Spintronics

We are a theoretical Condensed Matter group at the University of Regensburg with a special research focus on understanding Spin(Elec)tronics phenomena in nanoscale systems. Our current research activities comprise first-principles descriptions of the electronic properties and spin interactions of two-dimensional (magnetic) van-der-Waals multilayers, spin-orbit torques, excitons, as well as spin and transport properties of magnetic superconducting tunnel junctions including the recently intensively investigated supercurrent diode effect.

Our group (with Master student Sareh Bazyar, right) in October 2024

: RECENT RESEARCH HIGHLIGHTS :

Swapping Exchange and Spin-Orbit Induced Correlated Phases in Proximitized Bernal Bilayer Graphene

Combining sophisticated simulations and theoretical models, we discovered that we can electrically control the electronic behavior of Bernal bilayer graphene sandwiched between magnetic Cr₂Ge₂Te₆ and strong spin-orbit-coupling-inducing WS₂ layers to explore a wide range of correlated phases within a single system. Our research opens up novel possibilities to design such advanced EX-SO-tic van-der-Waals devices with tunable properties, simultaneously exploiting magnetic exchange (EX) and spin-orbit coupling (SO).

This work has been published in Physical Review B .

Emergence of Radial Rashba Spin-Orbit Fields in Twisted Van-Der-Waals Heterostructures

From first-principles calculations, we investigated the functional form of the spin-orbit fields that emerge in twisted van-der-Waals heterostructures consisting of graphene and WSe₂ multilayers. We proposed that the Rashba spin-orbit fields of these structures can be electrically tuned from the conventional – tangential to momentum – to a predominantly radial – parallel to momentum – spin texture. Such spin-orbit engineering could be useful to design spin-charge-conversion and spin-orbit-torque schemes, as well as for controlling correlated phases and superconductivity in van-der-Waals materials.

This work has been published in Physical Review B as Editors' Suggestion.

Link between Supercurrent Diode and Anomalous Josephson Effect Revealed by Gate-Controlled Interferometry

In collaboration with our experimental colleagues in Regensburg, we further investigated the supercurrent diode effect in two-dimensional electron gas (2DEG) Josephson junctions. We simultaneously explored the φ₀-shift (anomalous Josephson effect) and the supercurrent diode effect in the same system using a superconducting quantum interferometer. Electrostatic gating of the junction revealed a direct connection between the φ₀-shift and the diode effect. Our findings suggest that spin-orbit interaction?plays, together with a Zeeman field, a crucial role for the supercurrent diode effect to appear.

This work has been published in Nature Communications.

Emergent Correlated Phases in Rhombohedral Trilayer Graphene Induced by Proximity Spin-Orbit and Exchange Coupling

In collaboration with Dr. Denis Kochan from the Slovak Academy of Sciences in Bratislava, we studied the impact of proximity-induced spin-orbit (through proximity to a transition-metal dichalcogenide; TMDC) and exchange coupling (through proximity to CGT) on rhombohedral trilayer graphene. We identified a rich spectrum of correlated phases that originate, e.g., from valley–Zeeman coupling, and also unraveled a magnetocorrelation effect, which causes a strong sensitivity of the correlated phases to the relative magnetization orientations (parallel or antiparallel) of the proximitizing ferromagnetic (CGT) layers.

This work has been published in Physical Review Letters.

Charge Transfer and Asymmetric Coupling of MoSe₂ Valleys to the Magnetic Order of CrSBr

In an international collaboration, we investigated the control of the valley and excitonic properties of van-der-Waals heterostructures composed of two-dimensional transition-metal dichalcogenides (MoSe₂) and magnetic materials (CrSBr) exploiting proximity effects. We found a clear impact of the magnetic order of CrSBr on the optical properties of MoSe₂, the exciton and trion energies, and a valley g-factor reflecting asymmetric magnetic proximity interaction. Our first-principles calculations furthermore indicated that MoSe₂/CrSBr forms a broken-gap band alignment and supports charge transfer.

This work has been published in Nano Letters.

Electronic and Spin-Orbit Properties of hBN Encapsulated Bilayer Graphene

In collaboration with experimental colleagues from RWTH Aachen and Forschungszentrum Jülich, we employed first-principles calculations to model the electronic properties and spin-orbit coupling of bilayer graphene encapsulated by hexagonal boron-nitride (hBN). We extracted the spin-orbit coupling parameters fitting the computed band structures to a model Hamiltonian and furthermore studied the impact of twisting.

This work has been published in Physical Review B.

Sign Reversal of the Josephson Inductance Magnetochiral Anisotropy and 0–π-like Transitions in Supercurrent Diodes

After the first realization of a supercurrent diode based on two-dimensional electron gas (2DEG) Josephson junctions in the group of Prof. Dr. Christoph Strunk/Dr. Nicola Paradiso at UR, which we supported with theoretical calculations (Ref. 1, Ref. 2), we demonstrate in a recent work – again in close collaboration with our experimental colleagues – the peculiar role of 0–π-like transitions for the reversal (sign change) of both the supercurrent diode effect and the magnetochiral anisotropy. Our findings provide an important contribution to establish supercurrent diodes as the dissipationless counterparts of present-day semiconductor diodes and as essential building blocks of future electronic devices.

This work has been published in Nature Nanotechnology and Physical Review B.

The UR press release can be found here.

Proximity-Enhanced Valley Zeeman Splitting at the WS₂/Graphene Interface

Based on first-principles calculations, we characterized proximity effects occurring at the interfaces between the van-der-Waals material?WS₂ and graphene. We fitted the obtained bandstructures to an analytical model Hamiltonian and extracted the g-factors of the bilayer, suggesting a clear enhancement of the valley Zeeman effect due to proximity.

This work has been published in 2D Materials.

Signatures of Electric Field and Layer Separation Effects on the Spin-Valley Physics of MoSe₂/WSe₂ Heterobilayers: From Energy Bands to Dipolar Excitons

We explored the effects of electric fields and the separation of individual layers on the spin-valley physics of van-der-Waals?MoSe₂/WSe₂ heterobilayers using advanced first-principles methods. Within our recent work, we put a special focus on dipolar (interlayer) excitons – the exciton-forming electrons and holes are thereby localized in different layers –, for which multilayered van-der-Waals heterostructures provide a suitable platform to emerge.

This work has been published in Nanomaterials.

Strong Manipulation of the Valley Splitting upon Twisting and Gating in MoSe₂/CrI₃ and WSe₂/CrI₃ Van-Der-Waals Heterostructures

By means of first-principles calculations, we studied the impact of twisting and gating on the electronic properties of MoSe₂/CrI₃ and?WSe₂/CrI₃ van-der-Waals heterostructures. Fitting the ab-initio bandstructures to a well-established model Hamiltonian, we demonstrate that twisting and gating provide important control knobs to strongly tune the valley splitting.

This work has been published in Physical Review B.