Advancing Materials Research with New Breakthrough

Researchers are recording atomic-level electron mobility in solids for the first time with very high spatial and temporal precision, which advances materials research. Prof. Sebastian Loth and his colleagues' study findings have now been published in Nature Physics.

With the method we developed, we can make things visible that no one has seen before. This makes it possible to settle questions about the movement of electrons in solids that have been unanswered since the 1980s.

Sebastian Loth, Professor and Managing Director, Institute for Functional Matter and Quantum Technologies, University of Stuttgart

However, Loth’s group’s results have significant practical implications in the creation of novel materials.

Tiny Changes with Macroscopic Consequences

In metals, insulators, and semiconductors, the physical universe is straightforward. If only a few atoms are modified at the atomic level, the macroscopic characteristics remain constant. Metals, for example, that have been changed in this manner remain electrically conductive, whereas insulators do not.

However, the situation differs with more sophisticated materials, which can only be manufactured in the laboratory. Small alterations at the atomic level result in novel macroscopic behavior.

For example, some of these materials go from insulators to superconductors, which transmit electricity without losing heat. These changes can happen incredibly fast, within picoseconds, as they directly affect the passage of electrons through the material on an atomic scale. A picosecond is merely a trillionth of a second. It is in the same proportion to the blink of an eye as the blink of an eye is to a period of over 3000 years.

Recording the Movement of the Electron Collective

Loth's team has now discovered a means to study the behavior of these materials during such minute changes at the atomic level. Specifically, the scientists investigated a substance composed of the elements niobium and selenium in which one phenomenon could be observed reasonably unaltered: the collective migration of electrons in a charge density wave. Loth and his team examined how a single impurity can interfere with this collective movement.

For this reason, the Stuttgart researchers apply an incredibly brief electrical pulse to the material, lasting only one picosecond. The charge density wave presses against the impurity, causing nanometer-sized aberrations in the electron collective and resulting in highly complicated electron motion in the material for a brief period.

Important preliminary work for the now-presented results 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 previously conducted research before being appointed to the University of Stuttgart.

Developing Materials with Desired Properties

If we can understand how the movement of the electron collective is stopped, then we can also develop materials with desired properties in a more targeted manner.

Sebastian Loth, Professor and Managing Director, Institute for Functional Matter and Quantum Technologies, University of Stuttgart

Or, to put it another way, because there are no ideal materials without impurities, the microscopy approach created helps in understanding how impurities should be placed to accomplish the desired technical result.

“Design at the atomic level has a direct impact on the macroscopic properties of the material,” added Loth.

The phenomenon might be applied, for example, to ultra-fast switching materials in future sensors or electronic components.

An Experiment Repeated 41 Million Times Per Second

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

The new Stuttgart microscope does both by combining a scanning tunneling microscope, which resolves materials at the atomic level, with an ultrafast spectroscopy technology known as pump-probe spectroscopy.

To perform the necessary measurements, the laboratory setup must be exceptionally well shielded. Vibrations, noise, and air movement are all detrimental, as are variations in room temperature and humidity.

Loth noted, “This is because we measure extremely weak signals that are otherwise easily lost in the background noise.”

Furthermore, the team must repeat these measures frequently to acquire relevant results. The researchers were able to modify their microscope so that it repeated the experiment 41 million times per second, resulting in an exceptionally high signal quality.

Loth concluded, “Only we have managed to do this so far.”

Journal Reference:

Sheng, S., et. al. (2024) Terahertz spectroscopy of collective charge density wave dynamics at the atomic scale. Nature Physics. doi:10.1038/s41567-024-02552-7