Why do certain materials emit electrons with a certain energy? This has been a mystery for decades – scientists at TU Wien have found an answer.
This is quite common in physics: electrons leave a certain material, fly away and are then measured. Some materials emit electrons when exposed to light. These electrons are then called “photoelectrons”. So-called “Auger electrons” also play an important role in materials research – they can be emitted by atoms when an electron is first removed from one of the inner electron shells. Scientists at TU Wien have now succeeded in explaining a completely different type of electron emission that can occur in carbon 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 it,” says Prof. Wolfgang Werner from the Institute for Applied Physics. “There are materials that consist 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, all of which 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 electron state can form in between,” says Wolfgang Werner. “You can think of it as an electron that is continuously reflected back and forth between the two layers until it eventually penetrates the layer and escapes to the outside.”
The energy of these states actually fits well with the observed data – so people assumed there was a connection, but that alone was not 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 one would say: the transition probability is simply too low.”
Skipping strings 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 end points. In fact, they both create a wave that usually propagates from one side of the rope to the other. However, if the system is symmetrical and both children behave the same, the rope will only move up and down. The wave maximum always stays in the same place. We do not see any wave movement to the left or right, this is called a standing wave. “However, 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 change and the maximum position of the vibration moves.
Such symmetry breaks can also occur in the material. Electrons leave their place and begin to move, leaving a “hole” behind. Such electron-hole pairs disturb the symmetry of the material, and thus it can happen that the electrons 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 both execute at the same time, since the symmetry breaking causes the two states to “merge” (hybridize).
“In a certain way, it is about teamwork between the electrons reflected back and forth between two layers of material and the symmetry-breaking electrons,” says Prof. Florian Libisch from the Institute for Theoretical Physics. “Only if you look at them together can you explain that the material emits electrons with precisely this energy of 3.7 electron volts.”
Carbon materials such as the type of graphite analyzed in this research work play a major role today – for example the 2D material Graphbut also small-diameter carbon nanotubes, which also have remarkable properties. “The effect should occur with very different materials – wherever thin layers are held together by 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 through plasmon-induced breaking of the symmetry 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