Rhombohedral graphite as a model for quantum magnetism

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. (Source: Helmholtz-Zentrum Berlin – Presse releases) 

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.

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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.

The original press release can be found at:

Rhombohedral graphite as a model for quantum magnetism

The original publication can be found at:

Imre Hagymási, Mohammad Syahid Mohd Isa, Zoltán Tajkov, Krisztián Márity,László Oroszlány, János Koltai Assem Alassaf, Péter Kun, Konrád Kandrai, András Pálinkás, Péter Vancsó, Levente Tapasztó and Péter Nemes-Incze: Observation of competing, correlated ground states in the flat band of rhombohedral graphite. Science Advances (2022), DOI: 10.1126/sciadv.abo6879

Localization in the Helmholtz Research Field Information:

Helmholtz Research Field Information, Program 2: Natural, Artificial and Cognitive Information Processing, Topic 1: Quantum Materials

Contact:

Dr. Imre Hagymási
Department Theory of Novel Quantum Materials
Helmholtz-Zentrum Berlin für Materialien und Energie (HZB)
E-Mail: QM-ATQM-office@helmholtz-berlin.de

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