Charge-to-Spin Conversion by the Rashba-Edelstein Effect in Two-Dimensional van der Waals Heterostructures up to Room Temperature

The proximity of a transition-metal dichalcogenide (TMD) to graphene imprints a rich spin texture in graphene and complements its high-quality charge/spin transport by inducing spin-orbit coupling (SOC). Rashba and valley-Zeeman SOCs are the origin of charge-to-spin conversion mechanisms such as the Rashba-Edelstein effect (REE) and spin Hall effect (SHE). In this work, we experimentally demonstrate for the first time charge-to-spin conversion due to the REE in a monolayer WS2-graphene van der Waals heterostructure. We measure the current-induced spin polarization up to room temperature and control it by a gate electric field. Our observation of the REE and the inverse of the effect (IREE) is accompanied by the SHE, which we discriminate by symmetry-resolved spin precession under oblique magnetic fields. These measurements also allow for the quantification of the efficiencies of charge-to-spin conversion by each of the two effects. These findings are a clear indication of induced Rashba and valley-Zeeman SOC in graphene that lead to the generation of spin accumulation and spin current without using ferromagnetic electrodes. These realizations have considerable significance for spintronic applications, providing accessible routes toward all-electrical spin generation and manipulation in two-dimensional materials.

Enhancing magnetic ordering in Cr-doped Bi2Se3 using high-TC ferrimagnetic insulator

We report a study of enhancing the magnetic ordering in a model magnetically doped topological insulator (TI), Bi(2-x)Cr(x)Se(3), via the proximity effect using a high-TC ferrimagnetic insulator Y(3)Fe(5)O(12). The FMI provides the TI with a source of exchange interaction yet without removing the nontrivial surface state. By performing the elemental specific X-ray magnetic circular dichroism (XMCD) measurements, we have unequivocally observed an enhanced TC of 50 K in this magnetically doped TI/FMI heterostructure. We have also found a larger (6.6 nm at 30 K) but faster decreasing (by 80% from 30 to 50 K) penetration depth compared to that of diluted ferromagnetic semiconductors (DMSs), which could indicate a novel mechanism for the interaction between FMIs and the nontrivial TIs surface.

Spin-Split Band Hybridization in Graphene Proximitized with α-RuCl3 Nanosheets

Proximity effects induced in the two-dimensional Dirac material graphene potentially open access to novel and intriguing physical phenomena. Thus far, the coupling between graphene and ferromagnetic insulators has been experimentally established. However, only very little is known about graphene's interaction with antiferromagnetic insulators. Here, we report a low-temperature study of the electronic properties of high quality van der Waals heterostructures composed of a single graphene layer proximitized with α-RuCl₃. The latter is known to become antiferromagnetically ordered below 10 K. Shubnikov-de Haas oscillations in the longitudinal resistance together with Hall resistance measurements provide clear evidence for a band realignment that is accompanied by a transfer of electrons originally occupying the graphene's spin degenerate Dirac cones into α-RuCl₃ band states with in-plane spin polarization. Left behind are holes in two separate Fermi pockets, only the dispersion of one of which is distorted near the Fermi energy due to spin selective hybridization with these spin polarized α-RuCl₃ band states. This interpretation is supported by our density functional theory calculations. An unexpected damping of the quantum oscillations as well as a zero-field resistance upturn close to the Néel temperature of α-RuCl₃ suggest the onset of additional spin scattering due to spin fluctuations in the α-RuCl₃.

Proximity-Induced Superconductivity in Monolayer MoS2

Proximity effects in superconducting normal (SN) material heterostructures with metals and semiconductors have long been observed and theoretically described in terms of Cooper pair wave functions and Andreev reflections. Whereas the semiconducting N-layer materials in the proximity experiments to date have been doped and tens of nanometers thick, we present here a proximity tunneling study involving a pristine single-layer transition-metal dichalcogenide film of MoS₂ placed on top of a Pb thin film. Scanning tunneling microscopy and spectroscopy experiments together with parallel theoretical analysis based on electronic structure calculations and Green's function modeling allow us to unveil a two-step process in which MoS₂ first becomes metallic and then is induced into becoming a conventional s-wave Bardeen-Cooper-Schrieffer-type superconductor. The lattice mismatch between the MoS₂ overlayer and the Pb substrate is found to give rise to a topographic moiré pattern. Even though the induced gap appears uniform in location, the coherence peak height of the tunneling spectra is modulated spatially into a moiré pattern that is similar to but shifted with respect to the moiré pattern observed in topography. The aforementioned modulation is shown to originate from the atomic-scale structure of the SN interface and the nature of local atomic orbitals that are involved in generating the local pairing potential. Our study indicates that the local modulation of induced superconductivity in MoS₂ could be controlled via geometrical tuning, and it thus shows promise toward the integration of monolayer superconductors into next-generation functional electronic devices by exploiting proximity-effect control of quantum phases.

Effect of Distance on Photoluminescence Quenching and Proximity-Induced Spin-Orbit Coupling in Graphene/WSe2 Heterostructures

Spin-orbit coupling (SOC) in graphene can be greatly enhanced by proximity coupling it to transition metal dichalcogenides (TMDs) such as WSe₂. We find that the strength of the acquired SOC in graphene depends on the stacking order of the heterostructures when using hexagonal boron nitride ( h-BN) as the capping layer, i.e., SiO₂/graphene/WSe₂/ h-BN exhibiting stronger SOC than SiO₂/WSe₂/graphene/ h-BN. We utilize photoluminescence (PL) as an indicator to characterize the interaction between graphene and monolayer WSe₂ grown by chemical vapor deposition. We observe much stronger PL quenching in the SiO₂/graphene/WSe₂/ h-BN stack than in the SiO₂/WSe₂/graphene/ h-BN stack and, correspondingly, a much larger weak antilocalization (WAL) effect or stronger induced SOC in the former than in the latter. We attribute these two effects to the interlayer distance between graphene and WSe₂, which depends on whether graphene is in immediate contact with h-BN. Our observations and hypothesis are further supported by first-principles calculations, which reveal a clear difference in the interlayer distance between graphene and WSe₂ in these two stacks.

Proximity-Induced Superconductivity with Subgap Anomaly in Type II Weyl Semi-Metal WTe2

Due to the nontrivial topological band structure in type II Weyl semi-metal tungsten ditelluride (WTe₂), unconventional properties may emerge in its superconducting phase. While realizing intrinsic superconductivity has been challenging in the type II Weyl semi-metal WTe₂, the proximity effect may open an avenue for the realization of superconductivity. Here, we report the observation of proximity-induced superconductivity with a long coherence length along the c axis in WTe₂ thin flakes based on a WTe₂/NbSe₂ van der Waals heterostructure. Interestingly, we also observe anomalous oscillations of the differential resistance during the transition from the superconducting to the normal state. Theoretical calculations show excellent agreement with experimental results, revealing that such a subgap anomaly is the intrinsic property of WTe₂ in superconducting state induced by the proximity effect. Our findings enrich the understanding of the superconducting phase of type II Weyl semi-metals and pave the way for their future applications in topological quantum computing.

Two-dimensional ferromagnetic superlattices

Mechanically exfoliated two-dimensional ferromagnetic materials (2D FMs) possess long-range ferromagnetic order and topologically nontrivial skyrmions in few layers. However, because of the dimensionality effect, such few-layer systems usually exhibit much lower Curie temperature (T C) compared to their bulk counterparts. It is therefore of great interest to explore effective approaches to enhance their T C, particularly in wafer-scale for practical applications. Here, we report an interfacial proximity-induced high-T C 2D FM Fe3GeTe2 (FGT) via A-type antiferromagnetic material CrSb (CS) which strongly couples to FGT. A superlattice structure of (FGT/CS)n, where n stands for the period of FGT/CS heterostructure, has been successfully produced with sharp interfaces by molecular-beam epitaxy on 2-inch wafers. By performing elemental specific X-ray magnetic circular dichroism (XMCD) measurements, we have unequivocally discovered that T C of 4-layer Fe3GeTe2 can be significantly enhanced from 140 K to 230 K because of the interfacial ferromagnetic coupling. Meanwhile, an inverse proximity effect occurs in the FGT/CS interface, driving the interfacial antiferromagnetic CrSb into a ferrimagnetic state as evidenced by double-switching behavior in hysteresis loops and the XMCD spectra. Density functional theory calculations show that the Fe-Te/Cr-Sb interface is strongly FM coupled and doping of the spin-polarized electrons by the interfacial Cr layer gives rise to the T C enhancement of the Fe3GeTe2 films, in accordance with our XMCD measurements. Strikingly, by introducing rich Fe in a 4-layer FGT/CS superlattice, T C can be further enhanced to near room temperature. Our results provide a feasible approach for enhancing the magnetic order of few-layer 2D FMs in wafer-scale and render opportunities for realizing realistic ultra-thin spintronic devices.

Emergent Multifunctional Magnetic Proximity in van der Waals Layered Heterostructures

Proximity effect, which is the coupling between distinct order parameters across interfaces of heterostructures, has attracted immense interest owing to the customizable multifunctionalities of diverse 3D materials. This facilitates various physical phenomena, such as spin order, charge transfer, spin torque, spin density wave, spin current, skyrmions, and Majorana fermions. These exotic physics play important roles for future spintronic applications. Nevertheless, several fundamental challenges remain for effective applications: unavoidable disorder and lattice mismatch limits in the growth process, short characteristic length of proximity, magnetic fluctuation in ultrathin films, and relatively weak spin-orbit coupling (SOC). Meanwhile, the extensive library of atomically thin, 2D van der Waals (vdW) layered materials, with unique characteristics such as strong SOC, magnetic anisotropy, and ultraclean surfaces, offers many opportunities to tailor versatile and more effective functionalities through proximity effects. Here, this paper focuses on magnetic proximity, i.e., proximitized magnetism and reviews the engineering of magnetism-related functionalities in 2D vdW layered heterostructures for next-generation electronic and spintronic devices. The essential factors of magnetism and interfacial engineering induced by magnetic layers are studied. The current limitations and future challenges associated with magnetic proximity-related physics phenomena in 2D heterostructures are further discussed.

Proximity Effect Induced Spin Injection in Phosphorene on Magnetic Insulator

Black phosphorus is a promising candidate for future nanoelectronics with a moderate electronic band gap and a high carrier mobility. Introducing the magnetism into black phosphorus will widely expand its application scope and may present a bright prospect in spintronic nanodevices. Here, we report our first-principles calculations of spin-polarized electronic structure of monolayer black phosphorus (phosphorene) adsorbed on a magnetic europium oxide (EuO) substrate. Effective spin injection into the phosphorene is realized by means of interaction with the nearby EuO(111) surface, i.e., proximity effect, which results in spin-polarized electrons in the 3p orbitals of phosphorene, with the spin polarization at Fermi level beyond 30%, together with an exchange-splitting energy of ∼0.184 eV for conduction-band minimum of the adsorbed phosphorene corresponding to an energy region where only one spin channel is conductive. The energy region of these exchange-splitting and spin-polarized band gaps of the adsorbed phosphorene can be effectively modulated by in-plane strain. Intrinsically high and anisotropic carrier mobilities at the conduction-band minimum of the phosphorene also become spin-polarized mainly due to spin polarization of deformation potentials and are not depressed significantly after the adsorption. These extraordinary properties would endow black phosphorus with great potentials in the future spintronic nanodevices.

Magnetic Topological Insulator Heterostructures: A Review

Topological insulators (TIs) provide intriguing prospects for the future of spintronics due to their large spin-orbit coupling and dissipationless, counter-propagating conduction channels in the surface state. The combination of topological properties and magnetic order can lead to new quantum states including the quantum anomalous Hall effect that was first experimentally realized in Cr-doped (Bi,Sb)₂ Te₃ films. Since magnetic doping can introduce detrimental effects, requiring very low operational temperatures, alternative approaches are explored. Proximity coupling to magnetically ordered systems is an obvious option, with the prospect to raise the temperature for observing the various quantum effects. Here, an overview of proximity coupling and interfacial effects in TI heterostructures is presented, which provides a versatile materials platform for tuning the magnetic and topological properties of these exciting materials. An introduction is first given to the heterostructure growth by molecular beam epitaxy and suitable structural, electronic, and magnetic characterization techniques. Going beyond transition-metal-doped and undoped TI heterostructures, examples of heterostructures are discussed, including rare-earth-doped TIs, magnetic insulators, and antiferromagnets, which lead to exotic phenomena such as skyrmions and exchange bias. Finally, an outlook on novel heterostructures such as intrinsic magnetic TIs and systems including 2D materials is given.

Proximity Enhanced Hydrogen Evolution Reactivity of Substitutional Doped Monolayer WS2

The development of stable and low-cost catalysts with high reactivity to replace Pt-based ones is the central focus but challenging for hydrogen evolution reaction (HER). The incorporation of single atoms into two-dimensional (2D) supports has been demonstrated as an effective strategy because of the highly active single atomic sites and extremely large surface area of two-dimensional materials. However, the doping of single atoms is normally performed on the surface suffering from low stability, especially in acidic media. Moreover, it is experimentally challenging to produce monolayered 2D materials with atomic doping. Here, we propose a strategy to incorporate single foreign Fe atoms to substitute W atoms in sandwiched two-dimensional WS₂. Because of the charge transfer between the doped Fe atom and its neighboring S atoms on the surface, the proximate S atoms become active for HER. Our theoretical prediction is later verified experimentally, showing an enhanced catalytic reactivity of Fe-doped WS₂ in HER with the Volmer-Heyrovsky mechanism involved. We refer to this strategy as proximity catalysis, which is expected to be extendable to more sandwiched two-dimensional materials as substrates and transition metals as dopants.

Proximity-Dependent Switchable ATP Aptasensors Utilizing a High-Performance FRET Reporter

Here, fluorescent molecular rotors are employed to develop a new type of high-performance FRET system with large Stokes shift, high photostability, and pH insensitivity, showing great promise for use in proximity-dependent DNA aptasensors. Two carboxylated benzothiazole-based molecular rotors are synthesized, displaying bright green and red fluorescence once labeled to DNA. In the proximity state, an efficient FRET occurs between the two dyes, comparable to that of the most commonly used Cy3/Cy5 pair. Similar phenomena are also observed if naphthothiazole-based analogues are adopted. Our developed FRET pair is then attached to the two parts of a split ATP aptamer in a dimeric DNA nanoscaffold controlled by a bimolecular i-motif. In this way, a pH-switched proximity-induced fluorescent ATP aptasensor is constructed, with good sensitivity, selectivity, and reconfiguration. Furthermore, by altering the linker length of the switching unit, the proximity effect is investigated systematically, providing new insight into DNA proximity reactions and their roles in some physiological processes.

Spin Proximity Effects in Graphene/Topological Insulator Heterostructures

Enhancing the spin-orbit interaction in graphene, via proximity effects with topological insulators, could create a novel 2D system that combines nontrivial spin textures with high electron mobility. To engineer practical spintronics applications with such graphene/topological insulator (Gr/TI) heterostructures, an understanding of the hybrid spin-dependent properties is essential. However, to date, despite the large number of experimental studies on Gr/TI heterostructures reporting a great variety of remarkable (spin) transport phenomena, little is known about the true nature of the spin texture of the interface states as well as their role on the measured properties. Here, we use ab initio simulations and tight-binding models to determine the precise spin texture of electronic states in graphene interfaced with a Bi₂Se₃ topological insulator. Our calculations predict the emergence of a giant spin lifetime anisotropy in the graphene layer, which should be a measurable hallmark of spin transport in Gr/TI heterostructures and suggest novel types of spin devices.

Nanoparticle-Induced Anomalous Hall Effect in Graphene

Schemes for introducing magnetic properties into graphene are of fundamental interest and could enable the development of electrically controlled magnetic devices, thereby extending graphene's applications from conventional electronics to spintronics. Proximity-induced ferromagnetism (PIFM) has been reported for graphene coupled to adjacent ferromagnetic insulators (FMIs). PIFM from an FMI preserves graphene's high carrier mobility and does not introduce a parallel current path. However, few FMIs other than yttrium-iron-garnet are suitable for practical applications due to difficulties in their growth and deposition and to their typically low Curie temperatures. Furthermore, it is difficult to obtain a high-quality FMI/graphene interface by graphene transfer methods, which are essential for obtaining the required interfacial exchange coupling. Here, we report the observation of the anomalous Hall effect (AHE) in graphene proximity coupled to an array of magnetic nanoparticles. This observation of AHE in graphene in proximity to a discontinuous magnetic structure opens the door to realizing magnetic properties in graphene from a greatly expanded range of materials and offers new possibilities for realizing patterned spintronic devices and circuitry.

Gate-Tunable Proximity Effects in Graphene on Layered Magnetic Insulators

The extreme versatility of van der Waals materials originates from their ability to exhibit new electronic properties when assembled in close proximity to dissimilar crystals. For example, although graphene is inherently nonmagnetic, recent work has reported a magnetic proximity effect in graphene interfaced with magnetic substrates, potentially enabling a pathway toward achieving a high-temperature quantum anomalous Hall effect. Here, we investigate heterostructures of graphene and chromium trihalide magnetic insulators (CrI₃, CrBr₃, and CrCl₃). Surprisingly, we are unable to detect a magnetic exchange field in the graphene but instead discover proximity effects featuring unprecedented gate tunability. The graphene becomes highly hole-doped due to charge transfer from the neighboring magnetic insulator and further exhibits a variety of atypical gate-dependent transport features. The charge transfer can additionally be altered upon switching the magnetic states of the nearest CrI₃ layers. Our results provide a roadmap for exploiting proximity effects arising in graphene coupled to magnetic insulators.

Spin-Orbit-Induced Ising Ferromagnetism at a van der Waals Interface

Magnetocrystalline anisotropy, a key ingredient for establishing long-range order in a magnetic material down to the two-dimensional (2D) limit, is generally associated with spin-orbit interaction (SOI) involving a finite orbital angular momentum. Here we report strong out-of-plane magnetic anisotropy without orbital angular momentum, emerging at the interface between two different van der Waals (vdW) materials, an archetypal metallic vdW material NbSe₂ possessing Zeeman-type SOI and an isotropic vdW ferromagnet V₅Se₈. We found that the Zeeman SOI in NbSe₂ induces robust out-of-plane magnetic anisotropy in V₅Se₈ down to the 2D limit with a more than 2-fold enhancement of the transition temperature. We propose a simple model that takes into account the energy gain in NbSe₂ in contact with a ferromagnet, which naturally explains our observations. Our results demonstrate a conceptually new magnetic proximity effect at the vdW interface, expanding the horizons of emergent phenomena achievable in vdW heterostructures.

Encapsulated Silicene: A Robust Large-Gap Topological Insulator

The quantum spin Hall (QSH) effect predicted in silicene has raised exciting prospects of new device applications compatible with current microelectronic technology. Efforts to explore this novel phenomenon, however, have been impeded by fundamental challenges imposed by silicene's small topologically nontrivial band gap and fragile electronic properties susceptible to environmental degradation effects. Here we propose a strategy to circumvent these challenges by encapsulating silicene between transition-metal dichalcogenides (TMDCs) layers. First-principles calculations show that such encapsulated silicene exhibit a two-orders-of-magnitude enhancement in its nontrivial band gap, which is driven by the strong spin-orbit coupling effect in TMDCs via the proximity effect. Moreover, the cladding TMDCs layers also shield silicene from environmental gases that are detrimental to the QSH state in free-standing silicene. The encapsulated silicene represents a novel two-dimensional topological insulator with a robust nontrivial band gap suitable for room-temperature applications, which has significant implications for innovative QSH device design and fabrication.

Protein-Templated Reactions Using DNA-Antibody Conjugates

DNA-templated chemical reactions have found wide applications in drug discovery, programmed multistep synthesis, nucleic acid detection, and targeted drug delivery. The control of these reactions has, however, been limited to nucleic acid hybridization as a means to direct the proximity between reactants. In this work a system capable of translating protein-protein binding events into a DNA-templated reaction which leads to the covalent formation of a product is introduced. Protein-templated reactions by employing two DNA-antibody conjugates that are both able to recognize the same target protein and to colocalize a pair of reactant DNA strands able to undergo a click reaction are achieved. Two individual systems, each responsive to human serum albumin (HSA) and human IgG, are engineered and it is demonstrated that, while no reaction occurs in the absence of proteins, both protein-templated reactions can occur simultaneously in the same solution without any inter-system crosstalk.

Proximity Effect Induced Electronic Properties of Graphene on Bi₂Te₂Se

We report that the π-electrons of graphene can be spin-polarized to create a phase with a significant spin-orbit gap at the Dirac point (DP) using a graphene-interfaced topological insulator hybrid material. We have grown epitaxial Bi2Te2Se (BTS) films on a chemical vapor deposition (CVD) graphene. We observe two linear surface bands from both the CVD graphene notably flattened and BTS coexisting with their DPs separated by 0.53 eV in the photoemission data measured with synchrotron photons. We further demonstrate that the separation between the two DPs, Δ(D-D), can be artificially fine-tuned by adjusting the amount of Cs atoms adsorbed on the graphene to a value as small as Δ(D-D) = 0.12 eV to find any proximity effect induced by the DPs. Our density functional theory calculation shows the opening of a spin-orbit gap of ∼20 meV in the π-band, enhanced by 3 orders of magnitude from that of a pristine graphene, and a concomitant phase transition from a semimetallic to a quantum spin Hall phase when Δ(D-D) ≤ 0.20 eV. We thus present a practical means of spin-polarizing the π-band of graphene, which can be pivotal to advance graphene-based spintronics.

Transient Optical Modulation of Two-Dimensional Materials by Excitons at Ultimate Proximity

Controlling the optical response of two-dimensional (2D) layered materials is critical for their optoelectronic and photonic applications. Current transient optical modulation of 2D semiconductors is mainly based on the band filling effect, which requires internal exciton/charge occupation from photoexcitation or charge injection. However, 2D atomically thin layers exhibit a strong excitonic effect and environmental sensitivity, offering exciting opportunities to engineer their optical properties through an external dielectric or electronic environment. Here, using femtosecond transient absorption spectroscopy as a tool and transition-metal dichalcogenide (TMD) van der Waals heterostructures with type I band alignment, we show the transient absorption modulation of the TMD layer by excitons at ultimate proximity without direct photoexcitation or exciton/charge occupation. Further layer-dependent study indicates the presence of excitons reduces the exciton oscillator strength in adjacent layers through the electric field effect because of environmental sensitivity and proximity of 2D materials. This result demonstrates the transient optical modulation with decoupled light absorption and modulation components and suggests an alternative approach to control the optical response of 2D materials for optoelectronic and photonic applications.

Nanoparticle Shape Evolution and Proximity Effects During Tip-Induced Electrochemical Processes

Voltage spectroscopies in scanning probe microscopy (SPM) techniques are widely used to investigate the electrochemical processes in nanoscale volumes, which are important for current key applications, such as batteries, fuel cells, catalysts, and memristors. The spectroscopic measurements are commonly performed on a grid of multiple points to yield spatially resolved maps of reversible and irreversible electrochemical functionalities. Hence, the spacing between measurement points is an important parameter to be considered, especially for irreversible electrochemical processes. Here, we report nonlocal electrochemical dynamics in chains of Ag particles fabricated by the SPM tip on a silver ion solid electrolyte. When the grid spacing is small compared with the size of the formed Ag particles, anomalous chains of unequally sized particles with double periodicity evolve. This behavior is ascribed to a proximity effect during the tip-induced electrochemical process, specifically, size-dependent silver particle growth following the contact between the particles. In addition, fractal shape evolution of the formed Ag structures indicates that the growth-limiting process changes from Ag(+)/Ag redox reaction to Ag(+)-ion diffusion with the increase in the applied voltage and pulse duration. This study shows that characteristic shapes of the electrochemical products are good indicators for determining the underlying growth-limiting process, and emergence of complex phenomena during spectroscopic mapping of electrochemical functionalities.

Proximity-Induced Spin Polarization of Graphene in Contact with Half-Metallic Manganite

The role of proximity contact with magnetic oxides is of particular interest from the expectations of the induced spin polarization and weak interactions at the graphene/magnetic oxide interfaces, which would allow us to achieve efficient spin-polarized injection in graphene-based spintronic devices. A combined approach of topmost-surface-sensitive spectroscopy utilizing spin-polarized metastable He atoms and ab initio calculations provides us direct evidence for the magnetic proximity effect in the junctions of single-layer graphene and half-metallic manganite La0.7Sr0.3MnO3 (LSMO). It is successfully demonstrated that in the graphene/LSMO junctions a sizable spin polarization is induced at the Fermi level of graphene in parallel to the spin polarization direction of LSMO without giving rise to a significant modification in the π band structure.

Electronic Coupling between Graphene and Topological Insulator Induced Anomalous Magnetotransport Properties

It has been theoretically proposed that the spin textures of surface states in a topological insulator can be directly transferred to graphene by means of the proximity effect, which is very important for realizing a two-dimensional topological insulator based on graphene. Here we report the anomalous magnetotransport properties of graphene-topological insulator Bi₂Se₃ heterojunctions, which are sensitive to the electronic coupling between graphene and the topological surface state. The coupling between the p_z orbitals of graphene and the p orbitals of the surface states on the Bi₂Se₃ bottom surface can be enhanced by applying a perpendicular negative magnetic field, resulting in a giant negative magnetoresistance at the Dirac point up to about -91%. An obvious resistance dip in the transfer curve at the Dirac point is also observed in the hybrid devices, which is consistent with theoretical predictions of the distorted Dirac bands with nontrivial spin textures inherited from the Bi₂Se₃ surface states.

High-Temperature Anomalous Hall Effect in a Transition Metal Dichalcogenide Ferromagnetic Insulator Heterostructure

Integration of transition metal dichalcogenides (TMDs) on ferromagnetic materials (FM) may yield fascinating physics and promise for electronics and spintronic applications. In this work, high-temperature anomalous Hall effect (AHE) in the TMD ZrTe₂ thin film using a heterostructure approach by depositing it on a ferrimagnetic insulator YIG (Y₃Fe₅O₁₂, yttrium iron garnet) is demonstrated. In this heterostructure, significant anomalous Hall effect can be observed at temperatures up to at least 400 K, which is a record high temperature for the observation of AHE in TMDs, and the large R_AHE is more than 1 order of magnitude larger than those previously reported values in topological insulators or TMD-based heterostructures. A complicated interface with additional ZrO₂ and amorphous YIG layers is actually observed between ZrTe₂ and YIG. The magnetization of interfacial reaction-induced ZrO₂ and YIG is believed to play a crucial role in the induced high-temperature AHE in the ZrTe₂. These results present a promising system for the spintronic device applications, and it may shed light on the designing approach to introduce magnetism to TMDs at room temperature.

Proximity Effect Transfer from NbTi into a Semiconductor Heterostructure via Epitaxial Aluminum

We demonstrate the transfer of the superconducting properties of NbTi, a large-gap high-critical-field superconductor, into an InAs heterostructure via a thin intermediate layer of epitaxial Al. Two device geometries, a Josephson junction and a gate-defined quantum point contact, are used to characterize interface transparency and the two-step proximity effect. In the Josephson junction, multiple Andreev reflections reveal near-unity transparency with an induced gap Δ* = 0.50 meV and a critical temperature of 7.8 K. Tunneling spectroscopy yields a hard induced gap in the InAs adjacent to the superconductor of Δ* = 0.43 meV with substructure characteristic of both Al and NbTi.