In 1934, Eugene Wigner, a pioneer of quantum mechanics, theorized a strange kind of matter — a crystal made from electrons. The idea was simple; proving it wasn’t. Physicists tried many tricks over eight decades to nudge electrons into forming these so-called Wigner crystals, with limited success. In June, however, two independent groups of physicists reported in Nature the most direct experimental observations of Wigner crystals yet.
“Wigner crystallization is such an old idea,” said Brian Skinner, a physicist at Ohio State University who was not involved with the work. “To see it so cleanly was really nice.”
To make electrons form a Wigner crystal, it might seem that a physicist would simply have to cool them down. Electrons repel one another, and so cooling would decrease their energy and freeze them into a lattice just as water turns to ice. Yet cold electrons obey the odd laws of quantum mechanics — they behave like waves. Instead of getting fixed into place in a neatly ordered grid, wavelike electrons tend to slosh around and crash into their neighbors. What should be a crystal turns into something more like a puddle.
One of the teams responsible for the new work found a Wigner crystal almost by accident. Researchers in a group led by Hongkun Park at Harvard University were experimenting with electron behavior in a “sandwich” of exceptionally thin sheets of a semiconductor separated by a material that electrons could not move through. The physicists cooled this semiconductor sandwich to below −230 degrees Celsius and played around with the number of electrons in each of the layers.
The team observed that when there was a specific number of electrons in each layer, they all stood mysteriously still. “Somehow, electrons inside the semiconductors could not move. This was a really surprising find,” said You Zhou, lead author on the new study.
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