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In physics, electrons that leave a specific material, fly away and then are measured is a fairly common occurrence. Some materials emit electrons when irradiated by light. These 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 internal electronic shells.
However, researchers from TU Wien (Vienna) have now successfully described a completely different type of electron emission that can occur in carbon materials such as graphite. Although researchers have known about this electron emission for nearly five decades, the causes of this have not yet been elucidated.
Strange electrons with no explanation
Many researchers have already wondered. There are materials made up of atomic layers held together only by weak Van der Waals forces, such as graphite. And it has been found that this type of graphite emits very specific electrons, all of which have exactly the same energy, which is 3.7 electron volts..
Wolfgang Werner, Professor, Institute of Applied Physics, TU Wien
This electron emission cannot be explained by any known physical mechanism. However, the measured energy gave an indication of where to look: “If these atomically thin layers are on top of each other, a certain state of electrons can form in the middle“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 outside.. “
In general, the energy of these states is in good agreement with the observed data and thus the researchers assumed that there was some link, but this alone was not enough.
Electrons in these states shouldn’t actually reach the detector. In the language of quantum physics it would be said: the probability of transition is simply too low.
Dr Alessandra Bellissimo, Study Author, TU Wien
Skip cables and symmetry
To change this, it is essential to break the internal symmetry of the electronic states. “You can think of it as a jump rope“, Explained 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 don’t see any wave motion left or right, this is called a standing wave“, Werner added.
However, the scenario is different if the symmetry breaks because, for example, one of the children moves backwards: then there is a change in the dynamics of the rope and the maximum position of the swing moves.
There are possibilities of such symmetry breaks in the material. When the electrons leave their position and move, they leave a “hole”. Such electron-hole pairs disrupt the symmetry of the material, and thus, electrons could suddenly exhibit the properties of two different states at the same time.
Thus, it is possible to combine two advantages: firstly, there are 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. According to quantum mechanics, they can do both at the same time as the refraction of symmetry causes the two states to “merge” or hybridize.
In a way, it’s a teamwork between electrons reflected back and forth between two layers of the material and electrons breaking symmetry. Only when you look at them together can you explain that the material emits electrons of exactly this 3.7 electron volts energy.
Florian Libisch, Professor, Institute of Theoretical Physics, TU Vienna
Carbon materials, for example, the type of graphite examined in this study play a crucial role today – for example, the 2D graphene material – along with carbon nanotubes which 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 of these materials, this very special type of electron emission, which we can now explain for the first time, should play an important role.. “
Journal reference:
Werner, WSM, 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.
Source: https://www.tuwien.at/en/
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