Magnetoelectric (ME) effect, the phenomenon of inducing magnetization by application of an electric field or vice versa, holds great promise for magnetic sensing and switching applications 1 . Studies of the ME effect have so far focused on the control of the electron spin degree of freedom (DOF) in materials such as multiferroics 2 and conventional semiconductors 3 . Here, we report a new form of the ME effect based on the valley DOF in two-dimensional (2D) Dirac materials 4-6 . By breaking the three-fold rotational symmetry in single-layer MoS 2 via a uniaxial stress, we have demonstrated the pure electrical generation of valley magnetization in this material, and its direct imaging by Kerr rotation microscopy. The observed out-of-plane magnetization is independent of in-plane magnetic field, linearly proportional to the in-plane current density, and optimized when the current is orthogonal to the strain-induced piezoelectric field. These results are fully consistent with a theoretical model of valley magnetoelectricity driven by Berry curvature effects. Furthermore, the effect persists at room temperature, opening possibilities for practical valleytronic devices.Electrons in two-dimensional (2D) Dirac materials including gapped graphene and single-layer transition metal dichalcogenides (TMDs) possess a new two-fold valley degree of freedom (DOF) corresponding to the K and K' valleys of the Brillouin zone. The valley DOF carries orbital magnetic moment [4][5][6] . A net valley magnetization forms the basis for valley-based applications. Such magnetization can arise from either a finite population imbalance between the valleys (i.e. a net valley polarization) or a distribution difference between them without a population imbalance 5 . Whereas the former relaxes by intervalley scattering, the latter is largely limited by intravalley scattering. The presence of valley contrasting Berry curvatures in 2D Dirac materials, which can couple to external electromagnetic excitations, enables the control of valley magnetization [6][7][8][9][10][11][12][13][14][15][16][17] . Although the control of valley magnetization by circularly polarized light and by a vertical magnetic field has now become routine [6][7][8][10][11][12][13][14][15][16] , the development of practical valleytronic devices requires the pure electrical control of valley magnetization. The valley magnetoelectric (ME) effect is an attractive approach for this purpose.To realize the linear ME effect, a material has to possess broken time-reversal and spatial-inversion symmetries 1,18 . For an electrical conductor, time-reversal symmetry can be broken naturally by application of a bias voltage, under which dissipation through carrier scattering is caused by a charge current. The magnetoelectricity produced in this manner is known as the kinematic ME effect 19,20 . Single-layer TMDs such as MoS 2 , which are

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Twist engineering, or the alignment of two-dimensional (2D) crystalline layers with desired orientations, has led to tremendous success in modulating the charge degree of freedom in heteroand homo-structures, in particular, in achieving novel correlated and topological electronic phases in moiré electronic crystals 1,2 . However, although pioneering theoretical efforts have predicted nontrivial magnetism 3,4 and magnons 5 out of twisting 2D magnets, experimental realization of twist engineering spin degree of freedom remains elusive. Here, we leverage the archetypal 2D Ising magnet chromium triiodide (CrI3) to fabricate twisted double bilayer homostructures with tunable twist angles and demonstrate the successful twist engineering of 2D magnetism in them. Using linear and circular polarization-resolved Raman spectroscopy, we identify magneto-Raman signatures of a new magnetic ground state that is sharply distinct from those in natural bilayer (2L) and four-layer (4L) CrI3. With careful magnetic field and twist angle dependence, we reveal that, for a very small twist angle (~ 0.5 o ), this emergent magnetism can be well-approximated by a weighted linear superposition of those of 2L and 4L CI3 whereas, for a relatively large twist angle (~ 5 o ), it mostly resembles that of isolated 2L CrI3. Remarkably, at an intermediate twist angle (~ 1.1 o ), its magnetism cannot be simply inferred from the 2L and 4L cases, because it lacks sharp spin-flip transitions that are present in 2L and 4L CrI3 and features a dramatic Raman circular dichroism that is absent in natural 2L and 4L ones. Our results demonstrate the possibility of designing and controlling the spin degree of freedom in 2D magnets using twist engineering.

A moirésuperlattice formed by stacking two lattice mismatched transition metal dichalcogenide monolayers, functions as a diffusion barrier that affects the energy transport and dynamics of interlayer excitons (electron and hole spatially concentrated in different monolayers). In this work, we experimentally quantify the diffusion barrier experienced by interlayer excitons in hexagonal boron nitrideencapsulated molybdenum diselenide/tungsten diselenide (MoSe 2 / WSe 2 ) heterostructures with different twist angles. We observe the localization of interlayer excitons at low temperature and the temperature-activated diffusivity as a function of twist angle and hence attribute it to the deep periodic potentials arising from the moirésuperlattice. We further support the observations with theoretical calculations, Monte Carlo simulations, and a three-level model that represents the exciton dynamics at various temperatures.

Single-crystal materials with sufficiently low crystal symmetry and strong spin-orbit interactions can be used to generate novel forms of spin-orbit torques on adjacent ferromagnets, such as the out-of-plane antidamping torque previously observed in WTe 2 /ferromagnet heterostructures.Here, we present measurements of spin-orbit torques produced by the low-symmetry material β-MoTe 2 , which unlike WTe 2 retains bulk inversion symmetry. We measure spin-orbit torques on β-MoTe 2 /Permalloy heterostructures using spin-torque ferromagnetic resonance as a function of crystallographic alignment and MoTe 2 thickness down to the monolayer limit. We observe an outof-plane antidamping torque with a spin torque conductivity as strong as 1/3 of that of WTe 2 , demonstrating that the breaking of bulk inversion symmetry in the spin-generation material is not a necessary requirement for producing an out-of-plane antidamping torque. We also measure an unexpected dependence on the thickness of the β-MoTe 2 -the out-of-plane antidamping torque is present in MoTe 2 /Permalloy heterostructures when the β-MoTe 2 is a monolayer or trilayer thick, but goes to zero for devices with bilayer β-MoTe 2 .1 arXiv:1906.01068v1 [cond-mat.mes-hall] 3 Jun 2019Spin-orbit torques represent one of the most promising methods for manipulating emerging magnetic memory technologies [1]. When a charge current is applied to a material with large spin-orbit coupling, such as a heavy metal [2][3][4][5][6][7], topological insulator [8,9], or transition metal dichalcogenide (TMD) [10-16], a spin current generated through mechanisms such as the spin Hall or Rashba-Edelstein effects can be used to exert a torque on an adjacent ferromagnet. Recent work from several research groups has focused on understanding how a controlled breaking of symmetry in a spin-generating material / ferromagnet heterostructure can be used to tune the direction of the observed spin-orbit torques for optimal switching of magnetic devices [12][13][14][17][18][19][20][21][22][23][24]. For instance, the presence of magnetic order within a spin-generation layer can allow current-generated spin directions that are typically forbidden for highly-symmetric non-magnetic metals [17][18][19][20]. Similarly, our group has shown that by using WTe 2 as the spin-source material, a TMD with a low-symmetry crystal structure, it is possible to generate an out-of-plane antidamping torque [12,13] -the component of torque required for the most efficient mode of switching for magnets with perpendicular magnetic anisotropy, but forbidden in higher-symmetry materials. Only one other material, SrRuO 3 , has been shown to generate an out-of-plane antidamping spin-orbit torque [20], arising from symmetry breaking associated with magnetic order. Many questions remain regarding the mechanism and necessary conditions for generating a strong out-of-plane antidamping torque.In this work, we study the spin-orbit torques generated in TMD/ferromagnet heterostructures with a crystal symmetry that is distinct from WTe 2 i...

van der Waals (vdW) magnets are receiving ever-growing attention nowadays due to their significance in both fundamental research on low-dimensional magnetism and potential applications in spintronic devices. The high crystalline quality of vdW magnets is the key to maintaining intrinsic magnetic and electronic properties, especially when exfoliated down to the two-dimensional limit. Here, ultrahigh-quality air-stable vdW CrSBr crystals are synthesized using the direct solid–vapor synthesis method. The high single crystallinity and spatial homogeneity have been thoroughly evidenced at length scales from submm to atomic resolution by X-ray diffraction, second harmonic generation, and scanning transmission electron microscopy. More importantly, specific heat measurements of ultrahigh-quality CrSBr crystals show three thermodynamic anomalies at 185, 156, and 132 K, revealing a stage-by-stage development of the magnetic order upon cooling, which is also corroborated with the magnetization and transport results. Our ultrahigh-quality CrSBr can further be exfoliated down to monolayers and bilayers easily, providing the building blocks of heterostructures for spintronic and magneto-optoelectronic applications.

The strong excitonic effect in monolayer transition metal dichalcogenide (TMD) semiconductors has enabled many fascinating light-matter interaction phenomena. Examples include strongly coupled exciton-polaritons and nearly perfect atomic monolayer mirrors. The strong light-matter interaction also opens the door for dynamical control of mechanical motion through the exciton resonance of monolayer TMDs. Here we report the observation of exciton-optomechanical coupling in a suspended monolayer MoSe2 mechanical resonator. By moderate optical pumping near the MoSe2 exciton resonance, we have observed optical damping and anti-damping of mechanical vibrations as well as the optical spring effect. The exciton-optomechanical coupling strength is also gate-tunable. Our observations can be understood in a model based on photothermal backaction and gate-induced mirror symmetry breaking in the device structure. The observation of gate-tunable exciton-optomechanical coupling in a monolayer semiconductor may find applications in nanoelectromechanical systems (NEMS) and in exciton-optomechanics.

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