Researchers at ETH Zurich have created a crystal made entirely of electrons. The structures have been theorized for decades, but this marks the first time they’ve been experimentally confirmed in the lab.
Normally, electrons behave more or less like a liquid, flowing freely through a material. But in 1934, theoretical physicist Eugene Wigner predicted that a group of electrons could crystallize into a solid form under specific conditions, forming a phase now known as a Wigner crystal.
To do so, exactly the right balance needs to be struck between two forces affecting electrons: their electrostatic repulsion and their motional energy. The latter is the more powerful effect, causing electrons to bounce around at random, but if that could be reduced enough, Wigner proposed, then the repulsion could take over, locking the electrons into a uniform lattice.
But that proved trickier than it may sound. The density of electrons needs to be lowered beyond a certain point, they need to be confined in a “trap”, and they need to be cooled almost to absolute zero, to reduce outside influence on their movement.
Now, scientists at ETH Zurich have ticked off all those requirements to create a Wigner crystal. To confine the electrons, they used a one-atom-thick sheet of molybdenum diselenide, effectively restricting their movements to two dimensions. To control the number of electrons in this semiconductor, the team sandwiched this material between two graphene electrodes and applied a careful voltage. Finally, the whole system was cooled to close to absolute zero.
And sure enough, a Wigner crystal emerged. But observing it was another challenge entirely – the problem is that the separation between the electrons is so tiny, around 20 nanometers, that microscopes can’t even see it. Previous studies trying to create Wigner crystals had to rely on indirect methods to detect them, such as changes in current.
For the new study, the team used a new method. They shone light onto the material at a particular frequency to excite what are called “excitons” in the semiconductor, which emit light back. If Wigner crystals are present, then the excitons should appear stationery when they reflect the light back.
“A group of theoretical physicists led by Eugene Demler of Harvard University, who is moving to ETH this year, had calculated theoretically how that effect should show up in the observed excitation frequencies of the excitons – and that’s exactly what we observed in the lab,” says Ataç Imamoğlu, lead author of the study.
The research was published in the journal Nature.
Source: ETH Zurich
