Electrons Frozen Yet Free: A Quantum Breakthrough in Graphene

By twisting layers of graphene, researchers discovered a unique electronic crystal where electrons freeze in place yet allow current to flow along the edges without resistance.

This behavior is dictated by topology, much like a Möbius strip’s one-sided surface. The finding could have major implications for quantum computing.

Unveiling a New Quantum State in Graphene

Researchers from the University of British Columbia, the University of Washington, and Johns Hopkins University have discovered a new class of quantum states in specially engineered graphene. Their study, published in Nature, reveals the existence of topological electronic crystals in a unique material called twisted bilayer–trilayer graphene. This system is created by stacking ultra-thin graphene layers with a precise rotational twist, fundamentally altering their electronic behavior.

“The starting point for this work is two flakes of graphene, which are made up of carbon atoms arranged in a honeycomb structure. The way electrons hop between the carbon atoms determines the electrical properties of the graphene, which ends up being superficially similar to more common conductors like copper,” said Prof. Joshua Folk, a member of UBC’s Physics and Astronomy Department and the Blusson Quantum Matter Institute (UBC Blusson QMI).

Moiré Patterns and the Transformation of Electron Motion

“The next step is to stack the two flakes together with a tiny twist between them. This generates a geometric interference effect known as a moiré pattern: some regions of the stack have carbon atoms from the two flakes directly on top of each other, while other regions have the atoms offset,” Folks said.

“When electrons hop through this moiré pattern in the twisted stack, the electronic properties are totally changed. For example, the electrons slow way down, and sometimes they develop a twist in their motion, like the vortex in the water at the drain of a bathtub as it is draining out.”

A Surprising Discovery in the Lab

The breakthrough discovery reported in this study was observed by an undergraduate student, Ruiheng Su, from UBC, studying a twisted graphene sample prepared by Dr. Dacen Waters, a postdoctoral researcher in the lab of Prof. Matthew Yankowitz at the University of Washington. While working on the experiment in Folk’s lab, Ruiheng discovered a unique configuration for the device where the electrons in the graphene froze into a perfectly ordered array, locked in place yet twirling in unison like ballet dancers gracefully performing stationary pirouettes. This synchronized rotation gives rise to a remarkable phenomenon where electric current flows effortlessly along the edges of the sample while the interior remains insulating because the electrons are immobilized.

Electrons That Dance Yet Stay Frozen

Remarkably, the amount of current that flows along the edge is precisely determined by the ratio of two fundamental constants of nature—Planck’s constant and the charge of the electron. The precision of this value is guaranteed by a property of electron crystal known as topology, which describes the properties of objects that remain unchanged by modest deformations.

“Just as a donut cannot be smoothly deformed into a pretzel without first cutting it open, the circulating channel of electrons around the boundary 2D electron crystal remains undisturbed by disorder in its surrounding environment,” said Yankowitz.

“This leads to a paradoxical behavior of the topological electronic crystal not seen in conventional Wigner crystals of the past—despite the crystal forming upon freezing electrons into an ordered array, it can nevertheless conduct electricity along its boundaries.”

The Möbius Strip Connection

An everyday example of topology is the Möbius strip—a simple yet mind-bending object. Imagine taking a strip of paper, forming it into a loop, and taping the ends together. Now, take another strip, but before joining the ends, give it a single twist. The result is a Möbius strip, a surface with just one side and one edge. Amazingly, no matter how you try to manipulate the strip, you cannot untwist it back into a normal loop without tearing it apart.

The rotation of the electrons in the crystal is analogous to the twist in the Möbius strip, and leads to the remarkable characteristic of the topological electronic crystal never before seen in the rare cases where electron crystals have been observed in the past: edges where electrons flow without resistance, describe being locked in place within the crystal itself.

A Future Path Toward Quantum Computing

The topological electron crystal is not just a theoretical curiosity — it could play a key role in advancing quantum information technology. Scientists are exploring ways to combine this unique electron state with superconductivity, a step that could help create qubits for next-generation topological quantum computers.

Reference: “Moiré-driven topological electronic crystals in twisted graphene” by Ruiheng Su, Dacen Waters, Boran Zhou, Kenji Watanabe, Takashi Taniguchi, Ya-Hui Zhang, Matthew Yankowitz and Joshua Folk, 22 January 2025, Nature.
DOI: 10.1038/s41586-024-08239-6

Never miss a breakthrough: Join the SciTechDaily newsletter.
Follow us on Google and Google News.