Monolayers of group-VI transition metal dichalcogenides (TMDs) of the hexagonal polytype consist of a single layer of transition metal atoms sandwiched between two layers of chalcogen atoms in a trigonal prismatic structure (Fig. 1a). Because of the absence of inversion symmetry, electrons in monolayer TMDs at the K and the K' valley of the Brillouin zone possess finite but opposite Berry curvatures 3,13

Single-photon emitters play an essential role in quantum technologies, including quantum computing and quantum communications. Atomic defects in hexagonal boron nitride ( h-BN) have recently emerged as new room-temperature single-photon emitters in solid-state systems, but the development of scalable and tunable h-BN single-photon emitters requires external methods that can control the emission energy of individual defects. Here, by fabricating van der Waals heterostructures of h-BN and graphene, we demonstrate the electrical control of single-photon emission from atomic defects in h-BN via the Stark effect. By applying an out-of-plane electric field through graphene gates, we observed Stark shifts as large as 5.4 nm per GV/m. The Stark shift generated upon a vertical electric field suggests the existence of out-of-plane dipole moments associated with atomic defect emitters, which is supported by first-principles theoretical calculations. Furthermore, we found field-induced discrete modification and stabilization of emission intensity, which were reversibly controllable with an external electric field.

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

Color centers in 2-dimensional hexagonal boron nitride (h-BN) have recently emerged as stable and bright single-photon emitters (SPEs) operating at room temperature. In this study, we combine theory and experiment to show that vacancy-based SPEs selectively form at nano-scale wrinkles in h-BN with its optical dipole preferentially aligned to the wrinkle direction. By using density functional theory calculations, we find that the wrinkle's curvature plays a crucial role in localizing vacancy-based SPE candidates and aligning the defect's symmetry plane to the wrinkle direction. By performing optical measurements on SPEs created in h-BN single-crystal flakes, we experimentally confirm the wrinkle-induced generation of SPEs and their polarization alignment to the wrinkle direction. Our results not only provide a new route to controlling the atomic position