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, Priority Program SPP 2244 (Project No. 443416183), the European Union Horizon 2020 Research and Innovation Program under contract number 881603 (Graphene Flagship), and 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 layer form the valence band, while the upper-layer atoms form the conduction band. Reversing the electric-field direction, the gap remains unaffected, but the layer polarization can be swapped.

In our recent studies, we have demonstrated that there is already an intrinsic bandgap present in bilayer-graphene heterostructures due to the intrinsic dipole of the structure. The tunability of the gap and layer polarization with the electric field stays intact. Due to the layer polarization, combined with the short-range proximity effects (spin-orbit and exchange), we could demonstrate the fully electrical swapping of spin interactions in a WS₂/bilayer graphene/Cr₂Ge₂Te₆ heterostructure.

This work has been published in:

• Phys. Rev. B 108, 125126 (2023)
• Phys. Rev. B 105, 115126 (2022)
• Phys. Rev. B 104, 075126 (2021)
• Phys. Rev. Lett. 125, 196402 (2020)
• New J. Phys. 20, 073007 (2018)
• Phys. Rev. Lett. 119, 146401 (2017)

Superconducting nanojunctions are attracting enormous attention of researchers owing to their fascinating physical properties that make superconducting systems essential for modern technology – particularly for recently emerging quantum-computer architectures. To allow for an even greater tunability of their physical properties, and thereby offer more functionalities to potential applications, superconducting layers are often paired with magnetic components and regions that intrinsically induce strong spin-orbit coupling. Together with the long-range coherence of the superconducting part, these two fundamental spin interactions – magnetic exchange and spin-orbit coupling – manifest themselves in unique spectroscopic and transport characteristics, and were furthermore predicted to induce topological superconductivity that might harbor Majorana states.

In close collaboration with experimental groups, we are performing systematic and
comprehensive microscopic model calculations to unravel and further characterize the
transport ramifications of the interplay between superconductivity, magnetism, and spin-orbit coupling in different multi-component junctions.

This project is mainly funded by Deutsche Forschungsgemeinschaft (DFG) through the Research Grant 454646522 and the Collaborative Research Center SFB 1277.

: RECENT HIGHLIGHTS :

Supercurrent Diode Effect in 2DEG Josephson Junctions

Josephson junctions that are integrated into the two-dimensional electron gas (2DEG) of narrow quantum wells (e.g., InAs quantum wells) by means of the superconducting proximity effect have recently been the subject of a flurry of experimental and theoretical research activities. When combined with magnetic exchange – that could, for instance, be induced through applying an external magnetic field – in a certain way, the strong intrinsic spin-orbit coupling of the 2DEG imprints a nonreciprocal character on the supercurrent transport, resulting in supercurrent magnitudes that (substantially) differ for different polarities (transport directions). From this viewpoint, 2DEG magnetic Josephson junctions could act as the fully dissipationless counterparts of the yet in technological applications used semiconductor diodes – motivating to term the related effect supercurrent diode effect (SDE) – and become even more relevant to design superconducting transistors one day.

The Strunk Group in Regensburg successfully observed a giant SDE in an array of more than 2,000 Josephson junctions based on an InAs quantum well for the first time ever in 2021 (in a sample provided by Manfra’s group from Purdue University), and characterized its transport properties (magnetochiral anisotropy) through state-of-the-art inductance measurements. We supported the experimental studies with tight-binding KWANT simulations that fully reproduced (both qualitatively and quantitatively) the experimental observations.

After the initial work, we developed a microscopic model that connects the SDE characteristics with the spectral properties of the underlying current-carrying Andreev bound states of the Josephson junctions. We could demonstrate that the junction may undergo current-reversing 0–π-like transitions at strong enough magnetic exchange and, most surprisingly, that also the SDE can eventually reverse. In parallel to our theoretical studies, the Strunk group measured the field-dependence of the SDE and the obtained experimental data that are in good (qualitative and quantitative) agreement with our microscopic model. Moreover, the experiment confirmed the predicted reversal of the SDE at strong enough magnetic fields for the first time.

In close collaboration with our experimental colleagues, we are currently continuing to refine the model and try to answer some open questions that are still intensely debated among the community.

This work has been published in:

• arXiv:2308.01061
• arXiv:2303.11975
• Phys. Rev. B 108, 054522 (2023)
• Nat. Nanotechnol. 18, 1266 (2023)
• J. Phys. Condens. Matter 34, 154005 (2022)
• Nat. Nanotechnol. 17, 39 (2022)

The UR press release can be found here.

Signatures of Superconducting Triplet Pairing in Ni–Ga Bilayer Junctions

In close collaboration with Moodera’s group at MIT, we performed tight-binding KWANT simulations to understand quasiparticle transport through Ni–Ga bilayers. We showed that the observed rich, and initially extremely puzzling conductance variations (that we termed "conductance shoulders"), could serve as transport signatures of superconducting triplet pairings that are induced at the interface between the ferromagnetic Ni and the superconducting Ga films, establishing Ni–Ga bilayers as an interesting platform to further investigate long-range triplet superconductivity. Polarized-neutron-reflectometry data provided by Dr. Valeria Lauter from Oak Ridge National Laboratory confirmed the potential presence of nonuniform ferromagnetic order (domains) around the Ni–Ga interface, which could provide the microscopic explanation for the conversion of spin-singlet into polarized spin-triplet Cooper pairs.

This work has been published in New J. Phys. 24, 033046 (2022).

Interfacial Spin-Orbit Coupling in Ferromagnet/Superconductor-Like Junctions

Relativistic effects like spin-orbit coupling have a tremendous impact on electrical transport through nanoscale junctions, and are therefore at the heart of numerous recent research projects within the SFB 1277 at the Physics Department in Regensburg. We have been intensively studying the role of interfacial spin-orbit coupling, which is inevitably induced in all junctions through the space-inversion-symmetry breaking through thin (e.g., semiconducting) tunneling barriers, in ferromagnet/superconductor-like junctions.

The most interesting novel process that interfacial spin-orbit coupling gives rise to in superconducting junctions is the spin-flip ("unconventional") Andreev reflection, which triggers the conversion of spin-singlet into spin-triplet Cooper pairs and thereby generates spin-polarized supercurrents with astonishing physical consequences and properties. We elaborated on the ramifications of unconventional Andreev reflections on the charge and spin (Hall) transport properties of ferromagnet/superconductor junctions and superconductor/ferromagnet/superconductor Josephson junctions in the presence of interfacial Rashba and Dresselhaus spin-orbit couplings. In an experimental collaboration with Aliev’s group in Madrid, we demonstrated that our theory is capable of quantifying the strength of the interfacial spin-orbit couplings of Fe/V junctions through experimental conductance measurements.

We are currently continuing these studies extending our view to more advanced system geometries with more different layers and with a special focus on the distinct mechanisms that have been proposed to generate spin-polarized triplet supercurrents.

This work has been published in:

• Phys. Rev. B 104, 174504 (2021)
• Phys. Rev. B 101, 104508 (2020)
• Phys. Rev. Appl. 13, 014030 (2020)
• Phys. Rev. B 100, 060507(R) (2019)
• Phys. Rev. B 98, 134511 (2018)
• Phys. Rev. B 95, 024514 (2017)
• Phys. Rev. Lett. 115, 116601 (2015)