Electronic properties found in boron chains

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A Rice University team that simulated one-dimensional forms of boron is said to have found they possess unique properties. If the metallic ribbons of boron are stretched, they morph into antiferromagnetic semiconducting chains, and when released they fold back into ribbons.

"Our work on carbyne and with planar boron got us thinking that a one-dimensional chain of boron atoms is also a possible and intriguing structure," researcher Boris Yakobson said.

"We wanted to know if it is stable and what the properties would be. That's where modern theoretical-computational methods are impressive, because one can do pretty realistic assessments of non-existing structures.”

One-dimensional boron forms two well-defined phases – single-atom chains and two-atom-wide ribbons – which are linked by a ‘reversible phase transition’, meaning they can turn from one form to the other and back.

"Boron is very different from carbon," Yakobson said. "It prefers to form a double row of atoms, like a truss used in bridge construction. This appears to be the most stable, lowest-energy state.

"If you pull on it, it starts unfolding; the atoms yield to this monatomic thread. And if you release the force, it folds back," he said. "At the same time, it changes the electronic properties.

"That makes it an interesting combination: When you stretch it halfway, you may have a portion of ribbon and a portion of chain. Because one of them is metal and the other is a semiconductor, this becomes a one-dimensional, adjustable Schottky junction."

As a chain of atoms, the material is a strain-tuneable, wide-gap antiferromagnetic semiconductor. In an antiferromagnet, the atomic moments align in opposite directions. This coupling of magnetic state and electronic transport may be of interest to researchers studying spintronics.

According to Yakobson, one-dimensional boron's springiness is also interesting. "It's a special spring, a constant-force spring," he said.

This property could be useful in nanoscale sensors to gauge small forces.