New State of Matter

Physicists at Northeastern have discovered a new way to manipulate electric charge and they have accidentally discovered a new state of matter. And the changes to the future of our technology could be monumental.

“When such phenomena are discovered, imagination is the limit,” says Swastik Kar, an associate professor of physics. “It could change the way we can detect and communicate signals. It could change the way we can sense things and the storage of information, and possibilities that we may not have even thought of yet.”

The ability to move, manipulate, and store electrons is key to the vast majority of modern technology, whether we’re trying to harvest energy from the sun or play Plants vs. Zombies on our phone. In a paper published in Nanoscale, the researchers described a way to make electrons do something entirely new: Distribute themselves evenly into a stationary, crystalline pattern.

Nanoscale – Evidence of a purely electronic two-dimensional lattice at the interface of TMD/Bi2Se3 heterostructures

“I’m tempted to say it’s almost like a new phase of matter,” Kar says. “Because it’s just purely electronic.”

They are looking at new 2D materials. These materials are made up of a repeating pattern of atoms, like an endless checkerboard, and are so thin that the electrons in them can only move in two dimensions.

Stacking these ultra-thin materials can create unusual effects as the layers interact at a quantum level.

Kar and his colleagues were examining two such 2D materials, bismuth selenide and a transition metal dichalcogenide, layered on top of each other like sheets of paper. That’s when things started to get weird.

“At certain angles, these materials seem to form a way to share their electrons that ends up forming this geometrically periodic third lattice,” Kar says. “A perfectly repeatable array of pure electronic puddles that resides between the two layers.”

At first, Kar assumed the result was a mistake. The crystalline structures of 2D materials are too small to observe directly, so physicists use special microscopes that fire beams of electrons instead of light. As the electrons pass through the material, they interfere with each other and create a pattern. The specific pattern (and a bunch of math) can be used to recreate the shape of the 2D material.

When the resulting pattern revealed a third layer that couldn’t be coming from either of the other two, Kar thought something had gone wrong in the creation of the material or in the measurement process. Similar phenomena have been observed before, but only at extremely low temperatures. Kar’s observations were at room temperature.

When two repeating patterns or grids are offset, they combine to create a new pattern (you can replicate this at home by overlapping the teeth of two flat combs). Each 2D material has a repeating structure, and the researchers demonstrated that the pattern created when those materials are stacked determines where electrons will end up.

“That is where it becomes quantum mechanically favorable for the puddles to reside,” Kar says. “It’s almost guiding those electron puddles to remain there and nowhere else. It is fascinating.”

While the understanding of this phenomenon is still in its infancy, it has the potential to impact the future of electronics, sensing and detection systems, and information processing.

“The excitement at this point is in being able to potentially demonstrate something that people have never thought could exist at room temperature before,” Kar says. “And now, the sky’s the limit in terms of how we can harness it.”

When 2D materials are vertically stacked, new physics emerges from interlayer orbital interactions and charge transfer modulated by the additional periodicity of interlayer atomic registry (moiré superlattice). Surprisingly, relatively little is known regarding the real-space distribution of the transferred charges within this framework. Here we provide the first experimental indications of a real-space, non-atomic lattice formed by interlayer coupling induced charge redistribution in vertically stacked Bi2Se3/transition metal dichalcogenide (TMD) 2D heterostructures. Robust enough to scatter 200 keV electron beams, this non-atomic lattice generates selected area diffraction patterns that correspond excellently with simulated patterns from moiré superlattices of the parent crystals suggesting their location at sites of high interlayer atomic registry. Density functional theory (DFT) predicts concentrated charge pools reside in the interlayer region, located at sites of high nearest-neighbor atomic registry, suggesting the non-atomic lattices are standalone, reside in the interlayer region, and are purely electronic.

SOURCES- Northeastern University, Nanoscale Journal
Written By Brian Wang,