The electronic topology is generally related to the Berry curvature, which can induce the anomalous Hall effect in time-reversal symmetry breaking systems. Intrinsic monolayer transition metal dichalcogenides possesses two nonequivalent K and K′ valleys, having Berry curvatures with opposite signs, and thus vanishing anomalous Hall effect in this system. Here we report the experimental realization of asymmetrical distribution of Berry curvature in a single valley in monolayer WSe2 via applying uniaxial strain to break C 3v symmetry. As a result, although the Berry curvature itself is still opposite in K and K′ valleys, the two valleys would contribute equally to nonzero Berry curvature dipole. Upon applying electric field E , the emergent Berry curvature dipole D would lead to an out-of-plane orbital magnetization M ∝ D ⋅ E , which further induces an anomalous Hall effect with a linear response to E 2, known as nonlinear Hall effect. We show the strain modulated transport properties of nonlinear Hall effect in monolayer WSe2 with moderate hole-doping by gating. The second-harmonic Hall signals show quadratic dependence on electric field, and the corresponding orbital magnetization per current density M/J can reach as large as 60. In contrast to the conventional Rashba–Edelstein effect with in-plane spin polarization, such current-induced orbital magnetization is along the out-of-plane direction, thus promising for high-efficient electrical switching of perpendicular magnetization.
Optical metalens has been attracting more and more attention in recent years. To date, it is still difficult to simultaneously achieve wide field and broadband imaging in the visible region, which is very important in many applications, such as cameras, microscopy, and other imaging devices. In this paper, we design a double-layer metalens to achieve achromatic imaging over a field of view (FOV) of 60° in the visible light range of 470 nm to 650 nm, and its performance is verified by numerical simulations. The numerical aperture (NA) of the metalens is 0.61 and the average focusing efficiency is > 50% at normal incidence. The metalens has an additional advantage of polarization insensitivity.
Ionic liquid gating has proved to be effective in inducing emergent quantum phenomena such as superconductivity, ferromagnetism, and topological states. The electrostatic doping at two-dimensional interfaces relies on ionic motion, which thus is operated at sufficiently high temperature. Here, we report the in situ tuning of quantum phases by shining light on an ionic liquid-gated interface at cryogenic temperatures. The light illumination enables flexible switching of the quantum transition in monolayer WS 2 from an insulator to a superconductor. In contrast to the prevailing picture of photoinduced carriers, we find that in the presence of a strong interfacial electric field conducting electrons could escape from the surface confinement by absorbing photons, mimicking the field emission. Such an optical tuning tool in conjunction with ionic liquid gating greatly facilitates continuous modulation of carrier densities and hence electronic phases, which would help to unveil novel quantum phenomena and device functionality in various materials.
The proximity effect, which offers a proper route to extend the properties of 2D materials, is of great current interest. In hybrid systems formed by graphene and multiferroic materials, effective manipulation of the proximity effect is expected through magneto-electric coupling. In this work, we report the electrical control of the magnetic proximity effect in graphene/BiFeO3 heterostructures. The obvious ferroelectric gating effect on graphene is achieved using BiFeO3 as a top gate. The interfacial magnetic exchange field has a notable dependence on the top gate voltage, giving rise to an electrical modulation on Zeeman splitting and energy gap inside N = 0 Landau level of graphene. Our findings suggest graphene/BiFeO3 heterostructures provide a broad avenue for realization of future multiferroic electronics and spintronics.
Electronic correlation in a flat band has been a longstanding interest because of emergent phenomena such as Mott insulator and superconductivity. Besides recent Moiré superlattice, transition metal dichalcogenides (TMDs) may directly, e.g. by forming a charge density wave in 1T-TaS2, reconstruct a narrow band that exhibits a correlated insulator. Here we report an emergent insulator in electron doped monolayer WSe2, a prototypical TMDs with direct bandgaps. By detailed mapping a cascade of phases “band insulator-superconductor-emergent insulator-metal”, we can identify, besides the superconducting dome, a narrow miniband split from the conduction band, half filling of which coincides with the insulating state. The correlation picture is supported by a density wave that possesses an isolated flat band. Finally, through evolutionary changes within the same class of materials, multivalley population is suggested to account for enhanced superconductivity and the insulator. Our finding provides new opportunities to explore correlated physics within traditionally non-correlated materials.
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