Eugene Wigner
in the year 1934 theorized about a crystal solely made up of electrons. Nearly,
eighty seven years later, two mutually exclusive groups of physicists published
their recent experimental observation regarding the Wigner crystal in the
scientific journal Nature. One of the
team led by Ataç Imamoğlu, professor at the Institute for Quantum Electronics
at ETH Zurich and the other led by Hongkun Park at Harvard University came up
with these special crystals independently.
“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 (electron that arrange themselves in regular,
crystal-like patterns because of their mutual electrical repulsion), experts would 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.
How the two
teams achieved these crystals?
The Harvard
University group found these crystals by accident as they were actually
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 proceeded with cooling this semiconductor sandwich to
below −230 degrees Celsius and played around with the number of electrons in
each of the layers. They observed that when there was a specific number of
electrons in each layer, they all stood mysteriously still. But this behavior
happened only when the number of electrons in each layer was such that the top
and bottom crystal grids aligned: Smaller triangles in one layer had to exactly
fill up the space inside bigger ones in the other. Eventually they theorized
with the old idea of Wigner’s, Wigner had calculated that electrons in a flat
two-dimensional material would assume a pattern similar to a floor perfectly
covered with triangular tiles. This crystal would stop the electrons from moving
entirely.
The Harvard team made the crystal melt by forcing the
electrons to embrace their quantum wave nature. Wigner crystal melting is a
quantum phase transition — one that is similar to an ice cube becoming water,
but without any heating involved. The researchers blasted the semiconductor layers with laser
light to create a particle-like entity called an exciton. The material would
then reflect or re-emit that light. By analyzing the light, researchers could
tell whether the excitons had interacted with ordinary free-flowing electrons,
or with electrons frozen in a Wigner crystal.
The second
research team, led by Ataç Imamoğlu at the Swiss Federal Institute of
Technology Zurich, also used this technique to observe the formation of a
Wigner crystal.
Going
forward, the Harvard team plans on using their system to answer outstanding
questions about Wigner crystals and strongly correlated electrons. One open
question is what happens, exactly, when the Wigner crystal melts; competing
theories abound. Additionally, the team observed Wigner crystals in their
semiconductor sandwich at higher temperatures and for larger numbers of
electrons than theorists predicted. Investigating why this was the case could
lead to new insights about strongly correlated electron behavior.
