Researchers have discovered a new type of quantum state in a specially designed graphene structure. This finding, published in the journal Nature, describes the existence of topological electronic crystals within twisted bilayer-trilayer graphene. This innovative system is created by precisely twisting stacked layers of these two-dimensional materials.
Researchers from the University of British Columbia, the University of Washington, and Johns Hopkins University have discovered a new class of quantum states in a specially designed graphene structure. Their work, appearing today in Nature, details the finding of topological electronic crystals in twisted bilayer-trilayer graphene, formed by applying an accurate rotational twist to layers of two-dimensional materials stacked together.
“The foundation of this research involves two graphene flakes, which are composed of carbon atoms organized in a honeycomb lattice. The manner in which electrons move between these atoms dictates the graphene’s electrical characteristics, making it comparable to common conductors like copper,” explained Prof. Joshua Folk, associated with UBC’s Physics and Astronomy Department and the Blusson Quantum Matter Institute (UBC Blusson QMI).
“Next, we stack these two flakes together with a slight twist, which creates a geometric interference pattern called a moiré pattern: in certain areas, carbon atoms from the stacks directly align, while in others, they are slightly offset,” Folk added.
“As electrons navigate through this moiré configuration in the twisted stack, their electronic properties undergo significant changes. For instance, the electrons slow down considerably, and sometimes their path becomes twisted, much like water swirling in a bathtub drain,” he noted.
A remarkable aspect of this study was the observation made by undergraduate Ruiheng Su from UBC, who was examining a twisted graphene sample crafted by Dr. Dacen Waters, a postdoctoral researcher in Prof. Matthew Yankowitz’s lab at the University of Washington. While experimenting in Folk’s lab, Ruiheng discovered a unique device setup where the graphene electrons formed a perfectly ordered pattern, remaining immobilized yet spinning synchronously like ballet dancers performing poised pirouettes. This synchronized motion leads to a unique situation where an electric current can flow smoothly along the edges of the sample, while the interior remains insulating due to the electrons being locked in place.
Intriguingly, the current flowing along the edge is precisely determined by the ratio of two fundamental natural constants: Planck’s constant and the electron’s charge. The accuracy of this value is ensured by a feature of the electron crystal referred to as topology, which describes properties of objects that stay consistent despite minor changes.
“Just as you cannot transform a donut into a pretzel without cutting it first, the circulating channel of electrons around the boundary of the 2D electron crystal remains unaffected by irregularities in its surroundings,” stated Yankowitz.
“This creates an unusual behavior in the topological electronic crystal, unlike anything seen in traditional Wigner crystals: even though the crystal forms when electrons freeze into order, it can still conduct electricity along its edges,” he added.
An everyday model of topology can be found in the Möbius strip—a simple yet fascinating object. If you take a strip of paper, form it into a loop, and tape the ends together, you have a basic loop. However, if you take another strip and give it one twist before taping the ends, you create a Möbius strip, a surface with just one side and one edge. Remarkably, no matter how you manipulate this strip, you cannot revert it to a regular loop without tearing it apart.
The electron rotation within the crystal parallels the twist in the Möbius strip, leading to the unique feature of the topological electronic crystal that hasn’t been observed in other rare instances of electron crystals: the edges allow electron flow without resistance, while the electrons remain stationary within the crystal.
This topological electron crystal is not only intriguing conceptually, but it also paves the way for advancements in quantum information technology. This includes potential efforts to link the topological electron crystal with superconductivity, laying the groundwork for qubits in topological quantum computers.
