Research

We employ first-principles calculations to study the electronic, spin, optical, and magnetic properties of various solids, including bulk materials, 2D monolayers, and van-der-Waals heterostructures. First-principles calculations are a powerful tool allowing us to study the quantum many-body problem of hundreds of atoms. With this, we are able to calculate the bandstructure, density of states, magnetic moments, dipole matrix elements, etc. of a material of interest. All these information are then necessary to make realistic predictions about transport phenomena, light-matter interaction, proximity coupling, etc. that can be observed in experiments. Particular examples of materials that we currently investigate are: graphene, hexagonal boron-nitride (hBN), transition-metal dichalcogenides (MoS₂, MoSe₂, WS₂, WSe₂), transiton-metal trihalides (CrI₃, CrBr₃), topological insulators (Bi₂Te₃, Sb₂Te₃), and many more.

This project is funded by Deutsche Forschungsgemeinschaft (DFG) through the Collaborative Research Center SFB 1277 and the Priority Program SPP 2244 (Project No. 443416183), the European Union Horizon 2020 Research and Innovation Program under contract number 881603 (Graphene Flagship), and the FLAGERA project 2DSOTECH.

: RECENT HIGHLIGHTS :

Engineering Proximity Effects by Twisting and Gating

Van-der-Waals heterostructures composed of twisted monolayers promise great tunability of electronic, optical, and magnetic properties. The most prominent example is magic-angle twisted bilayer graphene, exhibiting magnetism and superconductivity due to strong correlations. Other platforms for correlated physics are offered by trilayer graphene and twisted transition-metal dichalcogenides.

Spin interactions, such as magnetism and spin-orbit coupling, can be induced in van-der-Waals heterostructures by means of the proximity effect. The prototype material for proximity physics is graphene since the Dirac states, which are ideally suited for spin transport, can be strongly modified. In particular, when graphene interacts with a magnetic semiconductor, the Dirac states are preserved within the bandgap of the substrate and experience Zeeman-like band splittings. When a transiton-metal dichalcogenide is employed as the substrate, strong spin-orbit coupling is induced in graphene. With twisting and gating, one can further tailor the proximity-induced spin interactions. Combining the spin interactions with the Dirac states of graphene, even topological states may be realized.

In particular, in graphene/Cr₂Ge₂Te₆ heterostructures, we demonstrated the reversal of the ferromagnetic exchange by twisting and the emergence of antiferromagnetic exchange in the Dirac states. In WSe₂/CrI₃ heterostructures, we illustrated the strong manipulation of the WSe₂ valley splitting, which arises due to the interaction with CrI₃. In graphene/WSe₂ heterostructures, we explored the tunability of the proximity spin-orbit coupling and spin-orbit fields of the Dirac states upon twisting.

This work has been published in:

• Phys. Rev. B 107, 035112 (2023)
• Phys. Rev. Lett. 128, 106401 (2022)
• Phys. Rev. B 104, 195156 (2021)
• Phys. Rev. Lett. 124, 136403 (2020)
• 2D Mater. 7, 015026 (2020)
• Phys. Rev. B 100, 085128 (2019)

Bandgap Engineering and Tunable Proximity Effects in Multilayer Graphene

Multilayer graphene stacks are particularly interesting due to the additional layer degree of freedom. For example, in Bernal bilayer graphene, the important low-energy states are formed by the non-dimer atoms of the two layers. Upon application of a transverse electric field, one introduces a potential difference between the layers and opens up a bandgap in the spectrum. The tunability of the gap is sizable and reaches about 100 meV for a field of 1 V/nm. Additionally, the low-energy bands are layer- and sublattice-polarized – say, the atoms of the lower laye