The study of quantum microscopy makes electrons visible in slow motion

Physicists at the University of Stuttgart under the leadership of Prof. Sebastian Loth is developing the quantum microscope that enables 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 enable scientists to develop materials in a much more targeted way than before. The researchers have published their findings in Nature Physics.

“With the method we have developed, we can do obvious things that no one has seen before,” says Prof. Loth, 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 motion of electrons in solids that have remained unanswered since the 1980s.” The findings of Loth’s group are also of great practical importance for the development of new materials.

Small changes with macroscopic consequences

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

However, the situation is different in the most advanced materials, which can only be produced in the laboratory – minimal changes at the atomic level cause new macroscopic behaviors. For example, some of these materials suddenly change from insulators to superconductors, ie. they conduct electricity without heat loss.

These changes can occur extremely quickly, within picoseconds, as they affect the movement of electrons through the material directly on the atomic scale. A picosecond is extremely short, only one trillionth of a second. It is in the same proportion as the closing of an eye, as is the closing of an eye with a period of more than 3000 years.

Recording the movement of the electron collective

Loth’s working group has now found a way to observe the behavior of these materials during such small changes at the atomic level. Specifically, the scientists studied a material composed of the elements niobium and selenium, in which an effect can be observed in a relatively undisturbed way: the collective movement of electrons in a wave of charge density.

Loth and his team investigated how a single impurity can stop this collective movement. To this end, the Stuttgart researchers apply an extremely short electrical pulse, lasting just one picosecond, to the material. The charge density wave presses against the impurity and sends nanometer-sized distortions into the electron beam, which cause very complex electron motions in the material for a short time.

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

Development of materials with desired properties

“If we can understand how the movement of the electron group is stopped, then we can also develop materials with the desired properties in a more targeted way,” explains Loth. Or to put it another way: Since there are no perfect materials without impurities, the developed microscopy method helps to understand how the impurities should be arranged to achieve the desired technical effect.

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

An experiment repeated 41 million times per second

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

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

To make the necessary measurements, the laboratory structure must be extremely protected. Vibrations, noises and air movement are harmful, as well as fluctuations in room temperature and humidity. “This is because we measure extremely weak signals that are otherwise easily lost in the background noise,” Loth points out.

In addition, the team must repeat these measurements very often to obtain 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 a particularly high signal quality. “We’re the only ones who have managed to do this so far,” says Loth.