Seoul Team Peels a Single Kagome Layer Whose Electrons Act as if Infinitely Heavy

Physicists at Seoul National University and partner institutions have made a one-layer-thick crystal in which electrons behave as if they have infinite mass, a long-sought setting for exotic quantum physics. The material, reported in Nature Physics, is a two-dimensional kagome lattice that keeps a tunable flat electronic band even as a single layer.

A kagome lattice is a pattern of triangles sharing corners with hexagonal gaps between them, named for a traditional Japanese basket weave. Its geometry can trap electrons in a flat band, a set of states with almost identical energies. "As opposed to a hexagonal lattice, the kagome lattice can host flat band electrons, which behave as if they have an extremely large effective mass," said Mingu Kang, the paper's senior author. In graphene's hexagonal lattice, by contrast, electrons behave as if they have no mass.

Flat bands are attractive because electrons in them interact strongly and can give rise to unconventional superconductivity and fractional quantum states. The problem, Kang said, is that most known kagome materials are three-dimensional, where coupling between layers and other complications "obscure the underlying kagome physics." The team wanted an ideal single-layer system with an isolated flat band, which they named kagomene.

They found it by brute-force search. The group built a database of roughly 183,000 materials, about 5,300 of which contain kagome lattices. They screened for compounds whose layers are held together by weak van der Waals forces, then used density functional theory calculations to check which candidates should peel apart. They synthesized the most promising ones and tested whether they could be exfoliated into atomically thin layers by standard mechanical methods. The winner was a new van der Waals kagome compound with the formula A2T3X4.

To see the electrons, the researchers used nano-ARPES, a technique that measures the electrons knocked out of a material by a tightly focused beam of light. The maps showed flat-band states near the Fermi level, the energy that sets which states are filled, in some of the thin samples. The team also demonstrated gate-engineering, using electric fields to tune how full the flat band is.

The headline number is density. "Our 2D kagome-based flat band system achieves more than two orders of magnitude higher flat band electron density compared to that of existing moiré superlattice-based flat band systems," Kang said, citing about 1 per square nanometer versus about 0.01. Moiré systems such as twisted graphene need artificial large superlattices to create flat bands. Here, he explained, the flat band "intrinsically arises from this unique lattice geometry."

The work is a materials discovery, not yet a demonstration of a new quantum phase. But a flat band this dense, in a single layer that can be gated, gives experimenters a cleaner platform for hunting superconductivity and fractional states that twisted systems can only reach at far lower electron density.