Spin-orbit proximity effect in graphene

Nature communications, 2015

The aim of valleytronics is to exploit confinement of charge carriers in local valleys of the energy bands of semiconductors as an additional degree of freedom in optoelectronic devices. Thanks to strong direct excitonic transitions in spin-coupled K valleys, monolayer molybdenum disulphide is a rapidly emerging valleytronic material, with high valley polarization in photoluminescence. Here we elucidate the excitonic physics of this material by light helicity-dependent photocurrent studies of phototransistors. We demonstrate that large photocurrent dichroism (up to 60%) can also be achieved in high-quality molybdenum disulphide monolayers grown by chemical vapour deposition, due to the circular photogalvanic effect on resonant excitations. This opens up new opportunities for valleytonic applications in which selective control of spin-valley-coupled photocurrents can be used to implement polarization-sensitive light-detection schemes or integrated spintronic devices, as well as bioch...

Phys. Chem. Chem. Phys., 2015

The lack of some spatial symmetries in planar devices with Rashba spin–orbit interactions opens up the possibility of producing spin polarized electrical currents in the absence of external magnetic fields or magnetic impurities.

npj 2D Materials and Applications

Van der Waals heterostructures composed of multiple few layer crystals allow the engineering of novel materials with predefined properties. As an example, coupling graphene weakly to materials with large spin–orbit coupling (SOC) allows to engineer a sizeable SOC in graphene via proximity effects. The strength of the proximity effect depends on the overlap of the atomic orbitals, therefore, changing the interlayer distance via hydrostatic pressure can be utilized to enhance the interlayer coupling between the layers. In this work, we report measurements on a graphene/WSe2 heterostructure exposed to increasing hydrostatic pressure. A clear transition from weak localization to weak antilocalization is visible as the pressure increases, demonstrating the increase of induced SOC in graphene.

Advanced Materials

Physical Review B, 2015

The angular momentum quantization of chiral gapless modes confined to a circularly shaped interface between two different topological phases is investigated. By examining several different setups, we show analytically that the angular momentum of the topological modes exhibits a highly chiral behavior, and can be coupled to spin and/or valley degrees of freedom, reflecting the nature of the interface states. A simple general 1D model, valid for arbitrarily shaped loops, is shown to predict the corresponding energies and the magnetic moments. These loops can be viewed as building blocks for artificial magnets with tunable and highly diverse properties.

Physical Review B

Graphene's electronic structure can be fundamentally altered when a substrate-or adatom-induced Kekulé superlattice couples the valley and isospin degrees of freedom. Here, we show that the band structure of Kekulétextured graphene can be re-engineered through layer stacking. We predict a family of Kekulé graphene bilayers that exhibit band structures with up to six valleys, and room-temperature Dirac quasiparticles whose masses can be tuned electrostatically. Fermi velocities half as large as in pristine graphene put this system in the strongly coupled regime, where correlated ground states can be expected.

Physical Review B

We present a methodology to address, from first principles, charge-spin interconversion in two-dimensional materials with spin-orbit coupling. Our study relies on an implementation of density functional theory based quantum transport formalism adapted to such purpose. We show how an analysis of the k-resolved spin polarization gives the necessary insight to understand the different charge-spin interconversion mechanisms. We have tested it in the simplest scenario of isolated graphene in a perpendicular electric field where effective tight-binding models are available to compare with. Our results show that the flow of an unpolarized current across a single layer of graphene produces, as expected, a spin separation perpendicular to the current for two of the three spin components (out-of-plane and longitudinal), which is the signature of the spin Hall effect. Additionally, it also yields an overall spin accumulation for the third spin component (perpendicular to the current), which is the signature of the Rashba-Edelstein effect. Even in this simple example, our results reveal an unexpected competition between the Rashba and the intrinsic spin-orbit coupling. Remarkably, the sign of the accumulated spin density does not depend on the electron or hole nature of the injected current for realistic values of the Rashba coupling.

Science Advances

We use time- and angle-resolved photoemission spectroscopy (tr-ARPES) to investigate ultrafast charge transfer in an epitaxial heterostructure made of monolayer WS2 and graphene. This heterostructure combines the benefits of a direct-gap semiconductor with strong spin-orbit coupling and strong light-matter interaction with those of a semimetal hosting massless carriers with extremely high mobility and long spin lifetimes. We find that, after photoexcitation at resonance to the A-exciton in WS2, the photoexcited holes rapidly transfer into the graphene layer while the photoexcited electrons remain in the WS2 layer. The resulting charge-separated transient state is found to have a lifetime of ∼1 ps. We attribute our findings to differences in scattering phase space caused by the relative alignment of WS2 and graphene bands as revealed by high-resolution ARPES. In combination with spin-selective optical excitation, the investigated WS2/graphene heterostructure might provide a platform ...

Scientific reports, 2018

Graphene has gigantic potential in the development of advanced spintronic devices. The interfacial interactions of graphene with semiconducting transition metal dichalcogenides improve the electronic properties drastically, making it an intriguing candidate for spintronic applications. Here, we fabricated bilayer graphene encapsulated by WSlayers to exploit the interface-induced spin-orbit interaction (SOI). We designed a dual gated device, where the SOI is tuned by gate voltages. The strength of induced SOI in the bilayer graphene is dramatically elevated, which leads to a strong weak antilocalization (WAL) effect at low temperature. The quantitative analysis of WAL demonstrates that the spin relaxation time is 10 times smaller than in bilayer graphene on conventional substrates. To support these results, we also examined Shubnikov-de Haas (SdH) oscillations, which give unambiguous evidence of the zero-field spin-splitting in our bilayer graphene. The spin-orbit coupling constants ...

Handbook of Graphene, 2019

Graphene/MoS 2 heterostructures are formed by combining the nanosheets of graphene and monolayer MoS 2. The electronic features of both constituent monolayers are rather well-preserved in the resultant heterostructure due to the weak van der Waals interaction between the layers. However, the proximity of MoS 2 induces strong spin orbit coupling effect of strength ∼1 meV in graphene, which is nearly three orders of magnitude larger than the intrinsic spin orbit coupling of pristine graphene. This opens a bandgap in graphene and further causes anticrossings of the spin-nondegenerate bands near the Dirac point. Lattice incommensurate graphene/MoS 2 heterostructure exhibits interesting moiré patterns which have been observed in experiments. The electronic bandstructure of heterostructure is very sensitive to biaxial strain and interlayer twist. Although the Dirac cone of graphene remains intact and no charge-transfer between graphene and MoS 2 layers occurs at ambient conditions, a strain-induced charge-transfer can be realized in graphene/MoS 2 heterostructure. Application of a gate voltage reveals the occurrence of a topological phase transition in graphene/MoS 2 heterostructure. In this chapter, we discuss the crystal structure, interlayer effects, electronic structure, spin states, and effects due to strain and substrate proximity on the electronic properties of graphene/MoS 2 heterostructure. We further present an overview of the distinct topological quantum phases of graphene/MoS 2 heterostructure and review the recent advancements in this field.

ACS Nano, 2021

We demonstrate a graphene−MoS 2 architecture integrating multiple field-effect transistors (FETs), and we independently probe and correlate the conducting properties of van der Waals coupled graphene−MoS 2 contacts with those of the MoS 2 channels. Devices are fabricated starting from high-quality single-crystal monolayers grown by chemical vapor deposition. The heterojunction was investigated by scanning Raman and photoluminescence spectroscopies. Moreover, transconductance curves of MoS 2 are compared with the current−voltage characteristics of graphene contact stripes, revealing a significant suppression of transport on the n-side of the transconductance curve. On the basis of ab initio modeling, the effect is understood in terms of trapping by sulfur vacancies, which counterintuitively depends on the field effect, even though the graphene contact layer is positioned between the backgate and the MoS 2 channel.

Physical Review B, 2016

We theoretically study scattering process and superconducting triplet correlations in a graphene junction comprised of ferromagnet-RSO-superconductor in which RSO stands for a region with Rashba spin orbit interaction. Our results reveal spin-polarized subgap transport through the system due to an anomalous equal-spin Andreev reflection in addition to conventional back scatterings. We calculate equal-and opposite-spin pair correlations near the F-RSO interface and demonstrate direct link of the anomalous Andreev reflection and equal-spin pairings arised due to the proximity effect in the presence of RSO interaction. Moreover, we show that the amplitude of anomalous Andreev reflection, and thus the triplet pairings, are experimentally controllable when incorporating the influences of both tunable strain and Fermi level in the nonsuperconducting region. Our findings can be confirmed by a conductance spectroscopy experiment and provide better insights into the proximity-induced RSO coupling in graphene layers reported in recent experiments 30,31,33,34.

Physical Review B, 2017

Using a wavefunction Dirac Bogoliubov-de Gennes method, we demonstrate that the tunable Fermi level of a graphene layer in the presence of Rashba spin orbit coupling (RSOC) allows for producing an anomalous nonlocal Andreev reflection and equal spin superconducting triplet pairing. We consider a graphene junction of a ferromagnet-RSOC-superconductor-ferromagnet configuration and study scattering processes, the appearance of spin triplet correlations, and charge conductance in this structure. We show that the anomalous crossed Andreev reflection is linked to the equal spin triplet pairing. Moreover, by calculating current cross-correlations, our results reveal that this phenomenon causes negative charge conductance at weak voltages and can be revealed in a spectroscopy experiment, and may provide a tool for detecting the entanglement of the equal spin superconducting pair correlations in hybrid structures.

Physical Review B, 2022

Starting from a low-energy effective Hamiltonian model, we theoretically calculate the dynamical optical conductivity and permittivity tensor of a magnetized graphene layer with Rashba spin-orbit coupling (SOC). Our results reveal a transverse Hall conductivity correlated with the nonreciprocal longitudinal conductivity. Further analysis illustrates that for intermediate magnetization strengths, the relative magnitudes of the magnetization and SOC can be identified experimentally by two well-separated peaks in the dynamical optical response (both the longitudinal and transverse components) as a function of photon frequency. Moreover, the frequencydependent permittivity tensor is obtained for a wide range of chemical potentials and magnetization strengths. Employing experimentally realistic parameter values, we calculate the circular dichroism of a representative device consisting of magnetized spin-orbit coupled graphene and a dielectric insulator layer, backed by a metallic plate. The results reveal that this device has different relative absorptivities for right-handed and left-handed circularly polarized electromagnetic waves. It is found that the magnetized spin-orbit coupled graphene supports strong handedness-switchings, effectively controlled by varying the chemical potential and magnetization strength with respect to the SOC strength.

Advanced materials (Deerfield Beach, Fla.), 2015

The first realization of a tunable band gap in monolayer WS2(1-x) Se2x is demonstrated. The tuning of the band gap exhibits a strong dependence of S and Se contents, as proven by PL spectroscopy. Because of its remarkable electronic structure, monolayer WS2(1-x) Se2x exhibits novel electrochemical catalytic activity and offers long-term electrocatalytic stability for the hydrogen evolution reaction.

physica status solidi (b), 2017

The tunability aspect of the dielectric properties induced by the substrate driven interactions (SDI) and the exchange field (M) due to the ferro-magnetic impurities in graphene monolayer on transition metal dichalcogenide is reported here. The interactions involve sub-latticeresolved, enhanced intrinsic spin-orbit couplings(SOC), the extrinsic Rashba spin-orbit coupling (RSOC), and the orbital gap related to the transfer of the electronic charge from graphene to the substrate. We obtain the gapped bands with a RSOC-dependent pseudo Zeeman field due to the interplay of SDI. This enables us to obtain an expression of the dielectric function in the finite doping case ignoring the spin-flip scattering events completely. We find that the stronger RSOC has foiling effect on the Thomas-Fermi screening length.This foiling effect, over a broad range of the exchange field values (0− 1meV), is an indication of the domination of the Dyakonov-Perel mechanism in the system over the Elliot-Yafet spin relaxation mechanism.The zero of the dielectric function corresponds to the plasmon frequency. We find that there is only one such frequency. The Plasmon dispersion yields the q 2/3 behavior and not the well known √q behavior. We also find that the plasmon frequency could be changed by the tuning of the chemical potential. I. INRODUCTION The isolation and production of the graphene in 2004[1,2] −a two-dimensional material with a single layer honeycomb lattice of carbon atoms− have generated substantial theoretical and experimental research activity[2]. As a material for application in the future nano-scale electronics and photonics, it has attracted great attention worldwide. The graphene was found to possess host of unusual properties, such as the high carrier mobility, the low resistivity, the large in-plane stiffness[3], the larger than metal optical absorption at the Plasmonresonance [4], and so on. The tunable plasmonic materials, like graphene, are required in various areas of photonics. Their applications range from broadband optical modulators, micro-ring resonator, nano-resonators, tunable terahertz meta-materials, tunable terahertz hybrid metal-graphene structure, real-time tunable lasing from nanocavity arrays, perfect absorber materials, and radiators to highly practicable (bio) chemical sensing [5-16]. The optical absorption (nearly 2.3% from the visible to near-infrared spectral windows) of pristine, pure graphene monolayer is featureless as the optical conductivity corresponds to a constant value σ 0 = e 2 /4ћ which is independent of any material parameters [17]. Such featureless -ness is a manifestation of the fact that Dirac fermions here are mass-less. However, these optical properties of graphene can be remarkably altered by changing the Fermi energy with the further addition of charge carriers [18].For example, doped graphene supports plasmons that are tunable, providing a novel platform for tunable devices. The large mobility of charge carriers in graphene makes high-speed tunable plasmonics possible by electrical gating of graphene. Despite these remarkable properties, it has not been possible to fully exploit the graphene's potential due to the difficulty of opening a reasonably sized insulating gap in its band structure. The absence of the gap is owing to graphene's weak spin-orbit coupling.It was shown by Kane and Mele [19] that the gap opens up in the spectrum if one includes intrinsic spin-orbit interactions(SOI)[19]. The spin degeneracy of the spectrum could be lifted through the Rashba SOI which depends upon an external electric field. The experimental finding of a gap of 0.26 eV when graphene is epitaxially grown on the SiC substrate[20] shows the way to yet another mode of gap (∆ orbital) opening. This gap increases as the sample thickness decreases. It has been proposed that the origin of this gap is the breaking of sub-lattice symmetry owing to the graphene-substrate interaction. In this communication we study the interesting and useful possibilities related to the dielectric properties of graphene on TMDC substrate. The possibilities follow on the heels of the engineering of the enhanced spin-orbit coupling (SOC) in graphene through interfacial effects via coupling to the suitable substrates, viz. a two dimensional transition metal dichalcogenide (TMDC). The graphene layer is also exchange(M)coupled to the magnetic impurities, e.g. Fe atoms deposited to the graphene surface. A direct, functional electric field control of magnetism at the nano-scale is needed for the effective demonstration of our results related to the exchange-field dependence. The magnetic multi-ferroics, like BiFeO 3 (BFO) have piqued the interest of the researchers worldwide with the promise of the coupling between the magnetic and electric order parameters.

Nature Communications, 2020

The promise of high-density and low-energy-consumption devices motivates the search for layered structures that stabilize chiral spin textures such as topologically protected skyrmions. At the same time, recently discovered long-range intrinsic magnetic orders in the two-dimensional van der Waals materials provide a new platform for the discovery of novel physics and effects. Here we demonstrate the Dzyaloshinskii–Moriya interaction and Néel-type skyrmions are induced at the WTe2/Fe3GeTe2 interface. Transport measurements show the topological Hall effect in this heterostructure for temperatures below 100 K. Furthermore, Lorentz transmission electron microscopy is used to directly image Néel-type skyrmion lattice and the stripe-like magnetic domain structures as well. The interfacial coupling induced Dzyaloshinskii–Moriya interaction is estimated to have a large energy of 1.0 mJ m−2. This work paves a path towards the skyrmionic devices based on van der Waals layered heterostructures.

Physical Review B, 2019

Topological antiferromagnetic (AFM) spintronics is an emerging field of research, which involves the topological electronic states coupled to the AFM order parameter known as the Néel vector. The control of these states is envisioned through manipulation of the Néel vector by spin-orbit torques driven by electric currents. Here we propose a different approach favorable for low-power AFM spintronics, where the control of the topological states in a two-dimensional material, such as graphene, is performed via the proximity effect by the voltage induced switching of the Néel vector in an adjacent magnetoelectric AFM insulator, such as chromia. Mediated by the symmetry protected boundary magnetization and the induced Rashba-type spin-orbit coupling at the interface between graphene and chromia, the emergent topological phases in graphene can be controlled by the Néel vector. Using density functional theory and tightbinding Hamiltonian approaches, we model a graphene/Cr2O3 (0001) interface and demonstrate non-trivial band gap openings in the graphene Dirac bands asymmetric between the K and K′ valleys. This gives rise to an unconventional quantum anomalous Hall effect (QAHE) with a quantized value of 2e 2 /h and an additional step-like feature at a value close to e 2 /2h, and the emergence of the spin-polarized valley Hall effect (VHE). Furthermore, depending on the Néel vector orientation, we predict the appearance and transformation of different topological phases in graphene across the 180° AFM domain wall, involving the QAHE, the valley-polarized QAHE and the quantum VHE (QVHE), and the emergence of the chiral edge state along the domain wall. These topological properties are controlled by voltage through magnetoelectric switching of the AFM insulator with no need for spin-orbit torques.

Physical Review B, 2015

Many of the exotic properties proposed to occur in graphene rely on the possibility of increasing the spin orbit coupling (SOC). By combining analytical and numerical tight binding calculations, in this work we study the SOC induced by heavy adatoms with active electrons living in p orbitals. Depending on the position of the adatoms on graphene different kinds of SOC appear. Adatoms located in hollow position induce spin conserving intrinsic like SOC whereas a random distribution of adatoms induces a spin flipping Rashba like SOC. The induced SOC is linearly proportional to the adatoms concentration, indicating the inexistent interference effects between different adatoms. By computing the Hall conductivity we have proved the stability of the topological quantum Hall phases created by the adatoms against inhomogeneous spin orbit coupling. For the case of Pb adatoms, we find that a concentration of 0.1 adatom per carbon atom generates SOC's of the order of ∼40meV .

npj Quantum Materials

Electron-hole asymmetry is a fundamental property in solids that can determine the nature of quantum phase transitions and the regime of operation for devices. The observation of electron-hole asymmetry in graphene and recently in twisted graphene and moiré heterostructures has spurred interest into whether it stems from single-particle effects or from correlations, which are core to the emergence of intriguing phases in moiré systems. Here, we report an effective way to access electron-hole asymmetry in 2D materials by directly measuring the quasiparticle self-energy in graphene/Boron Nitride field-effect devices. As the chemical potential moves from the hole to the electron-doped side, we see an increased strength of electronic correlations manifested by an increase in the band velocity and inverse quasiparticle lifetime. These results suggest that electronic correlations intrinsically drive the electron-hole asymmetry in graphene and by leveraging this asymmetry can provide alter...

npj 2D Materials and Applications

Proximity-induced spin–orbit coupling in graphene has led to the observation of intriguing phenomena like time-reversal invariant $${{\mathbb{Z}}}_{2}$$ Z 2 topological phase and spin-orbital filtering effects. An understanding of the effect of spin–orbit coupling on the band structure of graphene is essential if these exciting observations are to be transformed into real-world applications. In this research article, we report the experimental determination of the band structure of single-layer graphene (SLG) in the presence of strong proximity-induced spin–orbit coupling. We achieve this in high-mobility hexagonal boron nitride (hBN)-encapsulated SLG/WSe2 heterostructures through measurements of quantum oscillations. We observe clear spin-splitting of the graphene bands along with a substantial increase in the Fermi velocity. Using a theoretical model with realistic parameters to fit our experimental data, we uncover evidence of a band gap opening and band inversion in the SLG. Fur...

Sensors, 2023

Humidity Effect on Low-Temperature NH 3 Sensing Behavior of In 2 O 3 /rGO Composites under UV Activation.

Science Advances, 2021

Rashba spin-orbit coupling in atomically thin InSe is varied layer by layer and by tuning out-of-plane electric field.

Physical Review B, 2019

We introduce a pz-d coupling model Hamiltonian for the π-graphene/Au bands that predicts a rather large intrinsic spin-orbit (SO) coupling as are being reported in recent experiments and DFT studies. Working within the analytical Slater-Koster tight-binding approach we were able to identify the overlapping orbitals of relevance in the enhancement of the SO coupling for both, the sublattice symmetric (BC), and the ATOP (AC) stacking configurations. Our model effective Hamiltonian reproduces quite well the experimental spectrum for the two registries, and in addition, its shows that the hollow site configuration (BC), in which the A/B sites remain symmetric, yields the larger increase of the SO coupling. We also explore the Au-diluted case keeping the BC configuration and showed that it renders the preservation of the SO-gap with a similar SO interaction enhancement as the undiluted case but with a smaller graphene-gold distance.

Physical Review Materials, 2018

The spin-orbit coupling can lead to exotic states of matter and unexpected behavior of the system properties. In this paper, we investigate the influence of spin-orbit coupling induced by proximity effects on a monolayer of superconductor (with s-wave or d-wave pairing) placed on an insulating bulk. We show that the critical temperatures Tc of the superconducting states can be tuned by the spin-orbit coupling both in the case of on-site and inter-site pairing. Moreover, we discuss a possibility of changing the location of the maximal Tc from the half-filling into the underdoped or overdoped regimes.

Nature communications, 2015

Interfacial interactions allow the electronic properties of graphene to be modified, as recently demonstrated by the appearance of satellite Dirac cones in graphene on hexagonal boron nitride substrates. Ongoing research strives to explore interfacial interactions with other materials to engineer targeted electronic properties. Here we show that with a tungsten disulfide (WS2) substrate, the strength of the spin-orbit interaction (SOI) in graphene is very strongly enhanced. The induced SOI leads to a pronounced low-temperature weak anti-localization effect and to a spin-relaxation time two to three orders of magnitude smaller than in graphene on conventional substrates. To interpret our findings we have performed first-principle electronic structure calculations, which confirm that carriers in graphene on WS2 experience a strong SOI and allow us to extract a spin-dependent low-energy effective Hamiltonian. Our analysis shows that the use of WS2 substrates opens a possible new route ...

Physical Review B, 2022

Twisted bilayer graphene (TBG) realizes a highly tunable, strongly interacting system featuring superconductivity and various correlated insulating states. We establish gate-defined wires in TBG with proximity-induced spin-orbit coupling as (i) a tool for revealing the nature of correlated insulators and (ii) a platform for Majoranabased topological qubits. In particular, we show that the band structure of a gate-defined wire immersed in an 'inter-valley coherent' correlated insulator inherits electrically detectable fingerprints of symmetry breaking native to the latter. Surrounding the wire by a superconducting TBG region on one side and an inter-valley coherent correlated insulator on the other further enables the formation of Majorana zero modes-possibly even at zero magnetic field depending on the precise symmetry-breaking order present. Our proposal not only introduces a highly gate-tunable topological qubit medium relying on internally generated proximity effects, but can also shed light on the Cooper-pairing mechanism in TBG.

Applied Physics Reviews

Two-dimensional (2D) materials and their moiré superlattices represent a new frontier for quantum matter research due to the emergent properties associated with their reduced dimensionality and extreme tunability. The properties of these atomically thin van der Waals (vdW) materials have been extensively studied by tuning a number of external parameters such as temperature, electrostatic doping, magnetic field, and strain. However, so far pressure has been an under-explored tuning parameter in studies of these systems. The relative scarcity of high-pressure studies of atomically thin materials reflects the challenging nature of these experiments, but, concurrently, presents exciting opportunities for discovering a plethora of unexplored new phenomena. Here, we review ongoing efforts to study atomically thin vdW materials and heterostructures using a variety of high-pressure techniques, including diamond anvil cells, piston cylinder cells, and local scanning probes. We further addres...

Physics Letters, 2023

The effect of two-and three-body interactions on the modulation instability (MI) domain formation of a spin-orbit (SO) and Rabi-coupled Bose-Einstein condensate is studied within a quasi-one-dimensional model. To this aim, we perform numerical and analytical investigations of the associated dispersion relations derived from the corresponding coupled Gross-Pitaevskii equation. The interplay between the linear (SO and Rabi) couplings with the nonlinear cubic-quintic interactions are explored in the mixture, considering miscible and immiscible configurations, with a focus on the impact in the analysis of experimental realizations with general binary coupled systems, in which nonlinear interactions can be widely varied together with linear couplings.

Nano Research, 2016

The formation and control of a room-temperature magnetic order in two-dimensional (2D) materials is a challenging quest for the advent of innovative magnetic-and spintronic-based technologies. To date, edge magnetism in 2D materials has been experimentally observed in hydrogen (H)-terminated graphene nanoribbons and graphene nanomeshes (GNM), but the measured magnetization remain far too small to allow envisioning practical applications. Herein, we report experimental evidences of room-temperature large edge ferromagnetism in oxygen (O)-terminated few-layer black phosphorus nanomeshes (BPNMs). The magnetization values per unit area are 100 times larger than those reported for H-terminated GNMs, and magnetism is absent for H-terminated BPNMs. Magnetization measurements and First-principles simulations suggest that the origin of such magnetic order could stem from a ferromagnetic coupling between edge P with O atoms, resulting in a strong spin localization at edge valence band, and from an uniform oxidation of pore edges over large area without the formation of a bulk oxide. Our findings pave the way for realizing high-efficiency 2D flexible magnetic and spintronic devices without the use of rare magnetic elements.

Nature Physics, 2017

A large enhancement in the spin-orbit coupling of graphene has been predicted when interfacing it with semiconducting transition metal dichalcogenides. Signatures of such an enhancement have been reported but the nature of the spin relaxation in these systems remains unknown. Here, we unambiguously demonstrate anisotropic spin dynamics in bilayer heterostructures comprising graphene and tungsten or molybdenum disulphide (WS 2 , MoS 2). We observe that the spin lifetime varies over one order of magnitude depending on the spin orientation, being largest when the spins point out of the graphene plane. This indicates that the strong spin-valley coupling in the transition metal dichalcogenide is imprinted in the bilayer and felt by the propagating spins. These findings provide a rich platform to explore coupled spin-valley phenomena and offer novel spin manipulation strategies based on spin relaxation anisotropy in two-dimensional materials.

Chemical Society reviews, 2018

Since its discovery, graphene has been a promising material for spintronics: its low spin-orbit coupling, negligible hyperfine interaction, and high electron mobility are obvious advantages for transporting spin information over long distances. However, such outstanding transport properties also limit the capability to engineer active spintronics, where strong spin-orbit coupling is crucial for creating and manipulating spin currents. To this end, transition metal dichalcogenides, which have larger spin-orbit coupling and good interface matching, appear to be highly complementary materials for enhancing the spin-dependent features of graphene while maintaining its superior charge transport properties. In this review, we present the theoretical framework and the experiments performed to detect and characterize the spin-orbit coupling and spin currents in graphene/transition metal dichalcogenide heterostructures. Specifically, we will concentrate on recent measurements of Hanle preces...

Nanomaterials, 2021

The strain in hybrid van der Waals heterostructures, made of two distinct two-dimensional van der Waals materials, offers an interesting handle on their corresponding electronic band structure. Such strain can be engineered by changing the relative crystallographic orientation between the constitutive monolayers, notably, the angular misorientation, also known as the “twist angle”. By combining angle-resolved photoemission spectroscopy with density functional theory calculations, we investigate here the band structure of the WS2/graphene heterobilayer for various twist angles. Despite the relatively weak coupling between WS2 and graphene, we demonstrate that the resulting strain quantitatively affects many electronic features of the WS2 monolayers, including the spin-orbit coupling strength. In particular, we show that the WS2 spin-orbit splitting of the valence band maximum at K can be tuned from 430 to 460 meV. Our findings open perspectives in controlling the band dispersion of v...

Physical review, 2019

We propose an interband tunneling picture to explain and predict the interlayer twist angle dependence of the induced spin-orbit coupling in heterostructures of graphene and monolayer transition metal dichalcogenides (TMDCs). We obtain a compact analytic formula for the induced valley Zeeman and Rashba spin-orbit coupling in terms of the TMDC band structure parameters and interlayer tunneling matrix elements. We parametrize the tunneling matrix elements with few parameters, which in our formalism are independent of the twist angle between the layers. We estimate the value of the tunneling parameters from existing DFT calculations at zero twist angle and we use them to predict the induced spin-orbit coupling at non-zero angles. Provided that the energy of the Dirac point of graphene is close to the TMDC conduction band, we expect a sharp increase of the induced spin-orbit coupling around a twist angle of 18 degrees.

ACS Applied Materials & Interfaces, 2023

While technologically challenging, the integration of ferroelectric thin films with graphene spintronics potentially allows the realization of highly efficient, electrically tuneable, non-volatile memories through control of the interfacial spin-orbit driven interaction occuring at graphene/Co interfaces deposited on heavy metal supports. Here, the integration of ferroelectric Hf0.5Zr0.5O2 on graphene/Co/heavy metal epitaxial stacks is investigated via the implementation of several nucleation methods in atomic layer deposition. By employing in-situ Al2O3 as a nucleation layer sandwiched between Hf0.5Zr0.5O2 and graphene, the Hf0.5Zr0.5O2 demonstrates a remanent polarization (2Pr ) of 19.2 µC/cm 2 . Using an ex-situ, naturally oxidized sputtered Ta layer for nucleation, 2Pr could be controlled via the interlayer thickness, reaching maximum values of 28 µC/cm 2 with low coercive fields. Magnetic hysteresis measurements taken before and after atomic layer deposition show strong perpendicular magnetic anisotropy, with minimal deviations in the magnetization reversal pathways due to the Hf0.5Zr0.5O2 deposition process, thus pointing to a good preservation of the magnetic stack including single-layer graphene. X-ray diffraction measurements further confirm that the high-quality interfaces demonstrated in the stack remain unperturbed by the ferroelectric deposition and anneal. The proposed graphene-based ferroelectric/magnetic structures offer the strong advantages of ferroelectricity and ferromagnetism at room temperature, enabling the development of novel magneto-electric and non-volatile in-memory spin-orbit logic architectures with low power switching. 1

Physical Review B, 2015

Enhancement of the spin-orbit coupling in graphene may lead to various topological phenomena and also find applications in spintronics. Adatom absorption has been proposed as an effective way to achieve the goal. In particular, great hope has been held for indium in strengthening the spin-orbit coupling and realizing the quantum spin Hall effect. To search for evidence of the spin-orbit coupling in graphene absorbed with indium adatoms, we carry out extensive transport measurements, i.e., weak localization magnetoresistance, quantum Hall effect and non-local spin Hall effect. No signature of the spin-orbit coupling is found. Possible explanations are discussed.

AIP Advances

Coupling graphene’s excellent electron and spin transport properties with a higher spin–orbit coupling (SOC) material allows tackling the hurdle of spin manipulation in graphene due to the proximity to van der Waals layers. Here, we use magneto-transport measurements to study the electron spin resonance on a combined system of graphene and MoS2 at 1.5 K. The electron spin resonance measurements are performed in the frequency range of 18–33 GHz, which allows us to determine the g-factor in the system. We measure the average g-factor of 1.91 for our hybrid system, which is a considerable shift compared to that observed in graphene on SiO2. This is a clear indication of proximity induced SOC in graphene in accordance with theoretical predictions.

Nature Communications, 2021

Superconductor-ferromagnet interfaces in two-dimensional heterostructures present a unique opportunity to study the interplay between superconductivity and ferromagnetism. The realization of such nanoscale heterostructures in van der Waals (vdW) crystals remains largely unexplored due to the challenge of making atomically-sharp interfaces from their layered structures. Here, we build a vdW ferromagnetic Josephson junction (JJ) by inserting a few-layer ferromagnetic insulator Cr2Ge2Te6 into two layers of superconductor NbSe2. The critical current and corresponding junction resistance exhibit a hysteretic and oscillatory behavior against in-plane magnetic fields, manifesting itself as a strong Josephson coupling state. Also, we observe a central minimum of critical current in some JJ devices as well as a nontrivial phase shift in SQUID structures, evidencing the coexistence of 0 and π phase in the junction region. Our study paves the way to exploring sensitive probes of weak magnetism...

Physical review, 2016

Proximity effects resulting from depositing a graphene layer on a TMD substrate layer change the dynamics of the electronic states in graphene, inducing spin orbit coupling (SOC) and staggered potential effects. An effective Hamiltonian that describes different symmetry breaking terms in graphene, while preserving time reversal invariance, shows that an inverted mass band gap regime is possible. The competition of different perturbation terms causes a transition from an inverted mass phase to a staggered gap in the bilayer heterostructure, as seen in its phase diagram. A tight-binding calculation of the bilayer validates the effective model parameters. A relative gate voltage between the layers may produce such phase transition in experimentally accessible systems. The phases are characterized in terms of Berry curvature and valley Chern numbers, demonstrating that the system may exhibit quantum spin Hall and valley Hall effects.

Physical review, 2018

Depositing monolayer graphene on a transition metal dichalcogenide (TMD) semiconductor substrate has been shown to change the dynamics of the electronic states in graphene, inducing spin orbit coupling (SOC) and staggered potential effects. Theoretical studies on commensurate supercells have demonstrated the appearance of interesting phases, as different materials and relative gate voltages are applied. Here we address the effects of the real incommensurability between lattices by implementing a continuum model approach that does not require small-period supercells. The approach allows us to study the role of possible relative twists of the layers, and verify that the SOC transfer is robust to twists, in agreement with observations. We characterize the nature of the different phases by studying an effective Hamiltonian that fully describes the graphene-TMD heterostructure. We find the system supports topologically non-trivial phases over a wide range of parameter ranges, which require the dominance of the intrinsic SOC over the staggered and Rashba potentials. This tantalizing result suggests the possible experimental realization of a tunable quantum spin Hall phase under suitable conditions. We estimate that most TMDs used to date likely result in weak intrinsic SOC that would not drive the heterostructure into topologically non-trivial phases. Additional means to induce a larger intrinsic SOC, such as strain fields or heavy metal intercalation may be required.

Reviews of Modern Physics, 2020

After the first unequivocal demonstration of spin transport in graphene , surprisingly at room temperature, it was quickly realized that this novel material was relevant for both fundamental spintronics and future applications. Over the decade since, exciting results have made the field of graphene spintronics blossom, and a second generation of studies has extended to new two-dimensional (2D) compounds. This Colloquium reviews recent theoretical and experimental advances on electronic spin transport in graphene and related 2D materials, focusing on emergent phenomena in van der Waals heterostructures and the new perspectives provided by them. These phenomena include proximity-enabled spin-orbit effects, the coupling of electronic spin to light, electrical tunability, and 2D magnetism. I. Introduction 1 A. Initial spin transport experiments 2 B. Fundamentals of spin-orbit coupling in 2D crystals 4 II. State of the art in graphene spintronics 5 A. Spin-orbit coupling in graphene 5 1. hBN-Gr: Long-distance spin transport 5 2. Bilayer and few-layer graphene 7 3. The original Topological insulator 8 B. Corrugations and resonant impurities 8 C. Transition-metal dichalcogenides and graphene 9 D. Graphene/TMDC optospintronics 12 III. Current challenges and way forward 12 A. What is the dominant spin relaxation mechanism? 12 1. Intrinsic relaxation sources 13 2. Extrinsic sources: impurities 14