Electronic devices rely on materials whose electrical properties change with temperature, making them less stable in extreme conditions. A discovery by McGill University researchers that challenges conventional wisdom in physics suggests that bismuth, a metal, could serve as the foundation for highly stable electronic components.
The researchers observed a mysterious electrical effect in ultra-thin bismuth that remains unchanged across a wide temperature range, from near absolute zero (-273°C) to room temperature.
"If we can harness this, it could become important for green electronics," said Guillaume Gervais, a professor of physics at McGill and co-author of the study.
The finding could lead to the development of more efficient, stable and environmentally friendly electronic components and devices, including for space exploration and medical uses. Bismuth is non-toxic and biocompatible.
"We expected this effect to disappear once we increased the temperature, but it stubbornly refused; we kept going to room temperature and it was still there!" said Gervais. "I was so sure it would vanish that I even bet my students Oulin Yu and Frédéric Boivin a bottle of wine. It turned out I was wrong."
Inspired by a cheese grater
Published in Physical Review Letters, the study reports the observation of a temperature-independent anomalous Hall effect (AHE) in a 68-nanometre-thick flake of bismuth. This effect, which creates a voltage perpendicular to an applied current, is typically associated with materials that have magnetic properties. However, bismuth is diamagnetic, meaning it does not usually exhibit such behaviour.
To make the discovery, Gervais and his colleagues, including lead author and PhD candidate Oulin Yu, developed a new technique for creating ultra-thin bismuth. Inspired by a cheese grater, the team patterned microscopic trenches onto a semiconductor wafer, then mechanically shaved off thin layers of bismuth. They then tested these flakes under extreme magnetic fields - tens of thousands of times stronger than a fridge magnet - at the National High Magnetic Field Laboratory in Florida.
Breaking the rules of physics?
Previous studies suggested that bismuth should not exhibit AHE, making the team's findings all the more puzzling.
"I can't point to one theory that would explain this," said Gervais, "only bits and pieces of a potential explanation."
One hypothesis is that the atomic structure of bismuth constrains electron movement in a way that mimics the behaviour of topological materials, recently discovered exotic substances whose surfaces and interiors exhibit different properties. These materials could revolutionize computing.
The research team's next step is to explore whether bismuth's AHE can be converted into its quantum counterpart, the quantum anomalous Hall effect (QAHE). Such a breakthrough could pave the way for electronic devices that function at higher temperatures than previously possible.
About the study
Observation of Temperature-Independent Anomalous Hall Effect in Thin Bismuth from Near Absolute Zero to 300 K Temperature by Guillaume Gervais, Oulin Yu, et al., was published in Physical Review Letters.
The research was supported by the New Frontiers in Research Fund, the Natural Sciences and Engineering Research Council of Canada (NSERC), the Canadian Institute for Advanced Research, and the Fonds de recherche du Québec - Nature et technologies, Montreal-based CXC, as well as the National Science Foundation (NSF) for the work performed at the National High Magnetic Field Laboratory in Florida.
