Quantum microscopy study makes electrons visible in slow motion

Physicists at the University of Stuttgart, led by Professor Sebastian Loth, are developing quantum microscopy that 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 results in Physics of nature.

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

Tiny changes with macroscopic consequences

In the physical world of 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 changed in this way still conduct electricity, while insulators do not.

The situation is different for more advanced materials, which can only be produced in the laboratory: tiny changes at the atomic level lead to new macroscopic behavior. For example, some of these materials suddenly switch from being insulators to superconductors, i.e. they conduct electricity without heat loss.

These changes can happen extremely quickly, in picoseconds, because they influence the movement of electrons through matter directly at the atomic scale. A picosecond is extremely short, just one billionth of a second. It is about the same proportion to the blink of an eye as a blink of an eye with a period of over 3,000 years.

Recording the movement of the electronic collective

Loth’s working group has now found a way to observe how these materials behave during these 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 relatively unperturbed: the collective motion of electrons in a charge density wave.

Loth and his team investigated how a single impurity can stop this collective motion. To do this, the Stuttgart researchers apply an extremely short electrical pulse, lasting only one picosecond, to the material. The charge density wave is pressed against the impurity and sends nanometer-sized distortions into the electronic collective, causing an 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 the Structure and Dynamics of Matter (MPSD) in Hamburg, where Loth had conducted research before being appointed to the University of Stuttgart.

Developing materials with desired properties

“If we can understand how the movement of the electronic collective is stopped, we can then develop materials with the desired properties in a more targeted manner,” explains Loth. In other words: since there are no perfect materials without impurities, the developed microscopy method makes it possible to understand how the impurities must be arranged 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, for ultra-fast switching materials in future sensors or electronic components.

An experiment repeated 41 million times per second

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

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

In order to be able to carry out the necessary measurements, the laboratory must be extremely well protected. Vibrations, noise and air movements are harmful, as are fluctuations in ambient temperature and humidity. “This is because we are measuring extremely weak signals that are easily lost in the background noise,” Loth emphasizes.

In addition, the team has to repeat these measurements very often to obtain meaningful results. The researchers were able to optimize their microscope in such a way that it repeats the experiment 41 million times per second and thus achieves a particularly high signal quality. “We are the only ones who have succeeded in this so far,” says Loth.