No abstract

Two dimensional (2D) materials provide a unique platform for spintronics and valleytronics due to the ability to combine vastly different functionalities into one vertically-stacked heterostructure, where the strengths of each of the constituent materials can compensate for the weaknesses of the others.Graphene has been demonstrated to be an exceptional material for spin transport at room temperature, however it lacks a coupling of the spin and optical degrees of freedom. In contrast, spin/valley polarization can be efficiently generated in monolayer transition metal dichalcogenides (TMD) such as MoS 2 via absorption of circularly-polarized photons, but lateral spin or valley transport has not been realized at room temperature. In this letter, we fabricate monolayer MoS 2 /few-layer graphene hybrid spin valves and demonstrate, for the first time, the opto-valleytronic spin injection across a TMD/graphene interface. We observe that the magnitude and direction of spin polarization is controlled by both helicity and photon energy. In addition, Hanle spin precession measurements confirm optical spin injection, spin transport, and electrical detection up to room temperature. Finally, analysis by a one-dimensional driftdiffusion model quantifies the optically injected spin current and the spin transport parameters. Our results demonstrate a 2D spintronic/valleytronic system that achieves optical spin injection and lateral spin transport at room temperature in a single device, which paves the way for multifunctional 2D spintronic devices for memory and logic applications.Keywords: spintronics, valleytronics, graphene, transition metal dichalcogenides, optoelectronics 3 Spintronics and valleytronics, novel fields with large potential impacts in both fundamental science and technology, utilize the electron's spin and valley degrees of freedom, in addition to charge, for information storage and logic operations. In the past decade, experimental studies have established singlelayer and multilayer graphene as among the most promising materials for spintronics due to their high electronic mobility combined with low intrinsic spin-orbit coupling. Graphene exhibits room temperature spin diffusion length of up to tens of microns, substantially longer than conventional metals or semiconductors (<1 micron) [1][2][3][4] . However, graphene's lack of spin-dependent optical selection rules has made opto-spintronic functionality impossible, a substantial limitation for graphene.Fortunately, monolayer MoS 2 and related semiconducting transition metal dichalcogenides (TMDs) exhibit favorable characteristics for nanoscale opto-valleytronic and opto-spintronic applications [5][6][7] .TMDs have strong spin-orbit coupling due to the heavy metal atom and lack inversion symmetry in monolayer form, the combination of which allows complete simultaneous valley and spin polarization through absorption of circularly polarized light [8][9][10][11][12][13][14] . This originates from the valley-dependent optical selection rules of monolayer ...

In two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs), new electronic phenomena such as tunable bandgaps 1-3 and strongly bound excitons and trions emerge from strong many-body effects [4][5][6] , beyond the spin and valley degrees of freedom induced by spin-orbit coupling and by lattice symmetry 7 . Combining single-layer TMDs with other 2D materials in van der Waals heterostructures offers an intriguing means of controlling the electronic properties through these many-body effects, by means of engineered interlayer interactions [8][9][10] . Here, we use micro-focused angle-resolved photoemission spectroscopy (microARPES) and in situ surface doping to manipulate the electronic structure of single-layer WS 2 on hexagonal boron nitride (WS 2 /h-BN). Upon electron doping, we observe an unexpected giant renormalization of the spin-orbit splitting of the single-layer WS 2 valence band, from 430 meV to 660 meV, together with a bandgap reduction of at least 325 meV, attributed to the formation of trionic quasiparticles. These findings suggest that the electronic, spintronic and excitonic properties are widely tunable in 2D TMD/h-BN heterostructures, as these are intimately linked to the quasiparticle dynamics of the materials [11][12][13] . Coulomb interactions in 2D materials are several times stronger than in their 3D counterparts. In 2D TMDs, this is most directly evidenced by the presence of excitons with binding energies an order of magnitude higher than in the bulk 4 . Although the excitons in these 2D materials have been widely studied by optical techniques 13 , the impact of strong electron-electron interactions on the quasiparticle band structure remains unclear. Theory predicts that many-body effects will influence the spin-orbit splitting around the valenceband maximum (VBM) and conduction-band minimum (CBM) 14 . Although these should be observable by ARPES, a direct probe of many-body effects 15 , measurements so far have mainly focused on the layer-dependence of the single-particle spectrum and the direct bandgap transition in 2D TMD systems, including epitaxial single- . Unfortunately, the lateral size of mechanically assembled heterostructures is usually of the order of 10 μ m, much smaller than the beam spot of typical ARPES setups (≳ 100 μ m). Furthermore, sample charging on insulating bulk h-BN substrates would complicate ARPES experiments.We overcome these challenges as follows. We realize a high-quality 2D semiconductor-insulator interface by mechanically transferring a relatively large (~100 μ m) single-layer WS 2 crystal onto a thin flake of h-BN that has itself been transferred onto a degenerately doped TiO 2 substrate, as depicted in Fig. 1a. Sample charging is avoided because there is electrical contact from the continuous single-layer WS 2 flake to both the h-BN and the conductive TiO 2 . Figure 1b is an optical microscope image of the sample, including a flake of h-BN, approximately 100 μ m wide, surrounded by several transferred flakes of single-layer WS 2 on the Ti...

Abstract:We present an experimental study of spin transport in single layer graphene using atomic sheets of hexagonal boron nitride (h-BN) as a tunnel barrier for spin injection. While h-BN is expected to be favorable for spin injection, previous experimental studies have been unable to achieve spin relaxation times in the nanosecond regime, suggesting potential problems originating from the contacts. Here, we investigate spin relaxation in graphene spin valves with h-BN barriers and observe room temperature spin lifetimes in excess of a nanosecond, which provides experimental confirmation that h-BN is indeed a good barrier material for spin injection into graphene. By carrying out measurements with different thicknesses of h-BN, we show that few layer h-BN is a better choice than monolayer for achieving high non-local spin signals and longer spin relaxation times in graphene.* Author to whom correspondence should be addressed: kawakami.15@osu.edu 2 Graphene is a promising spin channel material for next generation spintronic devices due to the experimental demonstration of long spin diffusion lengths at room temperature 1-3 and theoretical predictions of long spin relaxation times 4,5 arising from the weak spin-orbit and hyperfine couplings 5,6 .However, experimentally measured spin relaxation times [1][2][3]7,8 in graphene are orders of magnitude shorter than theoretically predicted 4,5 . In graphene spin valves, the tunnel barrier plays a crucial role for spin injection by circumventing the problem of impedance mismatch 9 between graphene and the ferromagnetic electrodes. As demonstrated by Han et. al. 8 , high quality tunnel barriers are critical for obtaining higher spin relaxation times (τ s ) in graphene because barriers with pinholes or rough surface morphology can cause additional contact-induced spin relaxation, which has received a great deal of interest recently. [10][11][12][13][14] As opposed to growing oxide tunnel barriers on graphene, a thin insulating twodimensional (2D) van der Waals material can also be used as a tunnel barrier. A particular material of interest is single (or few) layer h-BN because of its various suitable properties 15 : large energy band gap ~5.97 eV, high crystallinity, spin filtering 16 , absence of pinholes and dangling bonds, atomic lattice similar to graphene, and chemical stability at ambient conditions. In addition, atomically clean vertical heterostructures of h-BN/graphene can be mechanically assembled using polymer-based transfer techniques 17,18 . The first experimental report demonstrating spin injection into graphene using a monolayer h-BN tunnel barrier showed τ s less than 100 ps 19 . This was followed by the work of Kamalakar et. al. 20,21 and Fu et. al. 22 , which used chemically grown h-BN barriers, yielding τ s ~ 500 ps. Another recent study using an encapsulated geometry 23 with graphene sandwiched between a thick bottom layer of h-BN and a monolayer of h-BN on top showed τ s less than 200 ps. As evident from these studies, graphene spin valve devices ...

Anomalous Nernst effect, a result of charge current driven by temperature gradient, provides a probe of the topological nature of materials due to its sensitivity to the Berry curvature near the Fermi level. Fe3GeTe2, one important member of the recently discovered two-dimensional van der Waals magnetic materials, offers a unique platform for anomalous Nernst effect because of its metallic and topological nature. Here, we report the observation of large anomalous Nernst effect in Fe3GeTe2. The anomalous Hall angle and anomalous Nernst angle are about 0.07 and 0.09 respectively, far larger than those in common ferromagnets. By utilizing the Mott relation, these large angles indicate a large Berry curvature near the Fermi level, consistent with the recent proposal for Fe3GeTe2 as a topological nodal line semimetal candidate. Our work provides evidence of Fe3GeTe2 as a topological ferromagnet, and demonstrates the feasibility of using twodimensional magnetic materials and their band topology for spin caloritronics applications.

No abstract

Graphene has remarkable opportunities for spintronics due to its high mobility and long spin diffusion length, especially when encapsulated in hexagonal boron nitride (h-BN). Here, we demonstrate gate-tunable spin transport in such encapsulated graphene-based spin valves with one-dimensional (1D) ferromagnetic edge contacts. An electrostatic backgate tunes the Fermi level of graphene to probe different energy levels of the spin-polarized density of states (DOS) of the 1D ferromagnetic contact, which interact through a magnetic proximity effect (MPE) that induces ferromagnetism in graphene. In contrast to conventional spin valves, where switching between high- and low-resistance configuration requires magnetization reversal by an applied magnetic field or a high-density spin-polarized current, we provide an alternative path with the gate-controlled spin inversion in graphene.

There is a substantial interest in the heterostructures of semiconducting transition metal dichalcogenides (TMDCs) amongst each other or with arbitrary materials, through which the control of the chemical, structural, electronic, spintronic, and optical properties can lead to a change in device paradigms. A critical need is to understand the interface between TMDCs and insulating substrates, for example high-κ dielectrics, which can strongly impact the electronic properties such as the optical gap. Here we show that the chemical and electronic properties of the single-layer (SL) TMDC, WS 2 , can be transferred onto high-κ transition metal oxide substrates TiO 2 and SrTiO 3 . The resulting samples are much more suitable for measuring their electronic and chemical structures with angle-resolved photoemission than their native-grown SiO 2 substrates. We probe the WS 2 on the micron scale across 100-micron flakes, and find that the occupied electronic structure is exactly as predicted for freestanding SL WS 2 with a strong spin-orbit splitting of 420 meV and a direct band gap at the valence band maximum. Our results suggest that TMDCs can be combined with arbitrary multi-functional oxides, which may introduce alternative means of controlling the optoelectronic properties of such materials.