Rhombohedral graphite as a model for quantum magnetism
Rhombohedral graphite (right) consists of staggered graphene layers. © 10.1126/sciadv.abo6879
The surface state of RG is visualized as red spheres centered on the carbon atoms in the top graphene layer. The size of the spheres is proportional to the density of electrons on the carbon atoms. It can also be called a 2D electron system.
Graphene is an extremely exciting material. Now a graphene variant shows another talent: rhombohedral graphite made of several layers slightly offset from each other could enlighten the hidden physics in quantum magnets.
Graphene materials are made of carbon atoms only, the basic shape is a honeycomb structure. But there are many variants with amazingly versatile properties. For example, stacks of graphene layers* can host a wide variety of quasiparticles and many-body phenomena: From Dirac fermions in single layers to exotic superconductivity in twisted double layers.
A stack of Honeycombs
In rhombohedral graphite (RG), the honeycomb layers are stacked on top of each other, with a specific shift or offset. This leads to a special electronic structure with very flat bands on the surface. Like in a topological insulator, charge carriers move freely at the surface only. Last year, it was shown that trilayers of RG also harbour ferromagnetism and unconventional superconductivity. And: The strength of the interactions increases with the number of layers.
Combination of experimental and theoretical results
A team from the Centre of Energy Research (Topology in Nanostructures, Momentum research group), Budapest, Hungary and at HZB has now examined the surface of multilayer RG samples under a scanning tunnelling microscope for the first time. They could precisely map the band structure and electronic properties and discovered unexpectedly rich many-body ground states. They also worked on various models of quantum physics in order to understand hidden processes and interactions in the samples.
Link between graphene-systems and quantum magnets
"The interesting thing about rhombohedral graphite is that this material also has so-called spin edge states, which occur in quantum magnets. The work thus connects two major areas of condensed matter: graphene-based systems and quantum magnets," says Dr Imre Hagymási, first author of the paper, which has now appeared in Science Advances.
Rhombohedrical graphene as a tunable platform
The study offers new insights into the interplay between topology and many-body physics and thus the chance to shed light on the physics in quantum magnets. At present, even simple quantum magnets are not fully understood. Yet quantum magnets also play a role in highly topical issues such as high-temperature cuprate superconductors. RG offers an alternative platform for the study of such correlated phenomena. A platform that is tunable by electric fields, strain, etc. and has a very simple crystal structure compared to other correlated materials. "These results are really helpful for the whole research field," says Hagymási.
*Note: Graphene is strictly one layer whereas the arrangement of graphene layers can be called graphite.
- Key technology for a future without fossil fuelsIn June and July 2025, catalyst researcher Nico Fischer spent some time at HZB. It was his sabbatical, he was relieved of his duties as Director of the Catalysis Institute in Cape Town for several months and was able to focus on research only. His institute is collaborating with HZB on two projects that aim to develop environmentally friendly alternatives using innovative catalyst technologies. The questions were asked by Antonia Rötger, HZB.
- 5000th patient treated with protons for eye tumoursFor more than 20 years, Charité – Universitätsmedizin Berlin and the Helmholtz-Zentrum Berlin (HZB) have been jointly offering proton radiation therapy for eye tumours. The HZB operates a proton accelerator in Berlin-Wannsee for this purpose, while Charité provides medical care for the patients. The 5000th patient was treated at the beginning of August.
- Iridium-free catalysts for acid water electrolysis investigatedHydrogen will play an important role, both as a fuel and as a raw material for industry. However, in order to produce relevant quantities of hydrogen, water electrolysis must become feasible on a multi-gigawatt scale. One bottleneck is the catalysts required, with iridium in particular being an extremely rare element. An international collaboration has therefore investigated iridium-free catalysts for acidic water electrolysis based on the element cobalt. Through investigations with various methods, among them experiments at the LiXEdrom at the BESSY II X-ray source in Berlin, they were able to elucidate processes that take place during water electrolysis in a cobalt-iron-lead oxide material as the anode. The study is published in Nature Energy.