In physics, electrons leaving a specific material, flying away, and then being measured is a quite common occurrence. Certain materials emit electrons upon irradiation by light. Such electrons are known as “photoelectrons.”
In the field of materials research, the so-called “Auger electrons” also play a crucial role—they are emitted by atoms if an electron is first eliminated from one of the inner electron shells.
However, researchers from TU Wien (Vienna) have now successfully described a totally different kind of electron emission that can occur in carbon materials like graphite. Although researchers have known this electron emission for almost five decades, what causes this has been unclear to date.
Strange Electrons Without Explanation
Many researchers have already wondered about this. There are materials that consist of atomic layers that are held together only 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 electron volts.
Wolfgang Werner, Professor, Institute of Applied Physics, TU Wien
This electron emission could not be explained by any known physical mechanism. However, the measured energy gave an indication of where to look: “If these atomically thin layers lie on top of each other, a certain electron state can form in between,” added Wolfgang Werner. “You can imagine it as an electron that is continuously reflected back and forth between the two layers until at some point it penetrates the layer and escapes to the outside.”
In general, the energy of these states is in good agreement with the observed data—and thus researchers assumed that there is some link, but that alone was not enough.
The electrons in these states should not actually reach the detector. In the language of quantum physics one would say: The transition probability is just too low.
Dr Alessandra Bellissimo, Study Author, TU Wien
Skipping Cords and Symmetry
To modify this, it is essential to break the internal symmetry of the electron states. “You can imagine this like rope skipping,” explained Wolfgang Werner.
“Two children hold a long rope and move the end points. Actually, both create a wave that would normally propagate from one side of the rope to the other. But if the system is symmetrical and both children behave the same way, then the rope just moves up and down. The wave maximum always remains at the same place. We don’t see any wave movement to the left or right, this is called a standing wave,” added Werner.
However, the scenario is different if the symmetry gets broken because, for instance, one of the children moves backward—then there is a change in the dynamics of the rope and the maximum position of the oscillation moves.
There are possibilities of such symmetry breaks in the material. When electrons vacate their position and move, they leave behind a “hole.” Such electron-hole pairs interrupt the material’s symmetry, and thus, the electrons could suddenly exhibit the properties of two different states at the same time.
Thus, two benefits can be combined: firstly, there is a huge number of such electrons, and secondly, their probability of reaching the detector is considerably high. In an absolutely symmetrical system, only one of the above cases would be feasible. As per quantum mechanics, they can do both simultaneously since the symmetry refraction makes the two states “merge,” or hybridize.
In a sense, it is teamwork between the electrons reflected back and forth between two layers of the material and the symmetry-breaking electrons. Only when you look at them together can you explain that the material emits electrons of exactly this energy of 3.7 electron volts.
Florian Libisch, Professor, Institute of Theoretical Physics, TU Wien
Carbon materials, for example, the kind of graphite investigated in this study have a crucial role at present—for instance, the 2D material graphene—together with carbon nanotubes that have extremely diameters and exhibit splendid properties.
“The effect should occur in very different materials—wherever thin layers are held together by weak Van der Waals forces,” added Wolfgang Werner. “In all these materials, this very special type of electron emission, which we can now explain for the first time, should play an important role.”
Werner, W. S. M., et al. (2020) Secondary Electron Emission by Plasmon-Induced Symmetry Breaking in Highly Oriented Pyrolytic Graphite. Physical Review Letters. doi.org/10.1103/PhysRevLett.125.196603.