2D Material with "Infinite-Mass" Electrons Realized for First Time... New Possibilities for Quantum Technology [Reading Science]

Kagomene monolayer realized after more than 40 years of research... Unusual electronic state confirmed
Electron density more than 220 times higher than in existing materials... Hopes rise for high-temperature quantum phenomena

In contrast to graphene, which has electrons that move as if they have no mass, researchers have realized a new two-dimensional material in which electrons behave as if they are infinitely heavy. The discovery confirms in a real material an unusual electronic state that has been studied theoretically since the 1980s. It raises the possibility of realizing some quantum phenomena, previously observed only at ultralow temperatures, at higher temperatures. However, further verification will be needed before quantum computers can be developed that operate at room temperature.

Seoul National University said on October 21 that a joint research team led by Professor Kang Mingu of its Department of Physics and Astronomy had, for the first time in the world, realized a two-dimensional material called Kagomene, which has the properties of “electrons with infinite mass.” Researchers from South Korea and abroad also took part, including teams led by Professor Jo Gilyoung of the Korea Advanced Institute of Science and Technology (KAIST) and Professor Park Jaehun of Pohang University of Science and Technology (POSTECH).

A schematic illustrating Kagomene, a two-dimensional material realized for the first time in the world by a Seoul National University research team, and the properties of “infinite-mass electrons” that emerge in the material. The electrons do not actually gain infinite mass; the term describes a phenomenon in which they become difficult to move at certain energy states. Provided by the research team.

The key to Kagomene lies in how its electrons move. In general, electrons move through a material depending on their energy. In Kagomene, however, a “flat band” appears, in which the electrons’ energy changes very little with their momentum. In this state, electrons find it difficult to move and behave as if they have infinite mass. Their actual mass does not become infinite.

This property strengthens interactions between electrons, creating conditions in which new quantum phenomena, such as superconductivity and unusual magnetism, may emerge. However, it has been difficult to achieve an ideal two-dimensional state in conventional kagome lattice materials because of the bonds between atomic layers and their complex electronic structures.

The research team identified a new kagome lattice structure in a van der Waals material, a class of materials with weak bonds between atomic layers. After isolating it as a single-atom-layer-thick sheet, the team analyzed the electrons’ energy structure using nano-angle-resolved photoemission spectroscopy (nano-ARPES) at a fourth-generation synchrotron radiation facility. The researchers directly confirmed that the flat band persists even in a monolayer.

In particular, the electron density that the flat band can accommodate was more than 220 times higher than in existing moire superlattice-based materials. This showed the potential for quantum phenomena driven by interactions between electrons to emerge at higher energies and temperatures, as the electrons can pack into a much smaller space.

Potential for High-Temperature Quantum Phenomena Beyond Ultralow Temperatures... Realization Remains a Future Challenge

The research team expects that the material could make it possible to realize unusual quantum states at higher temperatures than before. One example is an anyon. Anyons are quasiparticles in two-dimensional quantum systems with exchange properties unlike those of ordinary electrons or photons, and are being studied as candidates for implementing fault-tolerant quantum computing.

The team used theoretical calculations to analyze the possibility that a fractional quantum anomalous Hall state could emerge in Kagomene-family materials. The calculations suggested that the energy gap of this quantum state could be larger than in existing moire-based materials, potentially allowing it to remain stable at higher temperatures.

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

Seoul National University said that quantum phenomena previously observed only below 2.5 K (about -270.7 degrees Celsius) could potentially be realized in Kagomene at temperatures above 280 K (about 6.9 degrees Celsius). However, this remains a theoretical prospect; the research team has not actually realized anyons or quantum computing at that temperature.

Bong Hyeonggeun, a combined master's and doctoral student at Seoul National University and the first author of the study, said, “Our research team has unique expertise in South Korea in nano-angle-resolved photoemission spectroscopy based on a recently developed fourth-generation synchrotron radiation facility. This enabled us to directly measure electrons with infinite mass.”

Professor Kang Mingu of Seoul National University said, “Kagomene has limitless potential. We plan to pioneer research into fabricating next-generation quantum devices based on Kagomene and realizing key quantum phenomena and technologies at room temperature.”

The findings were published on October 15 in the international journal Nature Physics.

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