‘Zombie’ electrons trapped in 3D for the first time

Physicists at the Massachusetts Institute of Technology (MIT) have achieved a world first by trapping electrons in a three-dimensional crystal.

They believe the electrons in this ‘zombified’ state could act in coordinated, quantum ways. The resulting exotic behaviour such as superconductivity and unique forms of magnetism that may emerge suggest that trapping electrons in crystals could result in ultraefficient power lines, a new basis for quantum computing and smarter electronic devices.

Usually, electrons move freely through conducting materials. They bump into each other, but their overall movements are independent of the movements of other electrons in the lattice.

In new research published in Nature, researchers show how they successfully trapped electrons in a pure crystal.

By making the electrons settle into the same energy state, they begin to behave as one. This state is known as an electronic “flat band,” and is believed to be the result of the electrons “feeling” the quantum effects of the other electrons in the crystal.

The crystal was synthesised by physicists into an arrangement of atoms that resembles the woven patterns in “Kagome,” the Japanese art of basket-weaving.

“Now that we know we can make a flat band from this geometry, we have a big motivation to study other structures that might have other new physics that could be a platform for new technologies,” says author Joseph Checkelsky, an associate professor of physics at MIT.

Physicists have successfully trapped electrons in electronic flat-band states in 2D materials in recent years. But these electrons frequently escape into the third dimension, making flat-band states difficult to maintain in 3D.

The team based themselves on the interconnected triangular, hexagonal 2D lattices to synthesise a Kagome-like pattern in 3D.

Pyrochlore – a highly symmetric mineral – was used.

“We put certain elements together — in this case, calcium and nickel — melt them at very high temperatures, cool them down, and the atoms on their own will arrange into this crystalline, Kagome-like configuration,” Checkelsky explains.

Because the surface of the crystal is rugged, it is difficult to inspect close up to understand what is happening at specific locations.

To get around this, the researchers used angle-resolved photoemission spectroscopy (ARPES). This technique uses an ultrafocused beam of light to target specific locations on the surface. They were then able to measure the individual electron energies at those locations.

Swapping nickel out for rhodium and ruthenium should, in theory, shift the electrons’ flat band to zero energy – a superconducting state. Testing this out, the researchers found exactly this behaviour.

“This presents a new paradigm to think about how to find new and interesting quantum materials,” says co-author Riccardo Comin, also an MIT associate professor. “We showed that, with this special ingredient of this atomic arrangement that can trap electrons, we always find these flat bands. It’s not just a lucky strike. From this point on, the challenge is to optimise to achieve the promise of flat-band materials, potentially to sustain superconductivity at higher temperatures.”

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