Canadian physicists may have found a way to get around temperature’s effects which have been barriers to green electronics.
Bismuth is at the heart of the break-through. PhD candidate Oulin Yu of McGill University in Montreal and colleagues have discovered that this silvery metal can conduct electricity from near absolute zero (-273oC) to room temperature.
The metal is used in pharmaceuticals, cosmetics, glass, ceramics and atomic fire alarms and sits next to lead on the periodic table. The Geology Science website says it’s rare, with the world’s biggest deposits in China, but also in Bolivia, Canada, Mexico and Australia, where its often associated with other metal mining operations, including gold, lead, silver and zinc ores.
The team created wafer-thin bismuth flakes cheese-grater style, shaving 68 nanometer-thick slices into microscopic trenches in semiconductor wafers.
An electrical current was passed through these bismuth flakes exposed to a massive magnetic field tens of thousands of times stronger than a fridge magnet.
Yu and team were surprised to find the wafers showed an ‘anomalous Hall Effect’ (AHE).
A little physics. Usually when electrons flow through wire they bump into things, atoms or defects, the potholes and traffic lights of the electrical highway, wasting a lot of energy.
Now imagine a thin sheet of bismuth or other metal, like a flat road for electrons.
Introducing a giant magnet and extreme cold makes electrons flow only along the edges of the sheet, and in only one direction, like cars on a one-way street with no red lights or speed bumps. This flow happens without any resistance—a perfect highway for electrons.
This is called a ‘Quantum Hall Effect.’
In an ‘Anomalous Hall Effect’ (AHE) the same thing happens without a magnet, because the material used is itself magnetic. But bismuth is diamagnetic, which means it is repelled by a magnetic field — applying a magnet induces an opposite magnetic field in the metal.
Yu says that bismuth should not show an AHE.
“I can’t point to one theory that would explain this,” says coauthor Professor Guillaume Gervais, “only bits and pieces of a potential explanation.”
Bismuth could be showing similar effects to ‘topological’ materials, says Yu. The surfaces of topological substances show different properties to their interiors and could revolutionise computing.
Gervais was so certain that the AHE would disappear with rising temperature that he made a small wager with his team.
“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,” he says.
The discovery challenges conventional wisdom in physics, say the researchers, and has implications for low emissions green electronics.
“If we can harness this, it could become important for green electronics,” says Gervais. .
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).
Which could mean green electronics – that function at higher temperatures than previously possible.
The paper was published in Physical Review Letters
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