- NEWS
- 29 September 2021
This
is what a solid made of electrons looks like
Physicists have imaged elusive ‘Wigner
crystals’ for the first time.
This scanning tunneling microscope image of a graphene sheet reveals that a ‘Wigner crystal’ — a honeycomb arrangement of electrons — has formed inside a layered structure underneath. Credit: H. Li et al./Nature
If the conditions are
just right, some of the electrons inside a material will arrange themselves
into a tidy honeycomb pattern — like a solid within a solid. Physicists have
now directly imaged these ‘Wigner crystals’, named after the Hungarian-born
theorist Eugene Wigner, who first imagined them almost 90 years ago.
Researchers had
convincingly created Wigner crystals and measured their properties before, but
this is the first time that anyone has actually taken a snapshot of the
patterns, says study co-author Feng Wang, a physicist at the University of
California, Berkeley. “If you say you have an electron crystal, show me the
crystal,” he says. The results were published on 29 September in Nature1.
To create the Wigner crystals, Wang’s team built a device containing atom-thin layers of two similar semiconductors: tungsten disulfide and tungsten diselenide. The team then used an electric field to tune the density of the electrons that moved freely along the interface between the two layers.
In ordinary materials, electrons zoom around too quickly to be significantly affected by the repulsion between their negative charges. But Wigner predicted that if electrons travelled slowly enough, that repulsion would begin to dominate their behaviour. The electrons would then find arrangements that minimize their total energy, such as a honeycomb pattern. So Wang and his colleagues slowed the electrons in their device by cooling it to just a few degrees above absolute zero.
A mismatch between the
two layers in the device also helped the electrons to form Wigner crystals. The
atoms in each of the two semiconductor layers are slightly different distances
apart, so pairing them together creates a honeycomb ‘moiré pattern’, similar to
that seen when overlaying two grids. That repeating pattern created regions of
slightly lower energy, which helped the electrons settle down.
Graphene trick
The team used a
scanning tunnelling microscope (STM) to see this Wigner crystal. In an STM, a
metal tip hovers above the surface of a sample, and a voltage causes electrons
to jump down from the tip, creating an electric current. As the tip moves across
the surface, the changing intensity of the current reveals the location of
electrons in the sample.
Initial attempts to image the Wigner crystal by applying the STM directly on the double-layer device were unsuccessful, Wang says, because the current destroyed the fragile Wigner arrangements. So the team added a layer of graphene, a single-atom sheet of carbon, on top. The presence of the Wigner crystal slightly changed the electron structure of the graphene directly above, which was then picked up by the STM. The images clearly show the neat arrangement of the underlying Wigner electrons. As expected, consecutive electrons in the Wigner crystal are nearly 100 times farther apart than are the atoms in the semiconductor device’s actual crystals.
“I think that’s a
great advancement, being able to perform STM on this system,” says Carmen Rubio
Verdú, a physicist at Columbia University in New York City. She adds that the
same graphene-based method will enable STM studies of a number of other
interesting physical phenomena beyond Wigner crystals. Kin Fai Mak, a physicist
at Cornell University in Ithaca, New York, agrees. “The technique is
non-invasive to the state you want to probe. To me, it is a very clever idea.”
doi: https://doi.org/10.1038/d41586-021-02657-6
My two cents: Those frozen electrons reminds me of orange sections and autumnal toroidal fruit. Somewhat like the flower of life, but looks more like orange slices arranged in an impossible way, to me.
cyd
