For thousands of years, humans have been researching electric charge, and the findings have influenced modern society.
Our everyday lives rely on electric lights, tablets, vehicles, and machines, in respects that might never have been conceived by the first individuals to take care of a static shock or lightning bolt.
Now, Northeastern physicists have come up with a new way of manipulating electric charge. And the future changes in 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 power to transfer, control, and store electrons is fundamental to the overwhelming majority of industrial technology when we are attempting to extract electricity from the sun or playing Plants vs. Zombies on our computer. The researchers identified a way to make electrons do something completely different in a paper published in Nanoscale: Spread themselves uniformly into a stationary, crystalline structure.
“I’m tempted to say it’s almost like a new phase of matter,” Kar says. “Because it’s just purely electronic.”
The phenomenon appeared while the researchers conducted experiments on crystalline materials which are only a few thick atoms, known as 2D materials. These materials are composed of a repeated pattern of atoms, like an endless checkerboard, and are so thin that the electrons in them can move only in two dimensions. Stacking such ultra-thin materials may cause unexpected results when the layers communicate on a quantum level.
Kar and his collaborators studied two of such 2D compounds, bismuth selenide, and a transition metal dichalcogenide, stacked like sheets of paper on top of each other. That is when things began to get strange.
Electrons can repel each other — they’re charged negatively, so switch away from other items that are charged negatively. But that’s not what the electrons were doing in those layers. They formed a stationary pattern.
“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.”
Kar originally thought the result was a error. The crystalline structures in 2D objects are too fragile for simple examination, and physicists use different microscopes to shoot electron beams rather than light. They interact with each other when the electrons move through the material, and create a pattern. It is possible to use the specific pattern (and a bunch of math) to recreate the 2D material shape.
If the subsequent pattern showed a third layer which did not come from one of the other two, Kar figured that something had gone wrong with the substance production or in the measuring phase. Similar phenomena had previously been observed but only at extremely low temperatures.
“Have you ever walked into a meadow and seen an apple tree with mangoes hanging from it?” Kar asks. “Of course we thought something was wrong. This couldn’t be happening.”
But their findings stayed the same despite numerous studies and experiments led by doctoral student Zachariah Hennighausen. Between the 2D materials there was a new lattice-style pattern of charged spots developing. And with the orientation of the two sandwiching layers this pattern changed.
As Kar and his team were focusing on the experimental study, Arun Bansil, a respected university professor of physics at Northeastern, and Chistopher Lane, a doctoral student, were exploring the theoretical possibilities to understand how this could happen.
Electrons in a substance often jump around, says Bansil, because they are drawn in by the positively charged atomic nuclei and repelled by other negatively charged electrons. But everything in how such charges are spread out, in this situation, is pooling electrons in a particular fashion.
“They produce these regions where there are, if you like, ditches of some kind in the potential landscape, which are enough to force these electrons to create these puddles of charge,” Bansil says. “The only reason electrons will form into puddles is because there’s a potential hole there.”
These ditches are made, so to speak, by a combination of mechanical and physical quantum factors, Bansil says.
When two repeating patterns or grids are offset they combine to create a new pattern (by overlapping the teeth of two flat combs you can replicate this at home). Each 2D material has a repeating structure, and the researchers have shown that when those materials are stacked, the pattern created determines where electrons will end.
“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 this phenomenon ‘s understanding is still in its infancy, it has the potential to affect 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.”