2D Material Hosting "Infinite-Mass" Electrons Realized for First Time... New Possibilities for Quantum Technology [Reading Science] (Full Story)

Seoul National University-Led Team Develops "Kagomene" as Thin as Graphene... Directly Observes Flat Band
Electron Density More Than 220 Times Higher Than Existing Materials... Potential for Quantum Phenomena at 280 K Remains Theoretical

A distinctive two-dimensional material has been realized in which electrons barely move and behave as if they had infinite mass. Korean researchers have confirmed the electronic properties of a kagome lattice, studied theoretically for more than 40 years, in a material just one atomic layer thick, and succeeded in controlling them with an external voltage. The findings point to the possibility of realizing new quantum phenomena at higher temperatures than before.

A schematic illustration of Kagomene, a two-dimensional material first realized by a joint research team at Seoul National University, and the properties of “infinite-mass electrons” that emerge in the material. The electrons do not actually become infinitely massive; the term describes a phenomenon in which electrons have difficulty moving at certain energy states. Provided by the research team.

Seoul National University said on October 21 that a joint research team including Professor Kang Mingu of its Department of Physics and Astronomy, Professor Cho Gilyoung of the Department of Physics at the Korea Advanced Institute of Science and Technology (KAIST), and Professor Park Jaehun of the Department of Physics at Pohang University of Science and Technology (POSTECH) had realized a new two-dimensional kagome lattice material called “Kagomene.” The team directly observed a “flat band” in which electron energy remains nearly constant even in a single atomic layer. The findings were published on October 15 in the international journal Nature Physics.

In a phone interview with The Asia Business Daily, Professor Kang said, “Most of the kagome materials studied to date have been three-dimensional materials. An important achievement of this study is that we made them as thin as graphene, down to the atomic-layer level, and realized a genuine two-dimensional kagome lattice.”

A kagome lattice is an atomic arrangement in a mesh-like pattern of repeating triangles and hexagons. In this structure, the multiple paths along which electrons travel can interfere with one another, suppressing their movement. In particular, a flat band can form, in which energy changes very little even when the momentum of the electrons changes.

In a flat band, the effective mass of electrons becomes very large. This is where the expression “electrons with infinite mass” comes from. It does not mean that the mass of the electrons themselves actually increases to infinity. When electron motion is suppressed, interactions between electrons become relatively stronger, increasing the likelihood that new quantum phenomena will emerge.

Moving beyond the limits of 3D materials... Controlling electronic states with voltage

This is not the first time a kagome lattice material has been discovered. Related research has already been conducted on various metal compounds and artificial lattices. However, it has been difficult to realize and observe ideal two-dimensional kagome properties because of the bonds between atomic layers and complex electronic structures.

Structure and properties of a two-dimensional kagome lattice material developed by a joint research team at Seoul National University. Unlike conventional kagome materials, its weak interlayer bonding allows it to be exfoliated into a monolayer, where an unusual electronic state known as a "flat band" emerges. Courtesy of Nature Physics.

The research team addressed this problem by identifying a family of materials in which weak van der Waals forces act between atomic layers. They exfoliated the material to make a single layer about 0.7 nm thick (a nanometer is one-billionth of a meter) and analyzed its electronic structure using nano-angle-resolved photoemission spectroscopy (nano-ARPES). The team confirmed that the characteristic flat band of a kagome lattice remained even in a single layer.

The team also succeeded in controlling the electronic states using an external voltage. As the voltage changed, the material switched from an insulating state, in which electricity does not flow easily, to a metallic state. When the voltage was changed further, an insulating state appeared again. The team suggested that the newly observed insulating state may be related to interactions between electrons, but further research is needed to determine the exact cause.

The significance of this study lies not only in creating a new material, but also in establishing a basis for observing and controlling electronic states that had previously been predicted only in theory.

Electron density more than 220 times higher... Realizing high-temperature quantum phenomena remains a future challenge

The density of electrons in Kagomene’s flat band was more than 220 times higher than in existing moiré superlattice-based materials. A moiré superlattice is a structure in which atomic layers are twisted at a small angle or stacked to control their electronic structure. In contrast, Kagomene forms a flat band through the atomic arrangement of the material itself, allowing electrons to be distributed more densely.

Based on this, the research team theoretically analyzed the possibility of a distinctive quantum state called the “fractional quantum anomalous Hall state.” The calculated energy gap was about 14–29 millielectronvolts (meV), larger than the approximately 1–5 meV reported for existing moiré materials used for comparison in the paper. The findings suggest that the related quantum state may be maintained at higher temperatures.

Regarding the projection of 280 K (kelvin) presented in the press release, Professor Kang explained, “A millielectronvolt is a unit of energy. When converted to a temperature scale, it corresponds to roughly 140–290 K. That is why we mentioned the possibility of reaching up to around 280 K.”

280 K is about 6.9 degrees Celsius above zero. The study points to the possibility of realizing quantum phenomena previously studied at extremely low temperatures at much higher temperatures, although 280 K is still lower than typical room temperature. Nor does the value obtained by converting the energy gap into a temperature indicate the actual operating temperature of the quantum state.

Professor Kang Mingu of the Department of Physics and Astronomy at Seoul National University (left) and Bong Hyeonggeun, a doctoral student in the department. Provided by Seoul National University.

One of the phenomena attracting the research team’s attention is the “anyon.” Anyons are distinctive quasiparticles formed collectively by electrons and may have applications in topological quantum computing, which is relatively robust against external disturbances.

Professor Kang said, “Quantum computing using anyons has the advantage of being resistant to errors, but it is very difficult to find materials in which anyons emerge.” However, errors do not disappear completely in anyon-based quantum computing, and many challenges remain to be addressed before it can be implemented and controlled in practice.

The experiments in this study confirmed a single-layer kagome lattice, a flat band, and changes in electronic states in response to voltage. Whether anyons or related quantum states actually emerge at higher temperatures has yet to be verified.

“The potential of Kagomene is boundless,” Professor Kang said. “Going forward, we plan to pioneer research into fabricating next-generation quantum devices based on Kagomene and realizing key quantum phenomena and technologies at room temperature.”

This content was produced with the assistance of AI translation services.