In 1934, theoretical physicist Eugene Wigner proposed a new type of crystal.
If the density of negatively charged electrons could possibly be maintained under a sure stage, the subatomic particles could possibly be held in a repeating sample to create a crystal of electrons; this concept got here to be often known as a Wigner crystal.
The primary time a Wigner crystal was experimentally noticed was in 1979, when researchers measured an electron-liquid to electron-crystal part transition utilizing helium; since then, such crystals have been detected quite a few times.
Getting fidgety electrons to take a seat nonetheless sufficient to create the crystals is lots simpler stated than carried out, although. Now, a workforce of physicists has demonstrated a brand new technique – trapping the wiggly little brats between a pair of two-dimensional semiconducting tungsten layers.
Typical crystals – like diamonds or quartz – are shaped from a lattice of atoms organized in a fixed, three-dimensional repeating grid structure. In accordance with Wigner’s thought, electrons could possibly be organized similarly to type a stable crystal part, however provided that the electrons had been stationary.
If the density of the electrons is low sufficient, the Coulomb repulsion between electrons of the identical cost produces potential power that ought to dominate their kinetic power, ensuing within the electrons sitting nonetheless. Therein lies the problem.
“Electrons are quantum mechanical. Even should you do not do something to them, they’re spontaneously jiggling round on a regular basis,” said physicist Kin Fai Mak of Cornell College.
“A crystal of electrons would even have the tendency to only soften as a result of it is so laborious to maintain the electrons fastened at a periodic sample.”
Makes an attempt to create Wigner crystals due to this fact depend on some kind of electron entice, equivalent to powerful magnetic fields or single-electron transistors. In 2018, MIT scientists making an attempt to create a sort of insulator might have instead produced a Wigner crystal, however their outcomes left room for interpretation.
MIT’s entice was a graphene construction often known as a moiré superlattice, the place two two-dimensional grids are superimposed at a slight twist and bigger common patterns emerge, as seen within the instance picture above.
Now the Cornell workforce, led by physicist Yang Xu, has used a extra focused method with their very own moiré superlattice. For his or her two semiconducting layers, they used tungsten disulfide (WS2) and tungsten diselenide (WSe2) specifically grown at Columbia College.
When overlaid, these layers produced a hexagonal sample, permitting the workforce to manage the typical electron occupancy at any particular moiré website.
The subsequent step was to fastidiously place electrons in particular locations within the lattice, utilizing calculations to find out the occupancy ratio at which totally different preparations of electrons will type crystals.
The ultimate problem was how one can really see if their predictions had been appropriate, by observing the Wigner crystals or lack thereof.
“You’ll want to hit simply the fitting situations to create an electron crystal, and on the identical time, they’re additionally fragile,” Mak said.
“You want a great way to probe them. You do not actually wish to perturb them considerably whereas probing them.”
This downside was solved with insulating layers of hexagonal boron nitride. An optical sensor was positioned very near (however not touching) the pattern, at a distance of only one nanometre, separated by a boron nitride layer. This prevented electrical coupling between the sensor and the pattern, whereas sustaining sufficient proximity for prime detection sensitivity.
This association allowed the workforce to probe the pattern cleanly, they usually made their detection. Inside the moiré superlattice, electrons organized into a wide range of crystal configurations, together with triangular Wigner crystals, stripe phases and dimers.
This achievement would not simply have implications for learning electron crystals. The findings show the untapped potential of moiré superlattices for quantum physics analysis.
“Our research,” the researchers wrote in their paper, “lays the groundwork for utilizing moiré superlattices to simulate a wealth of quantum many-body issues which might be described by the two-dimensional prolonged Hubbard mannequin or spin fashions with long-range charge-charge and alternate interactions.”
The analysis has been printed in Nature.
Editor’s observe (13 Nov 2020): An earlier model of this text incorrectly implied that this was the primary time Wigner crystals have been created and noticed. We’ve got corrected this and apologise for the error.