PHOTON SCIENCE

Electrons determine the properties of all materials: They decide whether a metal conducts electricity, how a semiconductor operates or which magnetic effects occur. In some materials, electrons behave in a particularly unusual way: They switch between different states, strongly influence one another and can even cause a metal to suddenly become an insulator – a substance that no longer conducts electricity.

An international team led by Chul-Hee Min and Kai Rossnagel from Kiel University (CAU) has now deciphered a crucial mechanism. The researchers investigated a material based on a rare earth metal (thulium) in a compound of thulium, selenium and tellurium (TmSe_1-xTe_x). These metals exhibit special electronic properties that are used in many key technologies.

The team discovered a quasi-particle previously unknown in this material. It arises from the interaction between electrons and atoms and explains why the material changes its electrical properties. Parts of this research were carried out at PETRA III beamline P22 and P04 as well as at other synchrotron sources in the UK (Diamond Light Source) and in the US (Advanced Light Source). The researchers published their findings in the journal Physical Review Letters.

When metals suddenly become insulators

If the tellurium content in the compound TmSe_1-xTe_x rises to about 30 percent, the material stops conducting current and transforms from a semimetal into an insulator. These transitions fascinate physicists because they show that the properties of a material cannot be explained by its chemical composition alone. Electrons strongly influence each other, coupling to the vibrations of the crystal lattice – the regular network of atoms in the solid – and together forming particle-like states with new properties, known as quasi-particles.

The researchers examined the material at the atomic level to understand these processes. They carried out the measurements using high-resolution photoemission spectroscopy at various synchrotron radiation sources worldwide, including the Ruprecht Haensel Laboratory, a joint facility of Kiel University and DESY. They irradiated the sample with intense X-rays and measured the exit angles and energies of the electrons. The spectra show how strongly electrons are bound in certain states and provide information about the underlying interaction processes.

Discovering polarons

The spectroscopic measurements revealed new details about the movement of electrons in the material: A small additional signal kept appearing, which looked like a small bump next to the main signal. At first, the researchers thought it was a technical inaccuracy, but the signal reappeared in repeated measurements. This recurring phenomenon prompted the Kiel team to systematically investigate the history and behaviour of the material over a period of years – a search for clues that ultimately led to the discovery of the quasiparticles.

Lead author Chul-Hee Min, also scientist at the Norwegian University (NTNU) in Trondheim, began researching TmSe_1-xTe_x back in 2015. Initially, he was looking for topological surface states, but later his focus shifted to the electronic behaviour inside the material. For a long time, the additional signal next to the main peak remained an unsolved mystery.

Only after years of analysis and close collaboration with international theorists did the team identify the cause: The signal originates from polarons, quasi-particles in which an electron is closely coupled to the vibrations of the crystal lattice. The electron moves together with the distortion of the atoms, thus forming a new, composite particle.

In their research, the scientists used the periodic Anderson model, a theoretical model that describes how electrons interact with each other in such metals. By extending the model to include the coupling of electrons to the vibrations of the crystal lattice, they were able to accurately explain the spectroscopic measurements. “That was the decisive step,” explains Min. “As soon as we included this interaction in the calculations, the simulation and measurements matched perfectly.”

Polarons – Dance of electrons and atoms

A polaron can be described as a kind of ‘dance’ between an electron and the atoms surrounding it. In ordinary metals, electrons flow almost freely. In this material, however, they move together with slightly distorted atomic layers, comparable to a dent travelling through the crystal lattice. This coupling slows down the electrons, changes the electrical conductivity and explains the transition to an insulator.

"In quantum materials such as TmSe_1-xTe_x, whose exotic properties stem from the quantum mechanical properties of their electrons, this effect has not yet been experimentally proven," says Kai Rossnagel, also DESY lead scientist and Director at the Institute of Experimental and Applied Physics (IEAP) at Kiel University as well as spokesperson for the KiNSIS (Kiel Nano, Surface and Interface Science research centre). "The fact that we were able to make it visible here for the first time shows what interesting new phenomena are still to be discovered in the quantum cosmos of materials."

Potential for microelectronics and quantum technology

The findings extend beyond the material investigated. Similar coupling effects occur in many modern quantum materials – from high-temperature superconductors to 2D materials. In the future, researchers could use polarons in a targeted manner to control electronic, optical or magnetic material properties or to create entirely new states of matter.

"Such discoveries often arise from persistent basic research," says Rossnagel. "But they are exactly what can lead to new technologies in the long term."


(Partly from DESY News)


Reference:
C.-H. Min, Kai Roßnagel et al.: „Polaronic Quasiparticles in the Valence-Transition Compound TmSe_1-xTe_x“, Physical Review Letters (2025) DOI: 10.1103/PhysRevLett.135.186501