Of oranges and donuts: TU Chemnitz scientists investigate reversible switching of the quantum spin Hall insulator bismuthene

Chemnitz University of Technology research team investigates the synthesis and properties of bismuthene, a two-dimensional honeycomb structure made of bismuth, at the interface between graphene and silicon carbide - publication in renowned journal “Nature Communications”

Scientists from the Professorships of Experimental Physics with focus Technical Physics (Head: Prof. Dr. Thomas Seyller) and Theoretical Physics of Quantum Mechanical Processes and Systems (Head: Prof. Dr. Sibylle Gemming) at Chemnitz University of Technology are investigating the functionalization of low-dimensional electron gases as part of the research unit “Proximity-induced correlation effects in low-dimensional structures (FOR 5242)” (Spokesperson: Prof. Dr. Christoph Tegenkamp).

In their latest publication in the renowned journal “Nature Communications”, the research team led by Dr. Philip Schädlich, scientific associate at the Chair of Experimental Physics with focus Technical Physics, has demonstrated the synthesis of bismuthene, protected by graphene, in close cooperation with the Peter Grünberg Institute at Forschungszentrum Jülich. The synthesis is based on the process of intercalation - the introduction of bismuth atoms at the interface between graphene and the substrate material silicon carbide. However, this initially produces an electronically inactive “precursor” layer of bismuth atoms, which can be reversibly activated by additional intercalation of hydrogen to the quantum spin Hall insulator bismuthene.

The position is crucial

For a long time, the hydrogen-induced “switching on” of the quantum material was a mystery to researchers, but it is now clear: "The adsorption site, i.e. the position of the bismuth atoms in relation to the substrate, plays a decisive role. While in the “precursor” state each bismuth atom has bonds to three atoms of the substrate, in the bismuthene state it is only one atom," explains Niclas Tilgner, who played a key role in advancing the study as a PhD student. In this way, the characteristic in-plane bonds can form the honeycomb structure of bismuthene.

The solution was found with the help of the synchrotron-based measurement method of “X-ray standing wave imaging”, which the researchers used at the Diamond Light Source in Didcot, UK. The partners from Jülich are proven experts in this field. Prof. Dr. Christian Kumpf, group leader at Forschungszentrum Jülich, explains: "In this measurement technique, the superposition of incident and diffracted X-rays forms a standing wave whose phase can be varied via the photon energy used. In this way, photoelectrons are preferentially emitted from certain areas of the unit cell, enabling the atomic structure to be determined element-specifically and with a spatial resolution of less than a hundredth of a nanometer."

In this study, the researchers funded by the German Research Foundation (DFG) are also relying on a combination of experimental data and results from density functional theory (DFT). "The collaboration of partners from both experimental and theoretical physics makes it possible to reliably describe the complexity of such a system. Experimental structural data enables the modeling of the band structure, which in turn helps to interpret the results of photoelectron spectroscopy," says Dr. Philip Schädlich.

Topologically protected edge channels have the potential for dissipation-less current flow

With their research results, the scientists are making an important contribution to a highly topical issue in solid-state physics: the question of whether all materials with a band gap - i.e. electrical insulators - exhibit the same quantum physical properties as the vacuum - i.e. a state without any conductive structure. The surprising answer is: no. Because there is a whole class of new materials that behave completely differently despite their band gap - so-called topological insulators. Like ordinary insulators, these also have a band gap in their bulk and therefore do not conduct electricity. However, an astonishing effect occurs at their surfaces or edges - conductive channels are created here in which electrons can flow without dissipation. These edge channels are robust against perturbations such as impurities or small defects. They are therefore referred to as topologically protected states.

“Topology is not about shapes, but about the basic structure - for example, how many holes an object has,” explains Niclas Tilgner. An orange, for example, has zero holes, whereas a donut has one. This number - known as the genus - cannot be changed without fundamentally restructuring the object. In solid-state physics, there is a similar distinction between ordinary and topological insulators. When a material changes from one type to the other - metaphorically speaking from a donut to an orange - its band structure must change. This creates a transition region in which electrons can suddenly flow freely: the metallic edge channel. Quantum spin Hall insulators such as bismuthene are particularly fascinating. Their conductive edge channels are not only stable, but also spin-polarized: In this case, the electron spin determines the direction of movement of the electrons. These properties open up far-reaching prospects for current research in electronics and quantum physics.

Background: DFG research unit “Proximity-induced correlation effects in low dimensional structures” under the leadership of Chemnitz University of Technology

Phenomena such as the one described above are at the heart of the DFG research unit headed by Prof. Dr. Tegenkamp. The research unit, which has received over four million euros in funding, is dedicated to investigating correlation effects in 2D materials and is now looking forward to a second funding period. The objective is to manipulate 2D materials in a targeted manner in order to investigate exotic effects such as superconductivity, charge density waves, Mott states, the quantum Hall effect and Klein tunneling.

Publication: Niclas Tilgner, Christian Kumpf, Philip Schädlich et al: Reversible Switching of the environment-protected quantum spin Hall insulator bismuthene at the graphene/SiC interface, Nature Communications (2025).

DOI: https://doi.org/10.1038/s41467-025-60440-x

For further information, please contact Dr. Philip Schädlich, e-mail philip.schaedlich@physik.tu-chemnitz.de.

(Authors: Niclas Tilgner, Dr. Philip Schädlich, Christian Kumpf)

Mario Steinebach
28.07.2025

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