.

The first fermionic condensate was obtained on December 16, 2003 in the United States, thanks to a team of physicists from various universities and institutions. The experiment used about 500 thousand potassium-40 atoms subjected to a variable magnetic field and a temperature of 5 x 10 ^-8 Kelvin.

Superconducting magnet. Source: pixabay

That temperature is considered close to absolute zero and is vastly lower than the temperature of intergalactic space, which is about 3 Kelvin. The absolute zero of temperature is understood to be 0 Kelvin, which is equivalent to -273.15 degrees Celsius. So 3 Kelvin corresponds to -270.15 degrees Celsius.

Some scientists consider fermionic condensate to be the sex state of matter. The first four states are most familiar to everyone: solid, liquid, gas, and plasma.

Previously, a fifth state of matter had been obtained when a condensate of bosonic atoms was achieved. This first condensate was created in 1995 from a very dilute rubidium-87 gas cooled to 17 x 10 ^-8 Kelvin.

The importance of low temperatures

Atoms behave very differently at temperatures close to absolute zero, depending on the value of their intrinsic angular momentum, or spin.

This divides particles and atoms into two categories:

- The bosons, which are those with integer spin (1, 2, 3,…)

- Fermions, which are those with semi-integer spin (1/2, 3/2, 5/2,…).

Bosons do not have any restrictions, in the sense that two or more of them can occupy the same quantum state.

On the other hand, fermions fulfill the Pauli exclusion principle: two or more fermions cannot occupy the same quantum state, or in other words: there can only be one fermion per quantum state.

This fundamental difference between bosons and fermions makes fermionic condensates more difficult to obtain than bosonic ones.

For fermions to occupy all the lower quantum levels, it is necessary that they previously align in pairs, to form the so-called “Cooper pairs” that have bosonic behavior.

History, foundations and properties

Back in 1911, when Heike Kamerlingh Onnes was studying the resistance of mercury subjected to very low temperatures using liquid helium as a coolant, he found that upon reaching the temperature of 4.2 K (-268.9 Celsius) the resistance dropped abruptly to zero..

The first superconductor had been found in an unexpected way.

Without knowing it, HK Onnes had managed to put the conduction electrons all together at the lowest quantum level, a fact that in principle is not possible because the electrons are fermions.

It had been achieved that the electrons passed to the superfluid phase inside the metal, but since they have an electrical charge, they cause a flow of electrical charge with zero viscosity and consequently zero electrical resistance.

HK Onnes himself in Leiden, the Netherlands had found that the helium that he used as a refrigerant became superfluid when the temperature of 2.2 K (-270.9 Celsius) was reached.

Unknowingly, HK Onnes had for the first time succeeded in bringing together the helium atoms with which he cooled the mercury at their lowest quantum level. In passing, he also realized that when the temperature was below a certain critical temperature, the helium passed into the superfluid phase (zero viscosity).

The theory of superconductivity

Helium-4 is a boson and behaves as such, that is why it was possible to go from the normal liquid phase to the superfluid phase.

However neither of these is considered a fermionic or bosonic condensate. In the case of superconductivity, fermions, like electrons, were within the crystal lattice of mercury; and in the case of superfluid helium, it had passed from the liquid phase to the superfluid phase.

The theoretical explanation for superconductivity came later. It is the well-known BCS theory developed in 1957.

The theory states that electrons interact with the crystal lattice forming pairs that instead of repel each other, attract each other, forming "Cooper pairs" that act as bosons. In this way, the electrons as a whole can occupy the lowest energy quantum states, as long as the temperature is low enough.

How to produce a fermion condensate?

A legitimate fermion or boson condensate must start from a very dilute gas made up of fermionic or bosonic atoms, which is cooled in such a way that its particles all go to the lowest quantum states.

As this is much more complicated than getting a boson condensate, it is only recently that these types of condensates have been created.

Fermions are particles or conglomerates of particles with half whole spin. The electron, proton, and neutron are all ½ spin particles.

The nucleus of helium-3 (two protons and one neutron) behaves like a fermion. The neutral atom of potassium-40 has 19 protons + 21 neutrons + 19 electrons, which add up to the odd number 59, so it behaves as a fermion.

Mediator particles

The mediating particles of the interactions are bosons. Among these particles we can name the following:

- Photons (mediators of electromagnetism).

- Gluon (mediators of strong nuclear interaction).

- Bosons Z and W (mediators of weak nuclear interaction).

- Graviton (mediators of gravitational interaction).

Compound bosons

Among the compound bosons are the following:

- Deuterium nucleus (1 proton and 1 neutron).

- Helium-4 atom (2 protons + 2 neutrons + 2 electrons).

Whenever the sum of protons, neutrons and electrons of a neutral atom results in an integer the behavior will be boson.

How a fermionic condensate was obtained

A year before achieving the fermion condensate, the formation of molecules with fermionic atoms that formed tightly coupled pairs that behaved like bosons had been achieved. However this is not considered a pure fermionic condensate, but rather resembles a bosonic condensate.

But what was accomplished on December 16, 2003, by the team of Deborah Jin, Markus Greiner, and Cindy Regal from the JILA laboratory in Boulder, Colorado, was the formation of a condensate of pairs of individual fermionic atoms in a gas.

In this case the pair of atoms do not form a molecule, but move together in a correlated way. Thus, as a whole, the pair of fermionic atoms acts as a boson, hence their condensation has been achieved.

To achieve this condensation, the JILA team started from a gas with potassium-40 atoms (which are fermions), which was confined in an optical trap at 300 nanokelvin.

The gas was then subjected to an oscillating magnetic field to alter the repulsive interaction between atoms and turn it into an attractive one, through a phenomenon known as "Fesbach resonance."

Adjusting the parameters of the magnetic field appropriately makes it possible for the atoms to form Cooper pairs instead of molecules. Then it continues cooling to obtain the fermionic condensate.

Applications and examples

The technology developed to achieve fermionic condensates, in which atoms are practically manipulated almost individually, will allow the development of quantum computing, among other technologies.

It will also improve the understanding of phenomena such as superconductivity and superfluidity, allowing new materials with special properties. Furthermore, it has been discovered that there is an intermediate point between the superfluidity of the molecules and the conventional one through the formation of Cooper pairs.

The manipulation of ultracold atoms will allow us to understand the difference between these two ways of producing superfluids, which will surely result in the development of high-temperature superconductivity.

In fact, today there are superconductors that although they do not work at room temperature, they do work at temperatures of liquid nitrogen, which is relatively cheap and easy to obtain.

Extending the concept of fermionic condensates beyond the atomic fermion gases, numerous examples can be found where fermions collectively occupy low-energy quantum levels.

The first as already said are the electrons in a superconductor. These are fermions that align in pairs to occupy the lowest quantum levels at low temperatures, exhibiting collective bosonic-like behavior and reducing viscosity and resistance to zero.

Another example of fermionic grouping in low-energy states is quark condensates. Also the helium-3 atom is a fermion, but at low temperatures it forms Cooper pairs of two atoms that behave like bosons and exhibit superfluid behavior.

References

1. K Goral and K Burnett. Fermionic first for condensates. Recovered from: physicsworld.com
2. M Grainer, C Regal, D Jin. Fermi condensates. Retrieved from: users.physics.harvard.edu
3. P Rodgers and B Dumé. Fermions condensate makes its debut. Recovered from: physicsworld.com.
4. Wikiwand. Fermionic condensate. Recovered from Wikiwand.com
5. Wikiwand. Fermionic condensate. Recovered from Wikiwand.com