arXiv subjects

Klaus Zollner

Publications and source records attributed to Klaus Zollner.

At least 19 recordsLinked to original sources

arXiv2026

Resonant magnetic proximity hot spots in Co/hBN/graphene

Magnetic proximity effects in Co/hBN/graphene heterostructures are systematically analyzed via first-principles calculations, demonstrating a pronounced localized spatial variation of the induced spin polarization of graphene's Dirac states. The proximity-induced exchange coupling, magnetic moments, and tunneling spin polarization (TSP) are shown to depend sensitively on the atomic registry at the interfaces. We analyze more than twenty distinct stackings, including high- and low-symmetry configurations, and reveal that the spin splittings of graphene's Dirac bands span a wide range from 1 to 100 meV, depending on the local hybridization of Co $d_{z^2}$, hBN $p_z$, and graphene $p_z$ orbitals. The strongest proximity effects emerge at geometric resonances, or "proximity hot spots", where the three orbital states overlap maximally. The local spin polarization also depends sensitively on energy: Dirac states aligned with resonant Co orbitals experience the most pronounced exchange interaction. At these energies, the pseudospin Hamiltonian description of magnetic proximity effects breaks down. Outside these resonances, the pseudospin picture is restored. Our findings highlight the intrinsically local nature of proximity effects, governed by the spectral resonance and interlayer wavefunction overlap. We further quantify how additional hBN layers, interlayer twist, and multilayer graphene modify the proximity exchange and TSP, offering microscopic insight for designing spintronic van der Waals heterostructures with engineered interfaces and optimized spin transport.

cond-mat.mes-hall↗

arXiv2025

Radial Rashba spin-orbit fields in commensurate twisted transition-metal dichalcogenide bilayers

In commensurate twisted homobilayers, purely radial Rashba spin-orbit fields can emerge. We employ first-principles calculations to investigate the band structures and the spin-orbit fields close to the high-symmetry points $K$ and $\Gamma$ of several commensurate twisted transition-metal dichalcogenide homobilayers: WSe$_2$, NbSe$_2$, and WTe$_2$. The observed in-plane spin textures are mostly radial, and the main features are successfully reproduced using a model Hamiltonian based on two effective mass models including spin-orbit coupling, and a general (spin-conserving) interlayer coupling. Extracting the model Hamiltonian parameters through fitting of several twisted supercells, we find a twist angle dependency of the magnitude of the radial Rashba field, which is symmetric not only around the untwisted cases ($\Theta=0^\circ$ and $\Theta=60^\circ$), but also around $\Theta=30^\circ$. Furthermore, we observe that the interlayer coupling between the $K/K'$-points of the two layers decreases with the increase of the size of the commensurate supercells. Hence, peaks of high interlayer coupling can occur only for twist angles, where small commensurate supercells are possible. Exploring different lateral displacements between the layers, we confirm that the relevant symmetry protecting the radial Rashba is an in-plane 180$^\circ$ rotation axis. We additionally investigate the effects of atomic relaxation and modulation of the interlayer distance. Our calculations on WTe$_2$ bilayers show that their lack of $C_3$ symmetry results in spin textures that are neither radial nor tangential. Our results offer fundamental microscopic insights that are particularly relevant to engineering spin-charge conversion schemes based on twisted layered materials.

cond-mat.mes-hall↗

arXiv2025

Machine Learning Prediction of Magnetic Proximity Effect in van der Waals Heterostructures: From Atoms to Moir\'e

We introduce a machine learning framework that efficiently predicts large-scale proximity-induced magnetism in van der Waals heterostructures, overcoming the high computational cost of density functional theory (DFT). We apply it to graphene/\CGT, which exhibits a previously unrecognized dichotomy. Unlike the spin polarization at the Fermi level, which follows the pseudospin, the proximity-induced magnetic moments vary across carbon atoms, defying analytical modeling. To address this, we develop an ensemble-based regression model trained on DFT data and employ local environment descriptors to map the local ($\sim 2$\,nm$^2$) atomic-scale geometry to the carbon magnetic moments. Besides demonstrating locality, the model reveals rich magnetic moir\'e textures. Crucially, this method can be broadly applied to orbital and spin proximity effects that are highly sensitive to local atomic environments and are beyond analytical description.

cond-mat.mtrl-sci↗

arXiv2025

Spin injection and detection in all-van der Waals 2D devices

In this work we report efficient out-of-plane spin injection and detection in an all-van der Waals based heterostructure using only exfoliated 2D materials. We demonstrate spin injection by measuring spin-valve and Hanle signals in non-local transport in a stack of Fe$_3$GeTe$_2$ (FGT), hexagonal boron nitride (hBN) and graphene layers. FGT flakes form the spin aligning electrodes necessary to inject and detect spins in the graphene channel. The hBN tunnel barrier provides a high-quality interface between the ferromagnetic electrodes and graphene, eliminating the conductivity mismatch problem, thus ensuring efficient spin injection and detection with spin injection efficiencies of up to $P=40$\%. Our results demonstrate that FGT/hBN/graphene heterostructures form a promising platform for realizing 2D van der Waals spintronic devices.

cond-mat.mes-hall↗

arXiv2024

Effect of spin-dependent tunneling in a MoSe$_2$/Cr$_2$Ge$_2$Te$_6$ van der Waals heterostructure on exciton and trion emission

We study van der Waals heterostructures consisting of monolayer MoSe$_2$ and few-layer Cr$_2$Ge$_2$Te$_6$ fully encapsulated in hexagonal Boron Nitride using low-temperature photoluminescence and polar magneto-optic Kerr effect measurements. Photoluminescence characterization reveals a partial quenching and a change of the exciton-trion emission ratio in the heterostructure as compared to the isolated MoSe$_2$ monolayer. Under circularly polarized excitation, we find that the exciton-trion emission ratio depends on the relative orientation of excitation helicity and Cr$_2$Ge$_2$Te$_6$ magnetization, even though the photoluminescence emission itself is unpolarized. This observation hints at an ultrafast, spin-dependent interlayer charge transfer that competes with exciton and trion formation and recombination.

cond-mat.mes-hall↗

arXiv2024

Layer-selective spin-orbit coupling and strong correlation in bilayer graphene

Spin-orbit coupling (SOC) and electron-electron interaction can mutually influence each other and give rise to a plethora of intriguing phenomena in condensed matter systems. In pristine bilayer graphene, which has weak SOC, intrinsic Lifshitz transitions and concomitant van-Hove singularities lead to the emergence of many-body correlated phases. Layer-selective SOC can be proximity induced by adding a layer of tungsten diselenide (WSe2) on its one side. By applying an electric displacement field, the system can be tuned across a spectrum wherein electronic correlation, SOC, or a combination of both dominates. Our investigations reveal an intricate phase diagram of proximity-induced SOC-selective bilayer graphene. Not only does this phase diagram include those correlated phases reminiscent of SOC-free doped bilayer graphene, but it also hosts unique SOC-induced states allowing a compelling measurement of valley g-factor and a seemingly impossible correlated insulator at charge neutrality, thereby showcasing the remarkable tunability of the interplay between interaction and SOC in WSe2 enriched bilayer graphene.

cond-mat.mes-hall↗

arXiv2024

Emergence of radial Rashba spin-orbit fields in twisted van der Waals heterostructures

Rashba spin-orbit coupling is a quintessential spin interaction appearing in virtually any electronic heterostructure. Its paradigmatic spin texture in the momentum space forms a tangential vector field. Using first-principles investigations, we demonstrate that in twisted homobilayers and hetero-multilayers, the Rashba coupling can be predominantly radial, parallel to the momentum. Specifically, we study four experimentally relevant structures: twisted bilayer graphene (Gr), twisted bilayer WSe$_2$, and twisted multilayers WSe$_2$/Gr/WSe$_2$ and WSe$_2$/Gr/Gr/WSe$_2$. We show, that the Rashba spin-orbit field texture in such structures can be controlled by an electric field, allowing to tune it from radial to tangential. Such spin-orbit engineering should be useful for designing novel spin-charge conversion and spin-orbit torque schemes, as well as for controlling correlated phases and superconductivity in van der Waals materials.

cond-mat.mes-hall↗

arXiv2023

Quantum Spin Hall Effect in Magnetic Graphene

A promising approach to attain long-distance coherent spin propagation is accessing topological spin-polarized edge states in graphene. Achieving this without external magnetic fields necessitates engineering graphene band structure, obtainable through proximity effects in van der Waals heterostructures. In particular, proximity-induced staggered potentials and spin-orbit coupling are expected to form a topological bulk gap in graphene with gapless helical edge states that are robust against disorder. In this work, we detect the spin-polarized helical edge transport in graphene at zero external magnetic field, allowed by the proximity of an interlayer antiferromagnet, CrPS$_4$. We show the coexistence of the quantum spin Hall (QSH) states and magnetism in graphene, where the induced spin-orbit and exchange couplings also give rise to a large anomalous Hall (AH) effect. The detection of the QSH states at zero external magnetic field, together with the AH signal that persists up to room temperature, opens the route for practical applications of magnetic graphene in quantum spintronic circuitries.

cond-mat.mes-hall↗

arXiv2023

Twist- and gate-tunable proximity spin-orbit coupling, spin relaxation anisotropy, and charge-to-spin conversion in heterostructures of graphene and transition-metal dichalcogenides

We present a DFT-based investigation of the twist-angle dependent proximity spin-orbit coupling (SOC) in graphene/TMDC structures. We find that for Mo-based TMDCs the proximity valley-Zeeman SOC exhibits a maximum at around 15--20°, and vanishes at 30°, while for W-based TMDCs we find an almost linear decrease of proximity valley-Zeeman SOC when twisting from 0° to 30°. The induced Rashba SOC is rather insensitive to twisting, while acquiring a nonzero Rashba phase angle, $φ\in [-20;40]$°, for twist angles different from 0° and 30°. This finding contradicts earlier tight-binding predictions that the Rashba angle can be 90° in the studied systems. In addition, we study the influence of several tunability knobs on the proximity SOC for selected twist angles. By applying a transverse electric field in the limits of $\pm 2$ V/nm, mainly the Rashba SOC can be tuned by about 50\%. The interlayer distance provides a giant tunability, since the proximity SOC can be increased by a factor of 2--3, when reducing the distance by about 10\%. Encapsulating graphene between two TMDCs, both twist angles are important to control the interference of the individual proximity SOCs, allowing to precisely tailor the valley-Zeeman SOC in graphene, while the Rashba SOC becomes suppressed. Finally, based on our effective Hamiltonians with fitted parameters, we calculate experimentally measurable quantities such as spin lifetime anisotropy and charge-to-spin conversion efficiencies. The spin lifetime anisotropy can become giant, up to $10^4$, in encapsulated structures. The charge-to-spin conversion, which is due to spin-Hall and Rashba-Edelstein effects, can lead to twist-tunable non-equilibrium spin-density polarizations that are perpendicular and parallel to the applied charge current.

cond-mat.mes-hall↗

arXiv2023

Electronic and Spin-Orbit Properties of hBN Encapsulated Bilayer Graphene

Van der Waals (vdW) heterostructures consisting of Bernal bilayer graphene (BLG) and hexagonal boron nitride (hBN) are investigated. By performing first-principles calculations we capture the essential BLG band structure features for several stacking and encapsulation scenarios. A low-energy model Hamiltonian, comprising orbital and spin-orbit coupling (SOC) terms, is employed to reproduce the hBN-modified BLG dispersion, spin splittings, and spin expectation values. Most important, the hBN layers open an orbital gap in the BLG spectrum, which can range from zero to tens of meV, depending on the precise stacking arrangement of the individual atoms. Therefore, large local band gap variations may arise in experimentally relevant moiré structures. Moreover, the SOC parameters are small (few to tens of $μ$eV), just as in bare BLG, but are markedly proximity modified by the hBN layers. Especially when BLG is encapsulated by monolayers of hBN, such that inversion symmetry is restored, the orbital gap and spin splittings of the bands vanish. In addition, we show that a transverse electric field mainly modifies the potential difference between the graphene layers, which perfectly correlates with the orbital gap for fields up to about 1~V/nm. Moreover, the layer-resolved Rashba couplings are tunable by $\sim 5~μ$eV per V/nm. Finally, by investigating twisted BLG/hBN structures, with twist angles between 6$^{\circ}$ -- 20$^{\circ}$, we find that the global band gap increases linearly with the twist angle. The extrapolated $0^{\circ}$ band gap is about 23~meV and results roughly from the average of the stacking-dependent local band gaps. Our investigations give new insights into proximity spin physics of hBN/BLG heterostructures, which should be useful for interpreting experiments on extended as well as confined (quantum dot) systems.

cond-mat.mes-hall↗

arXiv2023

Low-scaling GW algorithm applied to twisted transition-metal dichalcogenide heterobilayers

The $GW$ method is widely used for calculating the electronic band structure of materials. The high computational cost of $GW$ algorithms prohibits their application to many systems of interest. We present a periodic, low-scaling and highly efficient $GW$ algorithm that benefits from the locality of the Gaussian basis and the polarizability. The algorithm enables $G_0W_0$ calculations on a MoSe$_2$/WS$_2$ bilayer with 984 atoms per unit cell, in 42 hours using 1536 cores. This is four orders of magnitude faster than a plane-wave $G_0W_0$ algorithm, allowing for unprecedented computational studies of electronic excitations at the nanoscale.

physics.chem-ph↗

arXiv2023

Twist-angle dependent proximity induced spin-orbit coupling in graphene/transition-metal dichalcogenide heterostructures

We investigate the proximity-induced spin-orbit coupling in heterostructures of twisted graphene and monolayers of transition-metal dichalcogenides (TMDCs) MoS$_2$, WS$_2$, MoSe$_2$, and WSe$_2$ from first principles. We identify strain, which is necessary to define commensurate supercells, as the key factor affecting the band offsets and thus magnitudes of the proximity couplings. We establish that for biaxially strained graphene the band offsets between the Dirac point and conduction (valence) TMDC bands vary linearly with strain, regardless of the twist angle. This relation allows to identify the apparent zero-strain band offsets and find a compensating transverse electric field correcting for the strain. The resulting corrected band structure is then fitted around the Dirac point to an established spin-orbit Hamiltonian. This procedure yields the dominant, valley-Zeeman and Rashba spin-orbit couplings. The magnitudes of these couplings do not vary much with the twist angle, although the valley-Zeeman coupling vanishes for 30$^{\circ}$ and Mo-based heterostructures exhibit a maximum of the coupling at around 20$^{\circ}$. The maximum for W-based stacks is at 0$^{\circ}$. The Rashba coupling is in general weaker than the valley-Zeeman coupling, except at angles close to 30$^{\circ}$. We also identify the Rashba phase angle which measures the deviation of the in-plane spin texture from tangential, and find that this angle is very sensitive to the applied transverse electric field. We further discuss the reliability of the supercell approach with respect to atomic relaxation (rippling of graphene), relative lateral shifts of the atomic layers, and transverse electric field.

cond-mat.mes-hall↗

arXiv2023

Semimetallic and semiconducting graphene-hBN multilayers with parallel or reverse stacking

We theoretically investigate 3D layered crystals of alternating graphene and hBN layers with different symmetries. Depending on the hopping parameters between the graphene layers, we find that these synthetic 3D materials can feature semimetallic, gapped, or Weyl semimetal phases. Our results demonstrate that 3D crystals stacked from individual 2D materials represent a synthetic materials class with emergent properties different from their constituents.

cond-mat.mtrl-sci↗

arXiv2023

Sensitivity of the MnTe valence band to orientation of magnetic moments

An effective model of the hexagonal (NiAs-structure) manganese telluride valence band in the vicinity of the A-point of the Brillouin zone is derived. It is shown that while for the usual antiferromagnetic order (magnetic moments in the basal plane) band splitting at A is small, their out-of-plane rotation enhances the splitting dramatically (to about 0.5 eV). We propose extensions of recent experiments (Moseley et al., Phys. Rev. Materials 6, 014404) where such inversion of magnetocrystalline anisotropy has been observed in Li-doped MnTe, to confirm this unusual sensitivity of a semiconductor band structure to magnetic order.

cond-mat.mtrl-sci↗

arXiv2023

Strong manipulation of the valley splitting upon twisting and gating in MoSe$_2$/CrI$_3$ and WSe$_2$/CrI$_3$ van der Waals heterostructures

We investigate the twist-angle and gate dependence of the proximity-induced exchange coupling in the monolayer transition-metal dichalcogenides (TMDCs) MoSe$_2$ and WSe$_2$ due to the vdW coupling to the ferromagnetic semiconductor CrI$_3$, from first-principles calculations. A model Hamiltonian, that captures the relevant band edges at the $K/K^{\prime}$ valleys of the proximitized TMDCs, is employed to quantify the proximity-induced exchange. Upon twisting from 0° to 30°, we find a transition of the TMDC valence band (VB) edge exchange splitting from about $-2$ to $2$ meV, while the conduction band (CB) edge exchange splitting remains nearly unchanged at around $-3$ meV. For the VB of WSe$_2$ (MoSe$_2$) on CrI$_3$, the exchange coupling changes sign at around 8° (16°). We find that even at the angles with almost zero spin splittings of the VB, the real-space spin polarization profile of holes at the band edge is highly non-uniform, with alternating spin up and spin down orbitals. Furthermore, a giant tunability of the proximity-induced exchange coupling is provided by a transverse electric field of a few V/nm. We complement our \textit{ab initio} results by calculating the excitonic valley splitting to provide experimentally verifiable optical signatures of the proximity exchange. Specifically, we predict that the valley splitting increases almost linearly as a function of the twist angle. Furthermore, the proximity exchange is highly tunable by gating, allowing to tailor the valley splitting in the range of 0 to 12 meV in WSe$_2$/CrI$_3$, which is equivalent to external magnetic fields of up to about 60 Tesla. Our results highlight the important impact of the twist angle and gating when employing magnetic vdW heterostructures in experimental geometries.

cond-mat.mes-hall↗

arXiv2022

Proximity effects in graphene on monolayers of transition-metal phosphorus trichalcogenides MPX$_3$

We investigate the electronic band structure of graphene on a series of two-dimensional magnetic transition-metal phosphorus trichalcogenide monolayers, MPX$_3$ with M={Mn,Fe,Ni,Co} and X={S,Se}, with first-principles calculations. A symmetry-based model Hamiltonian is employed to extract orbital parameters and sublattice resolved proximity-induced exchange couplings ($λ_{\textrm{ex}}^\textrm{A}$ and $λ_{\textrm{ex}}^\textrm{B}$) from the low-energy Dirac bands of the proximitized graphene. Depending on the magnetic phase of the MPX$_3$ layer (ferromagnetic and three antiferromagnetic ones), completely different Dirac dispersions can be realized with exchange splittings ranging from 0 to 10~meV. Surprisingly, not only the magnitude of the exchange couplings depends on the magnetic phase, but also the global sign and the type. Important, one can realize uniform ($λ_{\textrm{ex}}^\textrm{A} \approx λ_{\textrm{ex}}^\textrm{B}$) and staggered ($λ_{\textrm{ex}}^\textrm{A} \approx -λ_{\textrm{ex}}^\textrm{B}$) exchange couplings in graphene. From selected cases, we find that the interlayer distance, as well as a transverse electric field are efficient tuning knobs for the exchange splittings of the Dirac bands. More specifically, decreasing the interlayer distance by only about 10\%, a giant 5-fold enhancement of proximity exchange is found, while applying few V/nm of electric field, provides tunability of proximity exchange by tens of percent. We have also studied the dependence on the Hubbard $U$ parameter and find it to be weak. Moreover, we find that the effect of SOC on the proximitized Dirac dispersion is negligible compared to the exchange coupling.

cond-mat.mes-hall↗

arXiv2022

Engineering Proximity Exchange by Twisting: Reversal of Ferromagnetic and Emergence of Antiferromagnetic Dirac Bands in Graphene/Cr$_2$Ge$_2$Te$_6$

We investigate the twist-angle and gate dependence of the proximity exchange coupling in twisted graphene on monolayer Cr$_2$Ge$_2$Te$_6$ from first principles. The proximitized Dirac band dispersions of graphene are fitted to a model Hamiltonian, yielding effective sublattice-resolved proximity-induced exchange parameters ($λ_{\textrm{ex}}^\textrm{A}$ and $λ_{\textrm{ex}}^\textrm{B}$) for a series of twist angles between 0$^{\circ}$ and 30$^{\circ}$. For aligned layers (0$^{\circ}$ twist angle), the exchange coupling of graphene is the same on both sublattices, $λ_{\textrm{ex}}^\textrm{A} \approx λ_{\textrm{ex}}^\textrm{B} \approx 4$ meV, while the coupling is reversed at 30$^{\circ}$ (with $λ_{\textrm{ex}}^\textrm{A} \approx λ_{\textrm{ex}}^\textrm{B} \approx -4$ meV). Remarkably, at 19.1$^{\circ}$ the induced exchange coupling becomes antiferromagnetic: $λ_{\textrm{ex}}^\textrm{A} < 0, λ_{\textrm{ex}}^\textrm{B} > 0$. Further tuning is provided by a transverse electric field and the interlayer distance. The predicted proximity magnetization reversal and emergence of an antiferromagnetic Dirac dispersion make twisted graphene/Cr$_2$Ge$_2$Te$_6$ bilayers a versatile platform for realizing topological phases and for spintronics applications.

cond-mat.mes-hall↗

arXiv2022

Proximity spin-orbit and exchange coupling in ABA and ABC trilayer graphene van der Waals heterostructures

We investigate the proximity spin-orbit and exchange couplings in ABA and ABC trilayer graphene encapsulated within monolayers of semiconducting transition-metal dichalcogenides and the ferromagnetic semiconductor Cr$_2$Ge$_2$Te$_6$. Employing first-principles calculations we obtain the electronic structures of the multilayer stacks and extract the relevant proximity-induced orbital and spin interaction parameters by fitting the low-energy bands to model Hamiltonians. We also demonstrate the tunability of the proximity effects by a transverse electric field. Using the model Hamiltonians we also study mixed spin-orbit/exchange coupling encapsulation, which allows to tailor the spin interactions very efficiently by the applied field. We also summarize the spin-orbit physics of bare ABA, ABC, and ABB trilayers, and provide, along with the first-principles results of the electronic band structures, density of states, spin splittings, and electric-field tunabilities of the bands, qualitative understanding of the observed behavior and realistic model parameters as a resource for model simulations of transport and correlation physics in trilayer graphene.

cond-mat.mes-hall↗