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8 HEALTH AND LABORATORY MAGAZINE

A MATERIAL MADE

ENTIRELY OUT OF

ELECTRONS

No camera could have THIS IS THE
possibly imaged the crys- FIRST TIME
tal, so the scientists had
to get creative. Instead, SOMEONE
they managed to get their CLAIMING TO
snapshot using a scan-
ning tunneling micro- MAKE A WIGNER
scope (STM), which uses a CRYSTAL
probe to detect the exact
location of the gridlocked PRESENTED
electrons by observing how PROOF.
they respond to a tiny jolt
of electricity. It’s a move
that other scientists un- lished on 29 September in
related to the experiment Nature.
was particularly clever. travelled slowly enough, distances apart, so pairing structure of the graphene
To create the Wigner that repulsion would begin them together creates a directly above, which was
“I think that’s a great crystals, Wang’s team to dominate their behav- honeycomb ‘moiré pattern’, then picked up by the STM.
advancement, being able built a device containing iour. The electrons would similar to that seen when The images clearly show
to perform STM on this atom-thin layers of two then find arrangements overlaying two grids. That the neat arrangement
system,” Columbia Univer- similar semiconductors: that minimize their total repeating pattern created of the underlying Wigner
sity physicist Carmen Rubio tungsten disulfide and energy, such as a honey- regions of slightly lower electrons. As expected,
said. tungsten diselenide. The comb pattern. So Wang energy, which helped the consecutive electrons in
team then used an electric and his colleagues slowed electrons settle down. the Wigner crystal are
THIS IS WHAT A SOLID field to tune the density of the electrons in their device nearly 100 times farther
MADE OF ELECTRONS the electrons that moved by cooling it to just a few GRAPHENE TRICK apart than are the atoms in
LOOKS LIKE freely along the interface degrees above absolute the semiconductor device’s
between the two layers. zero. The team used a scanning actual crystals.
If the conditions are just tunnelling microscope
right, some of the elec- In ordinary materials, A mismatch between the (STM) to see this Wign- “I think that’s a great
trons inside a material will electrons zoom around too two layers in the device er crystal. In an STM, a advancement, being able
arrange themselves into quickly to be significantly also helped the electrons metal tip hovers above the to perform STM on this
a tidy honeycomb pattern affected by the repulsion to form Wigner crystals. surface of a sample, and a system,” says Carmen
— like a solid within a solid. voltage causes electrons Rubio Verdú, a physicist
Physicists have now direct- to jump down from the tip, at Columbia University in
ly imaged these ‘Wigner creating an electric current. New York City. She adds
crystals’, named after the As the tip moves across that the same graphene-
Hungarian-born theorist the surface, the changing based method will enable
Eugene Wigner, who first intensity of the current STM studies of a number of
imagined them almost 90 reveals the location of other interesting physical
years ago. electrons in the sample. phenomena beyond Wign-
er crystals. Kin Fai Mak, a
Researchers had con- Initial attempts to image physicist at Cornell Uni-
vincingly created Wigner the Wigner crystal by versity in Ithaca, New York,
crystals and measured applying the STM directly agrees. “The technique is
their properties before, but on the double-layer device non-invasive to the state
this is the first time that were unsuccessful, Wang you want to probe. To me, it
anyone has actually taken says, because the cur- is a very clever idea.”
a snapshot of the patterns, rent destroyed the fragile
says study co-author Feng This scanning tunneling microscope image of a graphene Wigner arrangements. So Nature 598, 21 (2021)
Wang, a physicist at the sheet reveals that a ‘Wigner crystal’ — a honeycomb the team added a layer
University of California, arrangement of electrons — has formed inside a layered of graphene, a single-at-
Berkeley. “If you say you structure underneath. Credit: H. Li et al./Nature om sheet of carbon, on
have an electron crystal, between their negative The atoms in each of top. The presence of the
show me the crystal,” he charges. But Wigner the two semiconductor Wigner crystal slightly
says. The results were pub- predicted that if electrons layers are slightly different changed the electron