Quantum microscopy study makes electrons visible in slow motion

Physicists at the University of Stuttgart, led by Professor Sebastian Loth, are developing a quantum microscope that will enable them for the first time to record the motion 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 more targeted way than before. The researchers have succeeded in Published Their results in Physics of nature.

“Thanks to the method we have developed, we can make visible things that no one has ever seen before. This makes it possible to settle questions about the movement of electrons in solids that have remained unanswered since the 1980s,” says Professor Loth, Managing Director of the Institute for Functional Materials and Quantum Technologies (FMQ) at the University of Stuttgart. The findings of the Loth group are also of very practical importance for the development of new materials.

Small changes have big consequences.

For 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 electrical conductors, while insulators are not.

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

These changes can happen very quickly, within picoseconds, affecting the movement of electrons through matter directly at the atomic level. A picosecond is extremely short, one trillionth of a second. That’s about the same as the blink of an eye, which is over 3,000 years.

Recording the movement of the electronic group

Now, Luth’s team has found a way to observe the behavior of these materials during such small changes at the atomic level. Specifically, the scientists studied a material made up of the elements niobium and selenium in which one effect can be observed in a relatively undisturbed way: the collective movement of electrons in a charge density wave.

Loth and his team investigated how a single impurity could stop this collective motion. To do this, the Stuttgart researchers applied an extremely short electrical pulse, lasting only a picosecond, to the material. The charge density wave is compressed onto the impurity and sends nanometer-scale distortions into the electron cluster, causing extremely complex electronic motion in the material for 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 Structure and Dynamics of Matter (MPSD) in Hamburg, where Löth conducted research before his appointment at the University of Stuttgart.

Development of materials with desired properties

“If we can understand how to stop the electron aggregation, we can also develop materials with desired properties in a more targeted way,” explains Loth. In other words: Since no material is perfect without impurities, the microscopy method developed helps us understand how impurities are arranged in order to achieve the desired technical effect.

“The design at the atomic level has a direct impact on the macroscopic properties of the material,” says Loth. This effect could be used, for example, in ultrafast materials in sensors or future electronic components.

An experiment repeated 41 million times per second

“There are well-established methods for visualizing individual atoms or their movements, but with these methods, you can achieve high spatial resolution or high temporal resolution,” Luth explains.

In order for the new microscope in Stuttgart to achieve both goals, the physicist and his team combine scanning tunneling microscopy, which analyzes materials at the atomic level, with an ultrafast spectroscopy method known as pump-and-probe spectroscopy.

To make the necessary measurements, the lab setup must be extremely well protected. Vibration, noise and air movement are harmful, as are fluctuations in room temperature and humidity. “This is because we are measuring very weak signals that are easily lost in background noise,” Loth says.

In addition, the team has to repeat these measurements very frequently in order to get meaningful results. The researchers were able to optimize their microscope in such a way that it can repeat the experiment 41 million times per second and thus achieve particularly high signal quality. “We’ve only been able to do this so far,” says Loth.