Electrons discover their individuality

Scientists are observing changes from cooperative to individual behavior with different fast electrons

Electrons between cooperative (coherent) and self-related behavior: If an electron is catapulted out of a nitrogen molecule at a relatively low velocity, it becomes cooperative: the waves, which are emitted by both atoms like a pseudo-pair, are superimposed. This remains the same when one of these electron waves is scattered at an atom. Self-related or like an individual, an electron behaves when it leaves the molecule quickly. If the electron now strikes the neighboring atom and is scattered at it, it recognizes from which atom it started. It then overlaps with its scattered wave. © Fritz Haber Institute / Uwe Becker
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Electrons have something in common with people: the more information they gain about their situation, the more they become aware of their individuality and the more collectivity becomes less important. In doing so, the cooperative consonance that binds the electrons in a fixed relationship to their environment is lost. That's what scientists found when they used X-rays to catapult electrons out of molecules of two nitrogen atoms.

If an electron accelerates only slightly, it does not recognize which of the two atoms it was emitted from, according to researchers from the Fritz Haber Institute of the Max Planck Society in the journal Nature Physics. Then it behaves as if it came from both atoms: It appears as a pseudo-pair, which behaves completely cooperatively. If the electron is fast enough, it knows its origin. Then it shows the characteristics of an individual.

At the transition between cooperative and individual behavior of an electron, the regime change between quantum physics and classical physics can also be studied. In addition, such transitions play a role in technically interesting materials such as superconductors and magnets as well as artificial molecules, which - built up of quantum dots - are to process data as components of future quantum computers.

Nitrogen molecule ionized with X-rays

In superconductors, electrons give up their solitary behavior and merge in pairs. These "Cooper pairs" conduct electricity without resistance. In magnetic materials, even all electrons, each of which resembles a small bar magnet, are rectified as if they were set up for a military formation. In order to better understand such materials and possibly improve their properties, physicists want to find out how electrons switch from cooperative to individual behavior and vice versa.

Uwe Becker and his colleagues at the Fritz-Haber-Institut of the Max Planck Society in Berlin have now studied this transition on individual electrons. They ionized a nitrogen molecule with X-rays and gradually increased the energy of the radiation. With the energy of the irradiated X-rays, the kinetic energy of the ejected electrons increases - they pick up more speed. In the quantum mechanical picture, in which electrons present themselves as particles and as waves, this means that their wavelengths, which physicists call de Broglie wavelengths, become shorter. The more energy a wave contains, the shorter its wave length. display

Perceptibility of the electrons in sight of the researchers

Analogously, how the wavelength of electromagnetic waves decides which details they can perceive in their surroundings also improves the perceptibility of the electrons as their de Broglie wavelength shrinks. The higher their energy, the more information they gain about their environment.

In the experiment of the Berlin physicists, this was clearly noticeable, as the wavelength of the electron sank below the distance between the two atoms of the nitrogen molecule. These two atoms are symmetrical, so the X-ray does not distinguish from which atom it catapults out the electron. And as long as the wavelength of the flying electron is greater than this atomic distance, the electron makes no difference as to which atom it comes from. It does not even know its place of origin.

Electron as a pseudo pair

Moreover, one electron even exhibits properties of a pair of electrons whose partners are started by both atoms. And that also corresponds to the reality, because the emitted electron is constantly tunneling back and forth between the two atoms - a quantum-mechanical way of locomotion, which opens up paths to paths that, according to classical physics, give them energy Are locked. When tunneling, the electron jumps extremely fast between the two nitrogen atoms back and forth.

"If it then leaves the molecule with low kinetic energy, ie relatively slowly, it can no longer tell which atom it actually started from, " explains Becker. In other words, the single electron appears as a pseudo-pair, half of which start from one atom at a time-one of the whimsical phenomena that is possible only in quantum physics.

The fact that the electron actually sets off on its journey as an electron pseudo-pair is recognized by physicists by a characteristic intensity pattern that the two halves of the electron generate when they are detected. As they behave like waves, they superimpose themselves into a characteristic interference pattern - just as the waves of two stones overlap and simultaneously fall into a pond. Such an interference pattern not only proves that one electron has started from two atoms. It also demonstrates the phase-locked and therefore cooperative behavior of the two "halves" of the electron.

When electrons know their origin

"We no longer observe this special interference pattern when the speed of the emitted electron exceeds a certain value, " says Becker. Then the de Broglie wavelength of the electron shrinks below the distance of the nitrogen atoms and the electron can now perceive like a "mountain iron microscope", from which atom it was emitted.

The idea for such a microscope had been formulated by Werner Heisenberg, where he assumed that particles could be localized by means of an energy-dependent impact process via its uncertainty relation, and thus could be resolved locally can. Since such a microscope could not be realized during his lifetime, he later distanced himself from his proposal. "Our experiment is one of the rare examples of such a microscope, " says Becker.

As the electrons now know their origin at high speed, a different effect comes into play as a result. Shortly after the electron starts flying or undulates, it hits the second atom - but now it can distinguish between the two atoms. Such an obstacle on the way of a wave acts as a starting point for a new wave front. Physicists refer to this phenomenon as scattering. It causes the one electron to run in the same direction in its original unscattered and scattered version.

Elucidate the structure of solids

The two waves of the original and the scattered electron from the same atom are now superimposed instead of the two waves emitted by different atoms. Physicists use this effect to elucidate the structure of solids in a process called "Extended X-ray Fine Structure, " EXAFS for short. This also creates an interference pattern that provides information about the nature and arrangement of the scattering atoms. However, this pattern differs from that of the two "halves" of an electron, which are emitted at low energies of the two atoms. Because the scattered electron behaves no longer cooperative to its neighbor electron, but only to his unsung predecessor.

Becker puts it this way: "The waves from both sides are no longer coherent and are therefore no longer in a fixed phase relation to each other." The latter means simply stated that the mountains and valleys of the two waves have no fixed distance to each other. Coherence is lost because the electron now recognizes which atom it came from: it behaves self-centered.

Parallels between physics and society

Not only to illustrate vividly the transition from the cooperative to the self-related behavior of the electrons, Becker draws a parallel between this physical phenomenon and certain social developments: "As soon as people get more information about themselves and their environment, they begin to reflect on their situation" explains the physicist. Their behavior, which was previously determined entirely by the coherent harmony with its environment, that is, with the collective, then becomes more self-centered.

This transition can currently be observed in different cultures, especially in the Middle East. The reason why, for example, the Taliban tried to deprive their people of education and information in such simple forms as popular music and entertainment films is the fear of this inevitable process, which fundamentally questions the existence of traditional, coherent social structures. However, this anxious attitude ignores the opportunities that a knowledge-based society offers for the future of a self-centered but nevertheless solidary society, says Becker.

(MPG, 25.08.2008 - DLO)