A “new type of electrons”

Why do some materials emit electrons with a very specific energy? It’s been a mystery for decades – scientists at TU Wien have found an answer.

This is something quite common in physics: electrons leave a certain material, they fly away and then they are measured. Some materials emit electrons when they are 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 from atoms if an electron is first removed from one of the inner electron shells. But now scientists at TU Wien (Vienna) have managed to explain a completely different type of electron emission, which can occur in carbonaceous materials such as graphite. This electron emission had been known for about 50 years, but its cause was still unclear.

Strange electrons with no explanation

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

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

The energy of these states actually matches the observed data well – so people assumed there was a connection, but that alone was not an explanation. “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 sounds like: the probability of transition is just too low.”

Skip the cords and symmetry

To change this, the internal symmetry of electronic states must be broken. “You can think of it as a jump rope,” says Wolfgang Werner. “Two children hold a long rope and move the ends. In fact, the two create a wave that would normally travel from one side of the rope to the other. But if the system is symmetrical and the two children behave the same, the rope simply moves up and down. The maximum wave always remains in the same place. We don’t see any wave movement to the left or right, this is called a standing wave. But if the symmetry is broken because, for example, one of the children recoils, the situation is different – then the dynamics of the string change, and the maximum position of the oscillation moves.

Such symmetry breaks can also occur in the material. The electrons leave their place and start to move, leaving a “hole” behind them. Such electron-hole pairs disturb the symmetry of the material, and so it can happen that the electrons suddenly have the properties of two different states simultaneously. 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 high enough. 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 sense, it’s a team effort between the electrons reflected back and forth between two layers of the material and the electrons breaking the symmetry,” explains Professor Florian Libisch of the Institute for Theoretical Physics. “It’s only when you look at them together that you can explain that the material emits electrons of exactly that 3.7 electron volts energy.”

Carbonaceous materials such as the type of graphite analyzed in this research work today play a major role – for example, the 2D material graphene, but also small diameter carbon nanotubes, which also have remarkable properties. “The effect should occur in very different materials – anywhere where thin layers are held together by weak Van der Waals forces,” says Wolfgang Werner. “In all these materials, this very particular 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, November 6, 2020, Physical examination letters.
DOI: 10.1103 / PhysRevLett.125.196603