Physicists at Johannes Gutenberg University Mainz (JGU) and the Max Planck Institute for Polymer Research (MPI-P) in Mainz have discovered that a phenomenon observed in certain molecular systems, namely the quantum freezing of molecular motion, is much more widespread than previously thought. For their study, the team used high-resolution X-ray scattering methods and computer simulations. The findings have important implications for understanding how organic semiconductor compounds function. The research team headed by JGU physicist Professor Silke Biermann is reporting their findings in the scientific journal Nature Physics.

Organic materials, such as those found in plastic electronics and organic photovoltaics, can be used as semiconductors just like silicon and other inorganic materials. Their semiconducting properties are dictated by how their molecules are arranged and how they move within the material.

It is well-known that the energy of the molecular vibrations in organic materials is an important factor in determining their thermal and electronic properties. However, it is not known to what extent the quantum character of these molecular vibrations affects these properties.

The researchers in Mainz showed that quantum effects can cause the molecular vibrations to become "frozen" at sufficiently low temperatures. This phenomenon, known as quantum freezing, has been observed before, but only in a few specific molecular systems.

Their goal was to investigate the quantum freezing behavior in a wider range of organic materials. "It is only then that meaningful predictions can be made about the extent to which quantum phenomena influence the properties of these organic materials," Biermann explained.

To achieve this goal, the researchers utilized high-resolution X-ray scattering methods to accurately determine the structure of the organic materials. The measurements were performed at the PETRA III storage ring at the German Electron Synchrotron (DESY) in Hamburg.

"Thanks to the high brilliance and focusability of the X-rays, we were able to determine the molecular structures in great detail, even at extremely low temperatures," said Daniel Tsivion, Ph.D. student in Biermann's group.

To analyze the data, the researchers collaborated with Matthias Schmidt at the MPI-P in Mainz. They developed sophisticated computer simulations, capable of reproducing the material's structure and simulating the dynamics of the molecules within.

The combined use of high-resolution X-ray experiments and computer simulations revealed that quantum freezing is a widespread phenomenon in organic materials, occurring in a variety of different classes of compounds. This finding is significant because it means that quantum effects must be taken into account when designing and predicting the properties of organic semiconductor materials—materials that are integral to advances in organic electronics and organic photovoltaics.

The research team is now planning to further explore quantum effects in organic materials, with the aim of understanding how these phenomena can be exploited to improve the performance and efficiency of organic electronic and optoelectronic devices.