Electrons Slow to a Crawl in a Quantum State Found in a Layered Magnet
University of Chicago physicists found millions of electrons moving together slowly yet coherently in Fe5GeTe2, a result that contradicts theory and persists up to 100 kelvin.
Physicists at the University of Chicago have found a strange quantum state in a layered magnetic crystal in which millions of electrons move together extremely slowly while staying quantum mechanically coherent. The finding, published October 1 in Science Advances, contradicts existing predictions about how the material behaves and points toward new kinds of memory devices.
The material is iron germanium telluride, written Fe5GeTe2. It belongs to a family called van der Waals magnets, crystals made of stacked layers that can be peeled down to atomically thin sheets. This one was discovered seven years ago.
The research team, led by Assistant Professor Shuolong Yang at the university's Pritzker School of Molecular Engineering, included postdoctoral scholars Gabriele Berruto and Qiang Gao. They used a technique called angle-resolved photoemission spectroscopy, which knocks electrons out of a material with light and measures their energy and direction, to map the electrons' behavior in a spot just 10 micrometers across.
What they saw was a flat electronic band. In an ordinary metal, electrons have a wide range of energies as they move, and they zip through the crystal. In a flat band, the range of energies tied to electrical conduction barely changes, so the electrons are effectively held in place and move very slowly. Yang's group found that the energy range associated with conduction "changed very little," which sets up conditions for the electrons to act collectively rather than as individuals.
That collective behavior is where the interesting physics lives. When electrons are this sluggish, their mutual repulsion dominates over their motion, and exotic phases can emerge, including some linked to unconventional magnetism and superconductivity. Here, the team identified an unexpected charge-ordered phase, in which electrons arrange themselves in a repeating pattern while retaining quantum coherence.
The coherent behavior persisted up to 100 kelvin, or about minus 173 degrees Celsius. That is far below room temperature, but high for a quantum state of this kind, which usually demands much colder conditions.
Flat bands have become a prized target in condensed-matter physics because slow electrons interact strongly with one another, and that is the setting where unconventional states of matter tend to appear. The Chicago result adds a magnetic, layered crystal to the list of places where researchers can study that behavior.
The result also conflicts with theoretical predictions about the material's magnetic interactions, suggesting that current models are missing something. The group is now testing whether lasers can switch Fe5GeTe2 between different quantum phases, an ability that would be useful for memory devices, and whether the slow-electron state survives when the crystal is thinned to a single atomic layer.
