The elusive excitony, the existence of which could not be experimentally proven for almost half a century, finally showed itself to researchers. This is reported in an article that a research team led by Peter Abbamonte published in the journal Science.
Let us recall this in a nutshell. It is convenient to describe the motion of electrons in a semiconductor using the concept of a hole - a place where an electron is missing. The hole, of course, is not a particle such as an electron or a proton. However, it behaves like a particle in many ways. For example, you can describe its movement and consider that it carries a positive electric charge. Therefore, objects such as a hole are called quasiparticles by physicists.
There are other quasiparticles in quantum mechanics. For example, a Cooper pair: a duet of electrons moving as a whole. There is also an exciton quasiparticle, which is a pair of an electron and a hole.
Excitons were theoretically predicted in the 1930s. Much later they were discovered experimentally. However, never before has a state of matter known as exciton been observed.
Let us explain what we are talking about. Both real particles and quasiparticles are divided into two large classes: fermions and bosons. The former include, for example, protons, electrons and neutrons, the latter - photons.
Fermions obey a physical law known as the Pauli exclusion principle: two fermions in the same quantum system (for example, two electrons in an atom) cannot be in the same state. By the way, it is thanks to this law that the electrons in the atom occupy different orbitals, and are not gathered by the whole crowd at the most "convenient" lower energy level. So it is precisely because of the Pauli principle that the chemical properties of the elements of the periodic table are as we know them.
Pauli's ban does not apply to bosons. Therefore, if it is possible to create a single quantum system from many bosons (as a rule, this requires an extremely low temperature), then the whole company happily accumulates in the state with the lowest energy.
Such a system is sometimes called a Bose condensate. Its special case is the famous Bose-Einstein condensate, where whole atoms act as bosons (we also wrote about this remarkable phenomenon). For his experimental discovery, the 2001 Nobel Prize in Physics was awarded.
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The already mentioned quasiparticle of two electrons (Cooper pair) is not a fermion, but a boson. The massive formation of such pairs leads to such a remarkable phenomenon as superconductivity. The unification of fermions into a quasiparticle-boson owes its appearance to superfluidity in helium-3.
Physicists have long dreamed of obtaining such a Bose condensate in a three-dimensional crystal (and not in a thin film), when electrons massively combine with holes to form excitons. After all, excitons are also bosons. It is this state of matter that is called excitony.
It is extremely interesting for scientists, like any state in which macroscopic volumes of matter exhibit exotic properties that can only be explained using quantum mechanics. However, it has not been possible to obtain this state experimentally so far. Rather, it was not possible to prove that it was received.
The fact is that in terms of those parameters that amenable to investigation using existing techniques (for example, the structure of a superlattice), excitonies are indistinguishable from another state of matter, known as the Peierls phase. Therefore, scientists could not say with certainty which of the two conditions they managed to obtain.
This problem was solved by the Abbamonte group. The researchers have perfected an experimental technique known as electron energy-loss spectroscopy (EELS).
In the course of this kind of research, physicists bombard matter with electrons, the energy of which lies in a previously known narrow range. After interacting with the sample, the electron loses some of its energy. By measuring how much energy certain electrons have lost, physicists draw conclusions about the substance under study.
The authors were able to add information to this technique. They found a way to measure not only the change in the energy of an electron, but also the change in its momentum. They named the new method M-EELS (the English word for momentum means “impulse”).
Scientists decided to test their innovation on crystals of titanium dichalcogenide dichlorohydrate (1T-TiSe2). To their surprise, at temperatures close to minus 83 degrees Celsius, they discovered clear signs of a state preceding the formation of excitonium - the so-called phase of soft plasmons. The results were reproduced on five different crystals.
“This result has cosmic significance,” Abbamonte said in a press release. “Since the term 'excitony' was coined in the 1960s by Harvard theoretical physicist Bert Halperin, physicists have tried to demonstrate its existence. Theorists debated whether it would be an insulator, an ideal conductor, or a superfluid - with some compelling arguments from all sides. Since the 1970s, many experimenters have published evidence for the existence of excitony, but their results have not been conclusive proof and are equally attributable to traditional structural phase transition.
It is too early to talk about the applications of excitonium in technology, but the method developed by scientists will allow investigating other substances to search for this exotic state and study its properties. In the future, this can lead to significant technical breakthroughs. Suffice it to recall, for example, that it was the discovery of superconductivity that enabled engineers to create super-strong magnets. And they gave the world both the Large Hadron Collider and bullet trains. And quantum effects are also used to create quantum computers. Even the most common computers would be impossible if quantum mechanics did not explain the behavior of electrons in a semiconductor. So the fundamental discovery made by Abbamonte's team could bring the most unexpected technological results.
Anatoly Glyantsev