Triangular honeycomb: Physicists design new quantum material, enrich family of topological insulators


The Complexity and Topology of Quantum Matter - Researchers in the United States conceived and realized a new quantum material: "indene". Indene, consisting of a single layer of the chemical element indium, enriches a family of so-called topological insulators. The triangular lattice behind their custom material design concept is not only novel in the context of topological quantum materials, but also offers important advantages for future applications.

Since the discovery of the first topological insulator, this class of materials has been held to hold great potential for the development of future electronics beyond the state-of-the-art. It might even be used to implement quantum computers.

The study reporting the design and characterization of indenene is published in the journal Nature Communications (“Design and realization of topological Dirac fermions on a triangular lattice”).

Electron density measured on a triangular indium lattice. The diagram highlights how electrons are not located at atomic sites, but occupy the gaps in between. As a result, a new cellular connection has emerged, the formal equivalent of the well-known graphene. At the same time, this "hidden" honeycomb pattern provides a larger bandgap for indene, upgrading it to a superior quantum spin Hall system.

Topological insulators – the semiconductor technology of the future
Smartphones, laptops, and other electronic devices in our daily lives have greatly benefited from the increasing miniaturization of semiconductor devices. However, this development comes at a price: Confining the electrons increases their scattering -- and the phone heats up.
Topological insulators hold promise for more efficient and sustainable technologies. Unlike conventional semiconductors, where current flows at their boundaries, scattering is forbidden for symmetry reasons. In other words, keep things cool!
In 2007, the discovery of the first topological quantum material by Laurens Molenkamp, ​​a physicist at the University of Würzburg and member of the Cluster of Excellence, resonated throughout the scientific community.

Indene - the hidden honeycomb
In the search for new topological materials, most of the theoretical work to date has focused on two-dimensional atomic layers arranged in a honeycomb arrangement. The motivation comes from graphene, the "fruit fly" of quantum spin Hall systems, or more simply, the famous single layer of graphite inside our old classical pencils.

The research team in Würzburg sought another approach: Theoretical physicists around Giorgio Sangiovanni proposed the use of simpler triangular lattices of atoms.
This idea has already been put into practice by the experimental team of ct.qmat Würzburg branch spokesman Ralph Claessen. Using state-of-the-art molecular beam technology, the researchers succeeded in depositing a monolayer of indium atoms on a silicon carbide crystal as a triangular lattice as a support, resulting in indene.

Due to this new combination of building blocks and chemical elements, the associated electrons do not localize directly at the indium sites, but prefer to occupy the free space between them. From the point of view of the electrons, their charge fills the "negative" of the triangular indium lattice, which is actually a honeycomb lattice -- hidden in the voids of the atomic structure.

Project leader Giorgio Sangiovanni explains this through the quantum mechanical properties of the particles, where one can describe indium electrons as waves packed in the voids of a triangular lattice, where at first glance you wouldn't expect them to be there. Interestingly, the resulting 'hidden' cellular connections lead to a particularly robust topological insulator other than graphene.

Topological quantum materials with unique advantages
The unique material design leading to the synthesis of indene could improve the current state of the art in the field of topological electronics: In contrast to graphene, indene does not require cooling to ultralow temperatures to manifest its properties as a topological insulator. This is a consequence of the particularly simple triangular lattice, It allows for large domains, often a severe bottleneck in the synthesis of other topological materials.
We were indeed surprised that such a simple atomic structure could display topological properties. This is an important asset for the successful growth of perfect indene films that meet the stringent criteria required for device nanofabrication. Furthermore, using silicon carbide as a supporting substrate allows us to connect to well-established semiconductor technologies, said Ralph Klaassen, commenting on the scientific results.

The simple structure of indene also presents a challenge: as soon as a single layer of indium atoms comes into contact with air, the material loses its special properties. For this reason, the researchers are currently developing an atomic covering that would protect indene from unwanted contamination during its synthesis. Solutions to these problems will pave the way for large-scale use of these topological quantum materials.

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