An international team, headed by Dr. Chul-Hee Min and Professor Kai Rossnagel at Kiel University (CAU), has successfully unraveled a significant mechanism. The researchers examined a material composed of a rare earth metal, specifically thulium, within a compound of thulium, selenium, and tellurium (TmSe1-xTex). These metals display unique electronic characteristics that are utilized in numerous essential technologies. The study was published in the journal Physical Review Letters.
Image Credit: DESY
Electrons play a crucial role in defining the characteristics of all materials: they determine if a metal can conduct electricity, how a semiconductor functions, and the nature of magnetic effects that arise. In certain materials, electrons exhibit particularly atypical behavior: they transition between various states, exert significant influence on each other, and can even lead to a metal abruptly transforming into an insulator (a material that ceases to conduct electricity).
The team identified a previously unrecognized quasi-particle within this material. This quasi-particle emerges from the interaction between electrons and atoms, providing an explanation for the alterations in the material's electrical properties.
When Metals Abruptly Transition to Insulators
If the tellurium concentration in the compound TmSe1-xTex increases to approximately 30 %, the material ceases to conduct electricity and changes from a semimetal to an insulator. These transitions captivate physicists as they demonstrate that a material's characteristics cannot be solely attributed to its chemical composition. Electrons exert a significant influence on one another, interacting with the vibrations of the crystal lattice (the orderly arrangement of atoms within the solid) and collectively creating particle-like states with novel properties, referred to as quasi-particles.
The researchers investigated the material at the atomic scale to gain insights into these processes. The team performed measurements utilizing high-resolution photoemission spectroscopy at multiple synchrotron radiation facilities across the globe, including the Ruprecht Haensel Laboratory, a collaborative establishment of Kiel University and DESY.
The sample was exposed to intense X-rays, and the exit angles and energies of the electrons were recorded. The resulting spectra illustrate the strength of electron binding in specific states and offer valuable information regarding the fundamental interaction processes.
Discovering Polarons
The spectroscopic measurements uncovered new insights regarding the movement of electrons within the material: a minor additional signal consistently emerged, resembling a small bump adjacent to the primary signal. Initially, the researchers suspected it to be a technical error; however, the signal reoccurred in subsequent measurements.
This persistent phenomenon motivated the Kiel team to conduct a thorough investigation into the history and behavior of the material over several years, a quest for clues that ultimately resulted in the identification of the quasiparticles.
Lead author Chul-Hee Min commenced research on TmSe1-xTex in 2015. At first, Min aimed to investigate topological surface states; however, Min’s attention later transitioned to the electronic behavior within the material. For an extended period, the additional signal adjacent to the primary peak remained an enigma.
It was only after several years of thorough analysis and extensive collaboration with international theorists that the team was able to determine the source: the signal is produced by polarons, which are quasi-particles formed when an electron is tightly coupled with the vibrations of the crystal lattice. The electron travels in conjunction with the distortion of the atoms, thereby creating a new, composite particle.
In their study, the researchers employed the periodic Anderson model, which is a theoretical framework that illustrates the interactions between electrons in these metals. By broadening the model to account for the coupling of electrons with the vibrations of the crystal lattice, they successfully provided an accurate explanation for the spectroscopic measurements.
That was the decisive step. As soon as we included this interaction in the calculations, the simulation and measurements matched perfectly.
Dr. Chul-Hee Min, Study Lead Author, Kiel University
Polarons: a Dance of Electrons and Atoms
A polaron can be characterized as a type of 'dance' involving an electron and the surrounding atoms. In typical metals, electrons move with relative freedom. However, in this material, they travel in conjunction with slightly distorted atomic layers, akin to a dent moving through the crystal lattice. This interaction results in a deceleration of the electrons, alters the electrical conductivity, and accounts for the transition to an insulator.
In quantum materials such as TmSe1-xTex, whose exotic properties stem from the quantum mechanical properties of their electrons, this effect has not yet been experimentally proven. 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.
Kai Rossnagel, Scientist, DESY
Kai Rossnagel is the director of the Institute of Experimental and Applied Physics (IEAP) at Kiel University and is also a Spokesperson for the KiNSIS (Kiel Nano, Surface and Interface Science research center).
Potential for Microelectronics and Quantum Technology
The results go beyond the specific materials studied. Comparable coupling effects are observed in various contemporary quantum materials, ranging from high-temperature superconductors to two-dimensional materials. In the future, researchers may utilize polarons strategically to manipulate electronic, optical, or magnetic properties of materials, or to develop completely new states of matter.
Such discoveries often arise from persistent basic research. But they are exactly what can lead to new technologies in the long term.
Kai Rossnagel, Scientist, DESY
Journal Reference:
Min, C. H., et al. (2025) Polaronic Quasiparticles in the Valence-Transition Compound TmSe1−xTex. Physical Review Letters. doi.org/10.1103/72dv-ynm2