In a examine revealed in Nature, scientists from Johns Hopkins College, the College of British Columbia, and the College of Washington have found a brand new class of quantum states in a specifically designed graphene construction: topological digital crystals in twisted bilayer–trilayer graphene, a system made by exactly rotating two-dimensional supplies stacked one on prime of the opposite.
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The start line for this work is 2 flakes of graphene, that are made up of carbon atoms organized in a honeycomb construction. The way in which electrons hop between the carbon atoms determines {the electrical} properties of the graphene, which finally ends up being superficially much like extra frequent conductors like copper.
Joshua People, Professor, Physics and Astronomy Division, College of British Columbia
“The subsequent step is to stack the 2 flakes along with a tiny twist between them. This generates a geometrical interference impact often called a moiré sample: some areas of the stack have carbon atoms from the 2 flakes immediately on prime of one another, whereas different areas have the atoms offset,” added People.
He additional added, “When electrons hop via this moiré sample within the twisted stack, the digital properties are completely modified. For instance, the electrons sluggish manner down, and typically they develop a twist of their movement, just like the vortex within the water on the drain of a tub as it’s draining out.”
The groundbreaking discovery revealed on this examine was made by Ruiheng Su, an undergraduate scholar at UBC, whereas investigating a twisted graphene pattern generated by Dr. Dacen Waters, a postdoctoral researcher in Prof. Matthew Yankowitz’s group on the College of Washington.
Ruiheng found a brand new configuration for the system whereas engaged on the experiment at People’s lab. The electrons within the graphene froze right into a flawlessly ordered array, caught in place however whirling in unison like ballet dancers gracefully performing stationary pirouettes. This synchronous rotation causes an interesting phenomenon by which electrical present flows easily alongside the pattern’s borders whereas the inside stays insulating attributable to electron immobilization.
The quantity of present flowing alongside the sting is exactly outlined by the ratio of two basic constants of nature: Planck’s fixed and the electron’s cost. This worth’s precision is assured by topology, a property of electron crystals that specifies the qualities of objects that stay intact even after minor deformations.
Simply as a donut can’t be easily deformed right into a pretzel with out first reducing it open, the circulating channel of electrons across the boundary 2D electron crystal stays undisturbed by dysfunction in its surrounding setting.
Matthew Yankowitz, Professor, Division of Physics, College of Washington
“This results in a paradoxical conduct of the topological digital crystal not seen in typical Wigner crystals of the previous—regardless of the crystal forming upon freezing electrons into an ordered array, it might probably nonetheless conduct electrical energy alongside its boundaries,” acknowledged Yankowitz.
The Möbius strip is a typical instance of topology—a easy but mind-bending object. Think about taking a strip of paper, folding it right into a loop, and taping the ends collectively. Now, seize one other strip and twist it as soon as earlier than attaching the ends. The end result is a Möbius strip, a floor with just one facet and edge. Regardless of how one tries to control the strip, untwisting it again into a standard loop with out tearing it aside is inconceivable.
The rotation of the electrons within the crystal is analogous to the twist within the Möbius strip. It leads to a outstanding characteristic of the topological digital crystal that has by no means been seen earlier than within the uncommon instances the place electron crystals have been noticed: edges the place electrons move with out resistance, indicating that they’re locked in place inside the crystal.
The topological electron crystal is just not solely fascinating conceptually, but it surely additionally opens up new avenues for breakthroughs in quantum data. Future makes an attempt to mix the topological electron crystal with superconductivity will present the premise of qubits for topological quantum computer systems.
Journal Reference:
Su, R., et al. (2025) Moiré-driven topological digital crystals in twisted graphene. Nature. doi.org/10.1038/s41586-024-08239-6
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