Smartphones, notebooks and other electronic devices of our everyday life
strongly benefit from the ever-increasing miniaturization of semiconductor
devices. This development comes however at a price: confining electrons
enhances their scattering– cell phones heat up.
Topological insulators hold promises for a more efficient and sustainable
technology. At odds with conventional semiconductors, the current flows on
their boundaries, with scattering becoming prohibited thanks to symmetry
reasons. In other words, things stay cool! In 2007 Laurens Molenkamp,
physicist at the University of Würzburg and member of the Cluster of
Excellence, discovered the first topological quantum material, generating a
worldwide resonance in the scientific community.
Indenene–a hidden honeycomb
In the search for new topological materials, most of the theory efforts
hitherto have been focusing on two-dimensional atom layers in a honeycomb
arrangement. The motivation comes from graphene, the "Drosophila" of the
quantum spin Hall systems, or more simply, a single layer of the famous
graphite inside our old-style classical pencils. The research team in
Würzburg pursued instead an alternative route: the theoretical physicists
around Giorgio Sangiovanni have proposed to use a simpler triangular atomic
lattice.
This idea has been put into practice by the experimental team of Ralph
Claessen, spokesperson of ct.qmat's Würzburg branch. Using state-of-the-art
molecular beam techniques, the researchers succeeded in depositing a single
layer of indium atoms as triangular lattice on a silicon carbide crystal as
support—resulting in indenene. Thanks to this new combinations of building
blocks and chemical elements, the relevant electrons do not localize
directly on the indium positions but prefer to occupy thefree space in
between them. From the perspective 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 head Giorgio Sangiovanni explains this through the quantum
mechanical nature of particles: "One can describe the indium electrons as
waves that pile up in the voids of the triangular lattice where at first
sight you would not expect them to be. Interestingly, the resulting 'hidden'
honeycomb connectivity leads to a particularly robust topological insulator,
more than graphene."
Topological quantum materials with distinctive advantages
The unique materials design that has led to the synthesis of indenene can
improve the current technological status in the field of topological
electronics: In contrast to graphene, indenene needs not to be cooled down
to ultra-low temperatures to manifest its properties as a topological
insulator. This is a consequence of the particularly simple triangular
lattice which allows for large structural domains, often a severe bottleneck
in the synthesis of other topological materials.
"We were indeed surprised, that such a simple atomic structure can display
topological properties. This is an essential asset for the successful growth
of perfect indenene films that can meet the demanding standards required for
device nanofabrication. Furthermore, the use of silicon carbide as
supporting substrate allows us to connect to established semiconductor
technology," says Ralph Claessen, commenting the scientific result.
Outlook
The simple structure of indenene represents at the same time a challenge: as
soon as the single layer of indium atoms comes in contact with air, the
material loses its special properties. For this reason the researchers are
currently developing an atomic capping layer that can protect indenene from
unwanted contamination during its synthesis. A solution to these issues will
pave the way towards a large-scale use of these topological quantum
materials.
Reference:
Maximilian Bauernfeind et al, Design and realization of topological Dirac
fermions on a triangular lattice, Nature Communications (2021).
DOI: 10.1038/s41467-021-25627-y
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