In 1934, theoretical physicist Eugene Wigner proposed the existence of a new type of crystal.
If the density of negatively charged electrons could be kept below a certain level, subatomic particles could be held in a repeating pattern, creating an electronic crystal; this idea became known as the Wigner crystal.
Much easier said than done, however. Electrons are fussy and it is extremely difficult to get them to stay in place. However, a group of physicists have now achieved this – by enclosing wiggling little hairpins between a pair of 2D semiconducting tungsten layers.
Common crystals such as diamonds or quartz are formed from a lattice of atoms forming a fixed, three-dimensional, repeating network structure. According to Wigner's idea, the electrons could be arranged in a similar way to form a solid crystalline phase, but only if the electrons were stationary.
If the electron density is low enough, the Coulomb repulsion between electrons with the same charge creates a potential energy that must dominate the kinetic energy, leaving the electrons stationary. This is the difficulty.
'Electrons are quantum mechanical. Even if you don't do anything with them, they spontaneously hesitate all the time, '' said physicist Keen Fay Mak from Cornell University.
“A crystal of electrons would actually have a tendency to melt because it is so difficult to keep electrons fixed in a periodic structure.”
Therefore, attempts to create Wigner crystals rely on a kind of electron trap, such as powerful magnetic fields or single-electron transistors, but physicists have not yet succeeded in complete crystallization. In 2018, MIT scientists trying to create a type of insulator instead created a Wigner crystal, but their results left room for interpretation.
(UCSD Department of Physics).
The trap of MIT was a graphene structure known as a moire superlattice, where two two-dimensional grids overlap each other with a slight twist and larger regular patterns appear, as shown in the image above.
Now, Cornell's team, led by physicist Yang Xu, has taken a more targeted approach with their own moiré superlattice. For their two semiconductor layers, they used tungsten disulfide (WS2) and tungsten diselenide (WSe2) specially grown at Columbia University.
When superimposed, these layers formed a hexagonal pattern, which allowed scientists to control the average electron mobility in any given moiré area.
The next step was to carefully place the electrons at specific locations on the lattice, using calculations to determine the degree of filling at which the various electron locations would form crystals.
The last problem was how to actually see if their predictions are correct by observing Wigner crystals or their absence.
“To create an electronic crystal, you need to create the right conditions, and at the same time, they respond to external influences,” said Mack.
'You need a good way to research them. Don't bother them too much by examining them. '
This problem was solved by using insulating layers of hexagonal boron nitride. The optical sensor was placed very close (but not touching) the sample, at a distance of only one nanometer, separated by a boron nitride layer. This prevented electrical communication between the probe and the sample while maintaining sufficient proximity for high detection sensitivity.
Inside a moire superlattice, electrons are arranged in various crystal configurations, including triangular Wigner crystals, stripe phases, and dimers.
This achievement is important not only for the study of electronic crystals. The data obtained demonstrate the untapped potential of moiré superlattices for research in the field of quantum physics.
“Our study,” the researchers wrote in their paper, “lays the foundation for the use of moiré superlattices to model quantum multi-body problems, which are described by the two-dimensional extended Hubbard model or spin models with long-range charge – charge and exchange interactions.”
The research is published in the journal Nature.
Sources: Photo: Isolating states in a superlattice that houses electrons. (Xu et al., Nature, 2020).
