A funny thing happened when Sergio de la Barrera and his colleagues started playing around with sheets of graphene – they created a new type of “quasicrystal” with fascinating properties that could help scientists explore exotic kinds of physics.
Layering two sheets of graphene atop one another, and then rotating them at a slight angle creates a unique pattern called a moiré superlattice that can exhibit interesting properties, such as becoming an insulator or a superconductor depending on how it is treated. When de la Barrera’s team added a third layer rotated at another angle they found that the two stacked moiré patterns interfered with each other in way that created a rare material called a quasicrystal.
“We weren’t trying to create quasicrystals, but we saw phenomena in the data that didn’t make sense and realized what it meant,” says de la Barrera, now at the University of Toronto.
Quasicrystals are a family of materials with a unique atomic structure somewhere between the ordered lattice of a crystal and the random arrangement of something like glass. Their pattern is quasiperiodic – meaning it looks like it is going to repeat, but then doesn’t, says Michael Widom, a condensed matter physicist at Carnegie Mellon University.
The most common quasicrystals are metal alloys, such as aluminum-manganese, but they also turn up in many other unexpected places, including hydrodynamic patterns in liquids, soft matter like colloids and polymers, and even interference patterns in light.
While quasicrystals do have some practical applications in hardening or strengthening metals, that part of the field hasn’t really flourished, says Widom. More interesting, he says, is how they have advanced scientific understanding, sparking a revolution in the fields of crystallography and metallurgy.
Widom says de la Barrera’s work is “a beautiful example of the convergence of different branches of science” – combining quasicrystals with graphene, one of the most active research areas in physics in recent years. “They have come together to produce something novel and intriguing,” he says.
By adjusting the twist angles between the three layers, the researchers were able to create quasicrystals with a variety of properties. Some were superconductors that also showed strong interactions between electrons, which was a surprising find, according to de la Barrera – there are not many other examples of quasicrystals that are superconductors, or many that show strong electronic interactions. “That’s a regime where you will find lots of interesting new electronic phases because the electrons are interacting with each other and can do more complex things that just conduct current,” he says.
De la Barrera is not sure that the discovery will have any near-term practical applications, though it could eventually help in the development of more efficient electronics. “The more we understand superconductivity and how it works, the easier it will be to design useful superconductors for everyday applications,” he says.
More immediately, the moiré quasicrystals provide a platform for learning more about the field. “Because the system is so engineerable we can use it to study how quasicrystals behave, and how electron interactions are affected by the quasicrystal,” says de la Barrera.
That is also what appeals to Widom. “It’s exciting because it’s going to need further study to understand,” he says. “And a scientist always likes to see that.”