Why do certain materials emit electrons with very specific energy? This has been a mystery for decades – scientists from TU Wien have found the answer.
This is something quite common in physics: electrons leave a certain material, they fly away and then are measured. Some materials emit electrons when irradiated with light. These electrons are then called “photoelectrons”
Strange electrons without explanation
“Many researchers have already wondered about this,” says Prof. Wolfgang Werner of the Institute of Applied Physics. “There are materials that consist of atomic layers that are held together only by weak van der Waals forces, such as graphite. And it was found that this type of graphite emits very specific electrons, all of which have exactly the same energy, namely 3.7 electron volts. “
No known physical mechanism can explain this electron emission. But at least the measured energy gives an indication of where to look: “If these atomically thin layers lie on top of each other, a certain electronic state can form between them,” says Wolfgang Werner. “You can imagine it as an electron that constantly bounces back and forth between the two layers, until at some point it penetrates the layer and escapes.”
The energy of these states actually fits well into the observed data – so people assumed there was some connection, but that alone was not the explanation. “The electrons in these states don’t really have to reach the detector,” said Dr. Alessandra Bellissimo, one of the authors of this publication. “In the language of quantum physics, one would say: The probability of transition is too low.”
Leakage of cords and symmetry
To change this, the internal symmetry of the electronic states must be broken. “You can think of it as jumping rope,” says Wolfgang Werner. “Two children hold a long rope and move the endpoints. In fact, they both create a wave that usually propagates from one side of the rope to the other. But if the system is symmetrical and both children behave in the same way, then the rope just moves up and down. The maximum wave always stays in the same place. We do not see any movement of the wave left or right, this is called a standing wave. “But if the symmetry is broken because, for example, one of the children is moving backwards, the situation is different – then the dynamics of the rope changes and the maximum position of the oscillation moves.
Such symmetry breaks can also occur in the material. The electrons leave their place and begin to move, leaving a “hole” behind. Such electron-hole pairs disrupt the symmetry of the material and thus 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 are a large number of such electrons, and on the other hand, their probability of reaching the detector is high enough. In a completely 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, this is a team effort between electrons reflected back and forth between two layers of material and electrons that break 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 with exactly that energy of 3.7 electron volts.”
Carbon materials such as the type of graphite analyzed in this research play an important role today – for example, 2D material graphene, but also carbon nanotubes with a small diameter, which also have remarkable properties. “The effect has to be seen in many different materials – wherever the thin layers are held together by the weak forces of Van der Waals,” says Wolfgang Werner. “In all these materials, this very special kind of e-broadcast, which we can now explain for the first time, has to play an important role.”
Reference: “Secondary electron emission by plasmon-induced symmetry violating strongly oriented pyrolytic graphite” by Wolfgang SM Werner, Vytautas Astashauskas, Philipp Ziegler, Alessandra Bellissimo, Giovanni Stephanie, Lucas Linhart and Florian Liebish 2020, 6 Physical examination letters.
DOI: 10.1103 / PhysRevLett.125.196603