An international team of researchers has solved a long-standing puzzle in quantum physics by discovering an elusive quasiparticle, called a polaron, in a unique rare-earth material.
The discovery, made by scientists from Kiel University and the DESY research centre, including Professor Kai Rossnagel, explains how this material can abruptly switch from a metal that conducts electricity to a perfect insulator.
The mystery centered on a compound made of thulium, selenium, and tellurium (TmSe₁₋ₓTeₓ). Physicists were baffled as to why this material would suddenly stop conducting current once the tellurium content reached approximately 30 percent—a change that its basic chemical composition could not explain.
Dance of electrons and atoms
The answer is not a simple particle but a composite entity. A polaron is formed when an electron strongly couples with the vibrations of the atoms surrounding it, creating a new, combined “particle-like” state.
“A polaron can be described as a kind of ‘dance’ between an electron and the atoms,” explained the researchers in a press release.
In this material, the electron moves together with a slight distortion in the crystal’s atomic structure, “comparable to a dent travelling through the crystal lattice.”
This coupling effectively slows the electrons down, ultimately causing the material to lose its conductivity and become an insulator.
Using intense X-rays
The team uncovered this phenomenon after years of persistent investigation. Using high-resolution photoemission spectroscopy at various global synchrotron facilities, they bombarded the material with intense X-rays to study the behavior of its electrons.
For years, a “small additional signal” kept appearing in their measurements—a “small bump” next to the main signal that the team initially dismissed as a technical error.
“The signal reappeared in repeated measurements,” the team noted, prompting a systematic investigation led by Dr. Chul-Hee Min, who began researching the material back in 2015.
The breakthrough came when the team collaborated with theorists. They adapted a standard theoretical framework, known as the periodic Anderson model, to include the coupling of electrons to the atomic vibrations.
“That was the decisive step,” explained Dr. Min. “As soon as we included this interaction in the calculations, the simulation and measurements matched perfectly.”
Implications for quantum materials
While polarons were a known theoretical concept, this study marks the first time they have been experimentally proven in this specific class of “quantum materials.”
Researchers believe these findings could have significant implications beyond this one compound. Similar coupling effects are thought to be at play in other advanced materials, including high-temperature superconductors and 2D materials.
“Such discoveries often arise from persistent basic research,” said Rossnagel. “But they are exactly what can lead to new technologies in the long term.”
The team’s success in identifying the polaron not only explains the material’s strange switch from metal to insulator but also confirms a key theoretical concept in a new class of materials.
This work opens a new avenue of research, allowing scientists to explore how this “dance” between electrons and atoms could be harnessed in other quantum systems.
The findings were published in the journal Physical Review Letters.