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Physicists from the University of Stuttgart, led by Prof. Sebastian Lott, have developed quantum microscopy, which allows them for the first time to record the movement of electrons at the atomic level with extremely high spatial and temporal resolution.

Their method has the potential to allow scientists to develop materials in a much more targeted way than before. The researchers published their findings in Natural physics.

“With the method we developed, we can make things visible that no one has seen before,” says Prof. Lott, managing director of the Institute for Functional Matter and Quantum Technologies (FMQ) at the University of Stuttgart. “This makes it possible to solve questions about the movement of electrons in solids that have been unanswered since the 1980s.” The findings of Lott’s group are also of great practical importance for the development of new materials.

Small changes with macroscopic consequences

With metals, insulators, and semiconductors, the physical world is simple. If you change a few atoms at the atomic level, the macroscopic properties remain unchanged. For example, metals modified in this way are still electrically conductive, while insulators are not.

However, the situation is different for more advanced materials that can only be produced in a laboratory – minute changes at the atomic level cause new macroscopic behavior. For example, some of these materials suddenly change from insulators to superconductors, i.e. they conduct electricity without heat loss.

These changes can happen extremely quickly, within picoseconds, because they affect the movement of electrons through the material directly on the atomic scale. A picosecond is extremely short, just one trillionth of a second. It is in the same proportion to the blink of an eye as it has been to the blink of an eye over a period of more than 3,000 years.

Recording the movement of the electronic collective

Lott’s group has already found a way to observe the behavior of these materials during such small changes at the atomic level. Specifically, the scientists studied a material consisting of the elements niobium and selenium, in which one effect can be observed in a relatively undisturbed manner: the collective motion of electrons in a charge density wave.

Lott and his team investigated how an impurity can stop this collective movement. To this end, the Stuttgart researchers applied an extremely short electrical pulse, lasting only one picosecond, to the material. The charge density wave presses against the impurity and sends nanometer-sized distortions into the electron collective, causing extremely complex electron movement in the material in a short time.

Important preliminary work for the results now presented was carried out at the Max Planck Institute for Solid State Research (MPI FKF) in Stuttgart and at the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) in Hamburg, where Lott conducted research before being appointed to University of Stuttgart.

Development of materials with desired properties

“If we can understand how the motion of the electron collective is stopped, then we can also develop materials with desired properties in a more targeted way,” explains Lott. Or to put it another way: since there are no ideal materials without impurities, the developed microscopy method helps to understand how the impurities must be arranged in order to achieve the desired technical effect.

“Design at the atomic level has a direct impact on the macroscopic properties of the material,” says Lott. The effect could be used, for example, for ultra-fast switching materials in future sensors or electronic components.

An experiment repeated 41 million times per second

“There are established methods for visualizing individual atoms or their movements,” explains Lott. “But with these methods, you can achieve either high spatial resolution or high temporal resolution.”

For the new Stuttgart microscope to achieve both, the physicist and his team combined a scanning tunneling microscope, which resolves materials at the atomic level, with an ultrafast spectroscopy method known as pump-probe spectroscopy.

In order to make the necessary measurements, the laboratory installation must be extremely well shielded. Vibration, noise and air movement are harmful, as are fluctuations in room temperature and humidity. “This is because we are measuring extremely weak signals that are otherwise easily lost in the background noise,” Lott points out.

Also, the team needs to repeat these measurements very often to get meaningful results. The researchers were able to optimize their microscope in such a way that it repeated the experiment 41 million times per second and thus achieved particularly high signal quality. “We’re the only ones who have been able to do this so far,” says Lott.