A “new kind of electrons”

Why do some materials emit electrons with a very specific energy? This has been a mystery for decades: scientists at TU Wien have found an answer.

It’s something quite common in physics: electrons leave a certain material, fly away, and then they are measured. Some materials emit electrons when irradiated with light. These electrons are then called “photoelectrons”. In materials research, so-called “Auger electrons” also play an important role: they can be emitted by atoms if an electron is first removed from one of the internal electronic shells. But now scientists from TU Wien (Vienna) have managed to explain a completely different type of electron emission, which can occur in carbon materials such as graphite. This electron emission had been known for about 50 years, but its cause was not yet clear.

Strange electrons with no explanation

“Many researchers have already wondered,” says prof. Wolfgang Werner of the Institute of Applied Physics. “There are materials made up of atomic layers held together only by weak Van der Waals forces, for example graphite. And it was found that this type of graphite emits very specific electrons, which all have exactly the same energy, namely 3.7 electron volts. “

No known physical mechanism could explain this electron emission. But at least the measured energy gave an indication of where to look: “If these atomically thin layers lie on top of each other, a certain state of electrons can form in the middle,” says Wolfgang Werner. “You can imagine it as an electron that is continually reflected back and forth between the two layers until at some point it penetrates the layer and escapes outside.”

Florian Libisch, Philipp Ziegler, Wolfgang Werner and Alessandra Bellissimo (left to right). Credit: Vienna University of Technology

The energy of these states actually fits well with the observed data, so people assumed there was some connection, but that alone wasn’t an explanation. “The electrons in these states shouldn’t actually reach the detector,” says Dr Alessandra Bellissimo, one of the authors of the current publication. “In the language of quantum physics it would say: the probability of transition is simply too low.”

Jump ropes and symmetry

To change this, the internal symmetry of the electronic states must be broken. “You can imagine it as a jump rope,” says Wolfgang Werner. “Two children hold a long rope and move the end points. In fact, they both create a wave that would normally propagate from one side of the string to the other. But if the system is symmetrical and both children behave the same, the rope simply moves up and down. The maximum of the wave always remains in the same place. We do not see any movement of the wave to the left or to the right, this is called a standing wave. “But if the symmetry breaks because, for example, one of the children is moving backwards, the situation is different – then the dynamics of the string changes. and the maximum swing position moves.

Such symmetry breaks can also occur in the material. The electrons leave their places and start moving, leaving a “hole” behind them. Such electron-hole pairs disturb the symmetry of the material, and therefore it can happen that electrons suddenly have the properties of two different states at the same time. In this way, two advantages can be combined: on the one hand, there is a large number of such electrons, and on the other hand, their probability of reaching the detector is sufficiently high. In a perfectly symmetrical system, only one or the other would be possible. According to quantum mechanics, they can do both at the same time, because the refraction of symmetry causes the two states to “merge” (hybridize).

“In a way, it’s a teamwork between the electrons reflected back and forth between two layers of the material and the electrons breaking the symmetry,” says prof. Florian Libisch of the Institute of Theoretical Physics. “Only when you look at them together can you explain that the material emits electrons of exactly this 3.7 electron volts energy.”

Carbon materials such as the type of graphite analyzed in this research work play an important role today, for example 2D material graphene, but also carbon nanotubes of tiny diameter, also endowed with remarkable properties. “The effect should occur in very different materials, wherever the thin layers are held together by the weak Van der Waals forces,” says Wolfgang Werner. “In all of these materials, this very special type of electron emission, which we can now explain for the first time, should play an important role.”

Reference: “Secondary Electron Emission by Plasmon-Induced Symmetry Breaking in Highly Oriented Pyrolytic Graphite” by Wolfgang SM Werner, Vytautas Astašauskas, Philipp Ziegler, Alessandra Bellissimo, Giovanni Stefani, Lukas Linhart and Florian Libisch, 6 November 2020, Physical Review Letters.
DOI: 10.1103 / PhysRevLett.125.196603