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The exquisite distortion in a Kekulé -Y (Kek-Y) superlattice merges the two inequivalent Dirac cones (from the K - and the K ′- points) into the highest symmetric Γ-point in the hexagonal Brillouin zone. Here, we report that UV circularly polarized light not only opens up a topological gap at the Γ-point, but also lifts the valley degeneracy at that point. Endowed with Floquet dynamics and by devising a scheme of high-frequency approximation, we propose that the left/right-handedness in polarized light offers the possibility to realize valley-selective circular dichroism in a Kek-Y-shaped graphene superlattice. In addition, the non-vanishing Berry curvature and enumeration of the valley-resolved Chern number C K / C K ′ = + 1 / − 1 enable us to assign two pseudospin flavors (up/down) with the two valleys. Thereby, the above observations confirm the topological transition, suggesting the ease of realizing the valley quantum anomalous Hall state within the photon-dressed Kek-Y. These findings further manifest a non-zero optical valley polarization that is maximal at the Γ-point. Our paper thus proposes an optically switchable topological valley filter, which is desired in the evolving landscape of valleytronics. [ABSTRACT FROM AUTHOR]

Moiré superlattices based on twisted transition metal dichalcogenide (TMD) heterostructures have recently emerged as a promising platform for probing novel and distinctive electronic phenomena in two-dimensional (2D) materials. By stacking TMD monolayers with a small twist angle, these superlattices create a periodic modulation of the electronic density of states, leading to the formation of mini bands. These mini bands can exhibit intriguing properties such as flat bands, correlated electron behavior, and unconventional superconductivity. This review provides a comprehensive overview of recent progress in Moiré superlattices formed from twisted TMD heterostructures. It covers the theoretical principles and experimental techniques for creating and studying these superlattices, and explores their potential applications in optoelectronics, quantum computing, and energy harvesting. The review also addresses key challenges, such as improving the scalability and reproducibility of the fabrication process, emphasizing the exciting opportunities and ongoing hurdles in this rapidly evolving field. [ABSTRACT FROM AUTHOR]

In this work, the effects of magnetic proximity coupling on electronic structures, valley polarization and magneto-crystal anisotropy of VI3/WSe2 heterostructure are studied in detail based on density functional theory. The results show that the spin-valley polarization characters of WSe2 monolayer can be induced by the proximity effect of ferromagnetic VI3 monolayer. Based on the effective Hamiltonian k ⋅ p model, a large valley polarization of 8.7 meV is observed, which is mainly contributed by the SOC effect and not magnetic exchange field. Meanwhile, the magneto-crystal anisotropy of VI3 layer is also obviously altered from the [100] to [001] magnetic axis compared to the isolated VI3 monolayer. The reduction in the interlayer space strengths the W–V atomic coupling, resulting in a significant increase in the spin and valley polarization. Compression strain shows weakening effect on the valley polarization, while the tensile strain largely enhances the valley polarization of VI3/WSe2 heterostructure, which can reach the maximum value of 29.1 meV at the tensile strain of 8%. The V spin direction (θ) has significant effect on valley polarization of WSe2 layer, which is positively correlated with the magnetization strength of V atom in the vertical direction. Furthermore, constructing the VI3/WSe2/VI3 sandwich heterostructure can induce a larger valley polarization, which can reach a maximum value of 32.4 meV. These results indicate that the VI3/WSe2 heterostructure is a potential candidate for valleytronics. [ABSTRACT FROM AUTHOR]

The valley degree of freedom, as a new information carrier, is important for basic physical research and the development of advanced devices. Herein, using first-principle calculations, we predict that two-dimensional Hf2TeIX (X = I, Br) monolayers harbor intriguing valley properties. Without considering spin–orbit coupling (SOC), the Hf2TeI2 monolayer has a semi-metallic nature, with Dirac cones located at the high-symmetry point K, and feature, with considerable Fermi velocity. When the SOC is taken into account, a band gap opening of 271 meV can be observed at the Dirac cones. More interestingly, the Hf2TeIBr monolayer exhibits intrinsic spatial inversion symmetry breaking, which leads to the emergence of valley-contrasting physics under SOC. This is demonstrated by the presence of spin–valley splitting and opposite Berry curvature at adjacent K points. Besides, the spin–valley splitting, the band gap and magnitude of the Berry curvature of the Hf2TeIBr monolayer can be effectively tuned by strain engineering. These findings contribute significantly to the design of valleytronic devices and extend opportunities for exploring two-dimensional valley materials. [ABSTRACT FROM AUTHOR]

Transition metal dichalcogenide (TMD) monolayers exhibit unique valleytronics properties due to the dependency of the coupled valley and spin state at the hexagonal corner of the first Brillouin zone. Precisely controlling valley spin-polarization via manipulating the electron population enables its application in valley-based memory or quantum technologies. This study uncovered the uncompensated spins of the antiferromagnetic nickel oxide (NiO) serving as the ferromagnetic (FM) order to induce valley spin-polarization in molybdenum disulfide (MoS2) monolayers via the magnetic proximity effect (MPE). Spin-resolved photoluminescence spectroscopy (SR-PL) was employed to observe MoS2, where the spin-polarized trions appear to be responsible for the MPE, leading to a valley magnetism. Results indicate that local FM order from the uncompensated surface of NiO could successfully induce significant valley spin-polarization in MoS2 with the depolarization temperature approximately at 100 K, which is relatively high compared to the related literature. This study reveals new perspectives in that the precise control over the surface orientation of AFMs serves as a crystallographic switch to activate the MPE and the magnetic sustainability of the trion state is responsible for the observed valley spin-polarization with the increasing temperature, which promotes the potential of AFM materials in the field of exchange-coupled Van der Waals heterostructures. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) materials have shown outstanding potential for new devices based on their interesting electrical properties beyond conventional 3D materials. In recent years, new concepts such as the valley degree of freedom have been studied to develop valleytronics in hexagonal lattice 2D materials. We investigated the valley degree of freedom of GaX and Janus GaXY (X, Y = S, Se, Te). By considering the spin–orbit coupling (SOC) effect in the band structure calculations, we identified the Rashba-type spin splitting in band structures of Janus Ga2SSe and Ga2STe. Further, we confirmed that the Zeeman-type spin splitting at the K and K' valleys of GaX and Janus Ga2XY show opposite spin contributions. We also calculated the Berry curvatures of GaX and Janus GaXY. In this study, we find that GaX and Janus Ga2XY have a similar magnitude of Berry curvatures, while having opposite signs at the K and K' points. In particular, GaTe and Ga2SeTe have relatively larger Berry curvatures of about 3.98 Å2 and 3.41 Å2, respectively, than other GaX and Janus Ga2XY. [ABSTRACT FROM AUTHOR]

2D transition metal dichalcogenides (TMDs) have emerged as a novel class of semiconductors with promising applications in optoelectronics, owing to their rich and tunable valley fine structure, known as valleytronics. Strain engineering in TMDs presents opportunities to tailor their valley fine structures and band alignment, which greatly expands the potential to investigate their intrinsic properties and improve device performance, thus opening a new field of straintronics. In this review, recent advances in strain‐engineered 2D TMDs are summarized, with a focus on new phenomena and applications enabled by precision tuning of valley physics. The underlying mechanisms and connections are delineated between strain‐induced modifications to the valley fine structures based on intravalley, intervalley, and interlayer band alignment in single and heterostructure TMDs. These insights allow targeted strain control strategies to be devised for optimizing optoelectronic characteristics. This review provides perspectives and guidance on the future directions of valley‐straintronics and flexible 2D optoelectronics using TMDs, highlighting the substantial promise of valley‐strain engineering in TMDs for fundamental valley physics studies as well as practical device applications. [ABSTRACT FROM AUTHOR]

The 2D ferrovalley semiconductor (FVSC) with spontaneous valley polarization offers an exciting material platform for probing Berry phase physics. How FVSC can be incorporated in valleytronic device applications, however, remain an open question. Here, the concept of metal/semiconductor (MS) contact into the realm of valleytronics is generalized. A half‐valley Ohmic contact is proposed based on FVSC/graphene heterostructure, where the two valleys of FVSC separately forms Ohmic and Schottky contacts with those of graphene, thus allowing current to be valley‐selectively injected through the 'Ohmic' valley while being blocked in the 'Schottky' valley. A theory of contact‐limited valley‐contrasting current injection is developed and such transport mechanism can produce gate‐tunable valley‐polarized injection current. Using RuCl2/graphene heterostructure as an example, a device concept of valleytronic barristor is illustrated, where high valley polarization efficiency and sizable current on/off ratio, can be achieved under experimentally feasible electrostatic gating conditions. These findings uncover contact‐limited valley‐contrasting current injection as an efficient mechanism for valley polarization manipulation, and reveals the potential of valleytronic MS contact as a functional building block of valleytronic device technology. [ABSTRACT FROM AUTHOR]

Valleytronic materials can provide new degrees of freedom to future electronic devices. In this work, the concepts of the ferrovalley metal (FVM) and valley gapless semiconductor (VGS) are proposed, which can be achieved in valleytronic bilayer systems by electric field engineering. In valleytronic bilayer systems, the interaction between out-of-plane ferroelectricity and A-type antiferromagnetism can induce layer-polarized anomalous valley Hall (LP-AVH) effect. The K and −K valleys of FVM are both metallic, and electron and hole carriers simultaneously exist. In the extreme case, the FVM can become VGS by analogizing spin gapless semiconductor (SGS). Moreover, it is proposed that the valley splitting enhancement and valley polarization reversal can be achieved by electric field engineering in valleytronic bilayer systems. Taking the bilayer RuBr2 as an example, our proposal is confirmed by the first-principle calculations. The FVM and VGS can be achieved in bilayer RuBr2 by applying electric field. With appropriate electric field range, increasing electric field can enhance valley splitting, and the valley polarization can be reversed by flipping electric field direction. To effectively tune valley properties by electric field in bilayer systems, the parent monolayer should possess out-of-plane magnetization, and have large valley splitting. Our results shed light on the possible role of electric field in tuning valleytronic bilayer systems, and provide a way to design the ferrovalley-related material by electric field. [ABSTRACT FROM AUTHOR]

The valley degree of freedom, as a new information carrier, is important for basic physical research and the development of advanced devices. Herein, using first-principle calculations, we predict that two-dimensional Hf2TeIX (X = I, Br) monolayers harbor intriguing valley properties. Without considering spin–orbit coupling (SOC), the Hf2TeI2 monolayer has a semi-metallic nature, with Dirac cones located at the high-symmetry point K, and feature, with considerable Fermi velocity. When the SOC is taken into account, a band gap opening of 271 meV can be observed at the Dirac cones. More interestingly, the Hf2TeIBr monolayer exhibits intrinsic spatial inversion symmetry breaking, which leads to the emergence of valley-contrasting physics under SOC. This is demonstrated by the presence of spin–valley splitting and opposite Berry curvature at adjacent K points. Besides, the spin–valley splitting, the band gap and magnitude of the Berry curvature of the Hf2TeIBr monolayer can be effectively tuned by strain engineering. These findings contribute significantly to the design of valleytronic devices and extend opportunities for exploring two-dimensional valley materials.

The capacity to manipulate magnetization in 2D dilute magnetic semiconductors (2D‐DMSs) using light, specifically in magnetically doped transition metal dichalcogenide (TMD) monolayers (M‐doped TX2, where M = V, Fe, and Cr; T = W, Mo; X = S, Se, and Te), may lead to innovative applications in spintronics, spin‐caloritronics, valleytronics, and quantum computation. This Perspective paper explores the mediation of magnetization by light under ambient conditions in 2D‐TMD DMSs and heterostructures. By combining magneto‐LC resonance (MLCR) experiments with density functional theory (DFT) calculations, we show that the magnetization can be enhanced using light in V‐doped TMD monolayers (e.g., V‐WS2, V‐WSe2). This phenomenon is attributed to excess holes in the conduction and valence bands, and carriers trapped in magnetic doping states, mediating the magnetization of the semiconducting layer. In 2D‐TMD heterostructures (VSe2/WS2, VSe2/MoS2), the significance of proximity, charge‐transfer, and confinement effects in amplifying light‐mediated magnetism is demonstrated. We attributed this to photon absorption at the TMD layer that generates electron–hole pairs mediating the magnetization of the heterostructure. These findings will encourage further research in the field of 2D magnetism and establish a novel design of 2D‐TMDs and heterostructures with optically tunable magnetic functionalities, paving the way for next‐generation magneto‐optic nanodevices. [ABSTRACT FROM AUTHOR]

Valleytronics has emerged as an interesting field of research in two-dimensional (2D) systems and uses the valley index or valley pseudospin to encode information. Spin–orbit coupling (SOC) and inversion symmetry breaking lead to spin-splitting of bands near the valleys. This property has promising device applications. In order to find a new 2D material useful for valleytronics, we have designed hexagonal planar monolayers of cadmium chalcogenides (Cd X, X = S, Se, Te) from the (111) surface of bulk Cd X zinc blende structure. The structural, dynamic, mechanical and thermal stability of these structures is confirmed. A band structure study reveals valence band local maxima (valleys) at the K and K′ symmetry points. The application of SOC initiates spin-splitting in the valleys that lifts the energy degeneracy and shows strong valley–spin coupling. To initiate stronger SOC, we substituted two Cd atoms in the almost-planar monolayers with Sn atoms, which increases the spin-splitting significantly. Zeeman-type spin-splitting is observed in the valley region and Rashba spin-splitting is observed at the Γ point for Sn-doped CdSe and CdTe monolayers. Berry curvature values are higher in all the Sn-doped monolayers than in the pristine monolayers. These newly designed monolayers are thus found to be suitable for valleytronics applications. Sn-doped monolayers show band inversion deep in the valence and conduction bands between Sn s and p and X p states but lack topological properties. [ABSTRACT FROM AUTHOR]

Monolayer transition metal dichalcogenides (TMDCs) have become a promising platform in valleytronics due to possessing the regulatable valley degrees of freedom. While, as a result of the rapid intervalley scattering, it is difficult to measure the PL valley polarization of monolayer TMDCs at room temperature, which limits their application in valleytronics devices. Here, we report a room temperature photoluminescence (PL) valley polarization up to 3.73% in an "optical bilayer" WS2 formed by transferring monolayer WS2 onto flat Ag film. Furthermore, in the transient absorption (TA) measurements, a remarkably long valley depolarization lifetime is found. Thus, we demonstrate the valley properties of such "optical bilayer" WS2 resemble actual bilayer WS2, in which the robust valley polarization can be attributed to the phonons depletion effect and the blocked interlayer hopping processes. These peculiar valley features in "optical bilayer" WS2 provide a particularly simple method to enhance valley control at room temperature. [ABSTRACT FROM AUTHOR]

Electrostatically confined quantum dots in bilayer graphene have shown potential as building blocks for quantum technologies. To operate the dots, e.g., as qubits, a precise understanding and control of the confined states and their properties is required. Herein, a large‐scale numerical characterization of confined quantum states in bilayer graphene dots is performed over an extensive range of gate‐tunable parameters such as the dot size, depth, shape, and the bilayer graphene gap. The dot states' orbital degeneracy, wave function distribution, and valley g‐factor are established and the parametric dependencies to achieve different regimes are provided. It is found that the dot states are highly susceptible to gate‐dependent confinement and material parameters, enabling efficient tuning of confined states and valley g‐factor modulation by quantum dot design. [ABSTRACT FROM AUTHOR]

Transition metal dichalcogenide (TMD) monolayers exhibit unique valleytronics properties due to the dependency of the coupled valley and spin state at the hexagonal corner of the first Brillouin zone. Precisely controlling valley spin-polarization via manipulating the electron population enables its application in valley-based memory or quantum technologies. This study uncovered the uncompensated spins of the antiferromagnetic nickel oxide (NiO) serving as the ferromagnetic (FM) order to induce valley spin-polarization in molybdenum disulfide (MoS2) monolayers via the magnetic proximity effect (MPE). Spin-resolved photoluminescence spectroscopy (SR-PL) was employed to observe MoS2, where the spin-polarized trions appear to be responsible for the MPE, leading to a valley magnetism. Results indicate that local FM order from the uncompensated surface of NiO could successfully induce significant valley spin-polarization in MoS2 with the depolarization temperature approximately at 100 K, which is relatively high compared to the related literature. This study reveals new perspectives in that the precise control over the surface orientation of AFMs serves as a crystallographic switch to activate the MPE and the magnetic sustainability of the trion state is responsible for the observed valley spin-polarization with the increasing temperature, which promotes the potential of AFM materials in the field of exchange-coupled Van der Waals heterostructures.

Two-dimensional (2D) materials have shown outstanding potential for new devices based on their interesting electrical properties beyond conventional 3D materials. In recent years, new concepts such as the valley degree of freedom have been studied to develop valleytronics in hexagonal lattice 2D materials. We investigated the valley degree of freedom of GaX and Janus GaXY (X, Y = S, Se, Te). By considering the spin–orbit coupling (SOC) effect in the band structure calculations, we identified the Rashba-type spin splitting in band structures of Janus Ga2SSe and Ga2STe. Further, we confirmed that the Zeeman-type spin splitting at the K and K’ valleys of GaX and Janus Ga2XY show opposite spin contributions. We also calculated the Berry curvatures of GaX and Janus GaXY. In this study, we find that GaX and Janus Ga2XY have a similar magnitude of Berry curvatures, while having opposite signs at the K and K’ points. In particular, GaTe and Ga2SeTe have relatively larger Berry curvatures of about 3.98 Å2 and 3.41 Å2, respectively, than other GaX and Janus Ga2XY.

Due to the short valley polarization time, it is hardly to separate opposite valley pseudospin of transition metal dichalcogenides (TMDs) for their practical applications in valleytronics. Coupling TMDs to unidirectional surface plasmon polariton (SPP) can overcome this obstacle. However, it is required to break the symmetry to induce the asymmetric coupling between valley exciton dipole and SPP to route valley exciton in previously proposed strategies. Herein, by utilizing a new mechanism that near-field interference can create directional SPP in symmetric nanostructures, we realize directional routing of valley exciton emission of monolayer WSe2 at room temperature with a symmetric nano-slits array. The near-field interference enabled directional SPP in our device not only render the exciton diffusion length increase from 0.9 to 3.0 μm, but also lead to a valley exciton separation length of 0.7 μm with degree of valley polarization up to 22 %. This valley excitons separation is attributed to the non-flat WSe2 in the nano-slits region, which makes the exciton dipoles present in-plane and out-of-plane simultaneously. Our work provides a convenient and promising strategy towards room temperature on-chip integrated valleytronic devices.

Abstract The capacity to manipulate magnetization in 2D dilute magnetic semiconductors (2D‐DMSs) using light, specifically in magnetically doped transition metal dichalcogenide (TMD) monolayers (M‐doped TX2, where M = V, Fe, and Cr; T = W, Mo; X = S, Se, and Te), may lead to innovative applications in spintronics, spin‐caloritronics, valleytronics, and quantum computation. This Perspective paper explores the mediation of magnetization by light under ambient conditions in 2D‐TMD DMSs and heterostructures. By combining magneto‐LC resonance (MLCR) experiments with density functional theory (DFT) calculations, we show that the magnetization can be enhanced using light in V‐doped TMD monolayers (e.g., V‐WS2, V‐WSe2). This phenomenon is attributed to excess holes in the conduction and valence bands, and carriers trapped in magnetic doping states, mediating the magnetization of the semiconducting layer. In 2D‐TMD heterostructures (VSe2/WS2, VSe2/MoS2), the significance of proximity, charge‐transfer, and confinement effects in amplifying light‐mediated magnetism is demonstrated. We attributed this to photon absorption at the TMD layer that generates electron–hole pairs mediating the magnetization of the heterostructure. These findings will encourage further research in the field of 2D magnetism and establish a novel design of 2D‐TMDs and heterostructures with optically tunable magnetic functionalities, paving the way for next‐generation magneto‐optic nanodevices.

Abstract Valley Hall effect is observed in asymmetric single‐layer and bilayer graphene systems. In single‐layer graphene systems, asymmetry is introduced by aligning graphene with hexagonal boron nitride (hBN) with a near‐zero twist angle, breaking the sub‐lattice symmetry. Although a similar approach is used in bilayer graphene to break the layer symmetry and thereby observe the valley Hall effect, the bilayer graphene is sandwiched with hBN on both sides in those studies. This study looks at a much simpler, non‐encapsulated structure where hBN is present only at the top of graphene. The crystallographic axes of both hBN and bilayer graphene are aligned. A clear signature of the valley Hall effect through non‐local resistance measurement (RNL) is observed. The observed non‐local resistance can be manipulated by applying a displacement field across the heterostructure. Furthermore, the electronic band structure and Berry curvature calculations validate the experimental observations.

The article explores the research on layer-controlled nonlinear terahertz valleytronics in the two-dimensional material PtSe2. The researchers demonstrate that terahertz nonlinearity can be achieved in PtSe2 by generating ultrafast photocurrents and manipulating the bandstructure valleys. They also observe layer-dependent circular dichroism, which allows for control of the emitted terahertz pulse. This research has implications for terahertz valleytronics, spintronics, and harmonic generation. The study emphasizes the role of the environment in inducing nonlinearity and circular dichroism in PtSe2. [Extracted from the article]

The valley pseudospin properties of electrons in two-dimensional hexagonal materials result in many fascinating physical phenomena, which opens up the new field of valleytronics. The valley-contrasting physics aims at distinguishing the valley degree of freedom based on valley-dependent effects. Here, we theoretically demonstrate that both of the valley-selective high harmonic generation and valley-selective electronic excitation can be achieved by using an orthogonal two-color (OTC) laser field in gapped graphene. It is shown that the asymmetry degrees of harmonic yields in the plateaus, cutoff energies of generated harmonics and electron populations from two different valleys can be precisely controlled by the relative phase of the OTC laser field. Thus, the selectivity of the dominant valley for the harmonic radiation and electronic polarization can be switched by adjusting the relative phase of the OTC laser field. Our work offers an all-optical route to produce the valley-resolved high harmonic emissions and manipulate the ultrafast valley polarization on a femtosecond timescale in condensed matter. [ABSTRACT FROM AUTHOR]

Monolayer group VI transition metal dichalcogenides (TMDs) have recently emerged as promising candidates for photonic and opto-valleytronic applications. The optoelectronic properties of these atomically-thin semiconducting crystals are strongly governed by the tightly bound electron-hole pairs such as excitons and trions (charged excitons). The anomalous spin and valley configurations at the conduction band edges in monolayer WS2 give rise to even more fascinating valley many-body complexes. Here we find that the indirect Q valley in the first Brillouin zone of monolayer WS2 plays a critical role in the formation of a new excitonic state, which has not been well studied. By employing a high-quality h-BN encapsulated WS2 field-effect transistor, we are able to switch the electron concentration within K-Q valleys at conduction band edges. Consequently, a distinct emission feature could be excited at the high electron doping region. Such feature has a competing population with the K valley trion, and experiences nonlinear power-law response and lifetime dynamics under doping. Our findings open up a new avenue for the study of valley many-body physics and quantum optics in semiconducting 2D materials, as well as provide a promising way of valley manipulation for next-generation entangled photonic devices.

Abstract Platinum diselenide (PtSe2) is a promising two‐dimensional (2D) material for the terahertz (THz) range as, unlike other transition metal dichalcogenides (TMDs), its bandgap can be uniquely tuned from a semiconductor in the near‐infrared to a semimetal with the number of atomic layers. This gives the material unique THz photonic properties that can be layer‐engineered. Here, we demonstrate that a controlled THz nonlinearity—tuned from monolayer to bulk PtSe2—can be realized in wafer size polycrystalline PtSe2 through the generation of ultrafast photocurrents and the engineering of the bandstructure valleys. This is combined with the PtSe2 layer interaction with the substrate for a broken material centrosymmetry, permitting a second order nonlinearity. Further, we show layer dependent circular dichroism, where the sign of the ultrafast currents and hence the phase of the emitted THz pulse can be controlled through the excitation of different bandstructure valleys. In particular, we show that a semimetal has a strong dichroism that is absent in the monolayer and few layer semiconducting limit. The microscopic origins of this TMD bandstructure engineering are highlighted through detailed DFT simulations, and shows the circular dichroism can be controlled when PtSe2 becomes a semimetal and when the K‐valleys can be excited. As well as showing that PtSe2 is a promising material for THz generation through layer controlled optical nonlinearities, this work opens up a new class of circular dichroism materials beyond the monolayer limit that has been the case of traditional TMDs, and impacting a range of domains from THz valleytronics, THz spintronics to harmonic generation.

Due to the short valley polarization time, it is hardly to separate opposite valley pseudospin of transition metal dichalcogenides (TMDs) for their practical applications in valleytronics. Coupling TMDs to unidirectional surface plasmon polariton (SPP) can overcome this obstacle. However, it is required to break the symmetry to induce the asymmetric coupling between valley exciton dipole and SPP to route valley exciton in previously proposed strategies. Herein, by utilizing a new mechanism that near-field interference can create directional SPP in symmetric nanostructures, we realize directional routing of valley exciton emission of monolayer WSe2 at room temperature with a symmetric nano-slits array. The near-field interference enabled directional SPP in our device not only render the exciton diffusion length increase from 0.9 to 3.0 μm, but also lead to a valley exciton separation length of 0.7 μm with degree of valley polarization up to 22 %. This valley excitons separation is attributed to the non-flat WSe2 in the nano-slits region, which makes the exciton dipoles present in-plane and out-of-plane simultaneously. Our work provides a convenient and promising strategy towards room temperature on-chip integrated valleytronic devices. [ABSTRACT FROM AUTHOR]

Coherent control and manipulation of quantum degrees of freedom such as spins forms the basis of emerging quantum technologies. In this context, the robust valley degree of freedom and the associated valley pseudospin found in two‐dimensional transition metal dichalcogenides is a highly attractive platform. Valley polarization and coherent superposition of valley states have been observed in these systems even up to room temperature. Control of valley coherence is an important building block for the implementation of valley qubit. Large magnetic fields or high‐power lasers have been used in the past to demonstrate the control (initialization and rotation) of the valley coherent states. Here, the control of layer–valley coherence via strong coupling of valley excitons in bilayer WS2 to microcavity photons is demonstrated by exploiting the pseudomagnetic field arising in optical cavities owing to the transverse electric–transverse magnetic (TE–TM)mode splitting. The use of photonic structures to generate pseudomagnetic fields which can be used to manipulate exciton‐polaritons presents an attractive approach to control optical responses without the need for large magnets or high‐intensity optical pump powers. [ABSTRACT FROM AUTHOR]

Exploiting the valley degrees of freedom as information carriers provides new opportunities for the development of valleytronics. Monolayer transition metal dichalcogenides (TMDs) with broken space-inversion symmetry exhibit emerging valley pseudospins, making them ideal platforms for studying valley electronics. However, intervalley scattering of different energy valleys limits the achievable degree of valley polarization. Here, we constructed WSe2/yttrium iron garnet (YIG) heterostructures and demonstrated that the interfacial magnetic exchange effect on the YIG magnetic substrate can enhance valley polarization by up to 63%, significantly higher than that of a monolayer WSe2 on SiO2/Si (11%). Additionally, multiple sharp exciton peaks appear in the WSe2/YIG heterostructures due to the strong magnetic proximity effect at the magnetic–substrate interface that enhances exciton emission efficiency. Moreover, under the effect of external magnetic field, the magnetic direction of the magnetic substrate enhances valley polarization, further demonstrating that the magnetic proximity effect regulates valley polarization. Our results provide a new way to regulate valley polarization and demonstrate the promising application of magnetic heterojunctions in magneto-optoelectronics. [ABSTRACT FROM AUTHOR]

The electronic band structure in crystalline semiconductors represents a relationship between crystal momentum and energy of the electrons and is often depicted by a series of extremum known as valleys. While the use of charge or spin degree of freedom for carrying information is prevalent in today's technology, the use of electron's valley degrees of freedom for encoding and processing information (valleytronics) has remained inaccessible for many decades because of inadequate coupling between external excitation and the valley. The advent of two-dimensional materials such as graphene and transition metal dichalcogenides has opened a new avenue for valleytronics which seeks to use valley degrees of freedom to carry and process information, analogues to spin-based spintronics and charge-based electronics. These materials have unique spin-valley locking, imparting a great ability to control and harness electronic valley degrees of freedom with light, electric, and magnetic fields. Here, we present a viewpoint on the fundamental aspects of valleytronics, emphasising valley contrasting characters of two-dimensional semiconductors. [ABSTRACT FROM AUTHOR]

Realizing valley asymmetric transmission(VAT) for incoming valley unpolarized Dirac carriers across an electrostatically modulated confinement potential(ECP) step, which is located at the interface of the unipolar ( $n-n^{^{\prime}}$ ) junction in dual-gated Bernal-bilayer graphene(BBG), has caught growing attention because it provides a platform to study the valley polarizer. Such a junction, however, seems to preclude from supporting vanishing VAT. Based on the effective two-band Hamiltonian of BBG with wave matching approach, we demonstrate theoretically that VAT through the n / n ′ interface vanishes in the following conditions: (i) the disappearing electrostatically modulated interlayer potential difference(EPD), or (ii) the invariance condition of the sum or difference of ECPs and EPDs, which physically corresponds to the zero band offset of the conduction or valence band in the band alignment of $n-n^{^{\prime}}$ junction. Guided by this, it is worth mentioning that the we can derive simple, analytical valley-independent expressions for the reflection and transmission probabilities. When the same invariant conditions occur, our calculations further show the appearance of the nearly vanishing VAT for the four-band model in bilayer graphene with ‘Mexican hat’-like band. As a byproduct, we apply these findings to suggest the best design for the valley-dependent selection of momenta based on dual-gated BBG connected to two pristine BBG electrodes in which ECPs of the two outer regions are distinguishable and show that the sign of valley polarization can be switched via varying EPD. We expect our results can provide guidance for future device applications relying on ballistic and valley-selective transport.

Valleytronics uses valleys, a novel quantum degree of freedom, to encode information. It combines other degrees of freedom, such as charge and spin, to produce a more comprehensive, stable, and efficient information processing system. Valleytronics has become an intriguing field in condensed matter physics due to the emergence of new two-dimensional materials in recent years. However, in nonmagnetic valleytronic materials, the valley polarization is transient and the depolarization occurs once the external excitation is withdrawn. Introduction of magnetic field is an effective approach to realizing the spontaneous valley polarization by breaking the time-reversal symmetry. In hexagonal magnetic valleytronic materials, the inequivalent valleys at the K and – K ( K ′) Dirac cones have asymmetric energy gaps and Berry curvatures. The time-reversal symmetry in nonmagnetic materials can be broken by applying an external magnetic field, adding a magnetic substrate or doping magnetic atoms. Recent theoretical studies have demonstrated that valleytronic materials with intrinsic ferromagnetism, now termed as ferrovalley materials, exhibit spontaneous valley polarization without the need for external fields to maintain the polarization. The coupling of the valley and spin degrees of freedom enables stable and unequal distribution of electrons in the two valleys and thus facilitating nonvolatile information storage. Hence, ferrovalley materials are promising materials for valleytronic devices. In this review, we first briefly overview valleytronics and its related properties, the ways to realize valley polarization in nonmagnetic valleytronic materials. Then we focus on the recent developments in two-dimensional ferrovalley materials, which can be classified according to their molecular formula and crystal structure: MX _2 ; M(XY) _2 , M(XY _2 ) and M(XYZ) _2 ; M _2 X _3 , M _3 X _8 and MNX _6 ; MNX _2 Y _2 , M _2 X _2 Y _6 and MNX _2 Y _6 ; and the Janus structure ferrovalley materials. In the inequivalent valleys, the Berry curvatures have opposite signs with unequal absolute values, leading to anomalous valley Hall effect. When the valley polarization is large, the ferrovalleys can be selectively excited even with unpolarized light. Intrinsic valley polarization in two-dimensional ferrovalley materials is of great importance. It opens a new avenue for information-related applications and hence is under rapid development.

The incidence of intra-flake heterogeneity of spectroscopic and electrical properties in chemical vapour deposited (CVD) WS2 flakes is explored in a multi-physics investigation via spatially resolved spectroscopic maps correlated with electrical, electronic and mechanical properties. The investigation demonstrates that the three-fold symmetric segregation of spectroscopic response, in topographically uniform WS2 flakes are accompanied by commensurate segmentation of electronic properties e.g. local carrier density and the differences in the mechanics of tip-sample interactions, evidenced via scanning probe microscopy phase maps. Overall, the differences are understood to originate from point defects, namely sulfur vacancies within the flake along with a dominant role played by the substrate. While evolution of the multi-physics maps upon sulfur annealing elucidates the role played by sulfur vacancy, substrate-induced effects are investigated by contrasting data from WS2 flake on Si and Au surfaces. Local charge depletion induced by the nature of the sample-substrate junction in case of WS2 on Au is seen to invert the electrical response with comprehensible effects on their spectroscopic properties. Finally, the role of these optoelectronic properties in preserving valley polarization that affects valleytronic applications in WS2 flakes, is investigated via circular polarization discriminated photoluminescence experiments. The study provides a thorough understanding of spatial heterogeneity in optoelectronic properties of WS2 and other transition metal chalcogenides, which are critical for device fabrication and potential applications. [ABSTRACT FROM AUTHOR]

The recent discovery of natural biaxial hyperbolicity in van der Waals crystals, such as α-MoO3, has opened up new avenues for mid-IR nanophotonics due to their deep subwavelength phonon polaritons. However, a significant challenge is the lack of active tunability of these hyperbolic phonon polaritons. In this work, we investigate heterostructures of graphene and α-MoO3 for actively tunable hybrid plasmon phonon polariton modes via electrostatic gating in the mid-infrared spectral region. We observe a unique propagation direction dependent hybridization of graphene plasmon polaritons with hyperbolic phonon polaritons for experimentally feasible values of graphene chemical potential. We further report an application to tunable valley quantum interference in this system with a broad operational bandwidth due to the formation of these hybrid modes. This work presents a lithography-free alternative for actively tunable, anisotropic spontaneous emission enhancement using a sub-wavelength thick naturally biaxial hyperbolic material.

The valley pseudospin properties of electrons in two-dimensional hexagonal materials result in many fascinating physical phenomena, which opens up the new field of valleytronics. The valley-contrasting physics aims at distinguishing the valley degree of freedom based on valley-dependent effects. Here, we theoretically demonstrate that both of the valley-selective high harmonic generation and valley-selective electronic excitation can be achieved by using an orthogonal two-color (OTC) laser field in gapped graphene. It is shown that the asymmetry degrees of harmonic yields in the plateaus, cutoff energies of generated harmonics and electron populations from two different valleys can be precisely controlled by the relative phase of the OTC laser field. Thus, the selectivity of the dominant valley for the harmonic radiation and electronic polarization can be switched by adjusting the relative phase of the OTC laser field. Our work offers an all-optical route to produce the valley-resolved high harmonic emissions and manipulate the ultrafast valley polarization on a femtosecond timescale in condensed matter.

Transition metal dichalcogenides (TMDCs) are ideal candidates to explore the manifestation of spin-valley physics under external stimuli. In this study, we investigate the influence of strain on the spin and orbital angular momenta, effective g -factors, and Berry curvatures of several monolayer TMDCs (Mo and W based) using a full ab initio approach. At the K -valleys, we find a surprising decrease of the conduction band spin expectation value for compressive strain, consequently increasing the dipole strength of the dark exciton by more than one order of magnitude (for ∼ 1 % â€" 2 % strain variation). We also predict the behavior of direct excitons g -factors under strain: tensile (compressive) strain increases (decreases) the absolute value of g -factors. Strain variations of ∼1% modify the bright (A and B) excitons g -factors by ∼0.3 (0.2) for W (Mo) based compounds and the dark exciton g -factors by ∼0.5 (0.3) for W (Mo) compounds. Our predictions could be directly visualized in magneto-optical experiments in strained samples at low temperature. Additionally, our calculations strongly suggest that strain effects are one of the possible causes of g -factor fluctuations observed experimentally. By comparing the different TMDC compounds, we reveal the role of spinâ€"orbit coupling (SOC): the stronger the SOC, the more sensitive are the spin-valley features under applied strain. Consequently, monolayer WSe2 is a formidable candidate to explore the role of strain on the spin-valley physics. We complete our analysis by considering the side valleys, Î" and Q points, and by investigating the influence of strain in the Berry curvature. In the broader context of valley- and strain-tronics, our study provides fundamental microscopic insights into the role of strain in the spin-valley physics of TMDCs, which are relevant to interpret experimental data in monolayer TMDCs as well as TMDC-based van der Waals heterostructures. [ABSTRACT FROM AUTHOR]

The integration of magnetic material with semiconductors has been fertile ground for fundamental science as well as of great practical interest toward the seamless integration of information processing and storage. We create van der Waals heterostructures formed by an ultrathin ferromagnetic semiconductor CrI3 and a monolayer of WSe2. We observe unprecedented control of the spin and valley pseudospin in WSe2, where we detect a large magnetic exchange field of nearly 13 T and rapid switching of the WSe2 valley splitting and polarization via flipping of the CrI3 magnetization. The WSe2 photoluminescence intensity strongly depends on the relative alignment between photoexcited spins in WSe2 and the CrI3 magnetization, because of ultrafast spin-dependent charge hopping across the heterostructure interface. The photoluminescence detection of valley pseudospin provides a simple and sensitive method to probe the intriguing domain dynamics in the ultrathin magnet, as well as the rich spin interactions within the heterostructure.

Heterostructures made from van der Waals (vdW) materials provide a template to investigate a wealth of proximity effects at atomically sharp two-dimensional (2D) heterointerfaces. In particular, near-field charge and energy transfer in vdW heterostructures made from semiconducting transition metal dichalcogenides (TMD) have recently attracted interest to design model 2D “donor–acceptor” systems and new optoelectronic components. Here, using Raman scattering and photoluminescence spectroscopies, we report a comprehensive characterization of a molybedenum diselenide ($\mathrm{MoSe}_2$) monolayer deposited onto hexagonal boron nitride (hBN) and capped by mono- and bilayer graphene. Along with the atomically flat hBN susbstrate, a single graphene epilayer is sufficient to passivate the $\mathrm{MoSe}_2$ layer and provides a homogenous environment without the need for an extra capping layer. As a result, we do not observe photo-induced doping in our heterostructure and the $\mathrm{MoSe}_2$ excitonic linewidth gets as narrow as 1.6 meV, approaching the homogeneous limit. The semi-metallic graphene layer neutralizes the 2D semiconductor and enables picosecond non-radiative energy transfer that quenches radiative recombination from long-lived states. Hence, emission from the neutral band edge exciton largely dominates the photoluminescence spectrum of the $\mathrm{MoSe}_2$/graphene heterostructure. Since this exciton has a picosecond radiative lifetime at low temperature, comparable with the non-radiative transfer time, its low-temperature photoluminescence is only quenched by a factor of $3.3 \pm 1$ and $4.4 \pm 1$ in the presence of mono- and bilayer graphene, respectively. Finally, while our bare $\mathrm{MoSe}_2$ on hBN exhibits negligible valley polarization at low temperature and under near-resonant excitation, we show that interfacing $\mathrm{MoSe}_2$ with graphene yields a single-line emitter with degrees of valley polarization and coherence up to ${\sim }$ 15 %.

The concept of valley originates from two degenerate but nonequivalent energy bands at the local minimum in the conduction band or local maximum in the valence band. Manipulating the valley states for information storage and processing develops a brand-new electronicsâ€"valleytronics. Broken inversion symmetry is a necessary condition to produce pure valley currents. The polycrystalline two-dimensional materials (graphene, silicene, monolayer group-VI transition metal dichalcogenides, etc) with pristine grains stitched together by disordered grain boundaries (GBs) are the natural inversion-symmetry-broken systems and the candidates in the field of valleytronics. Different from their pristine forms, the Dirac valleys on both sides of GBs are mismatched in the momentum space and induce peculiar valley transport properties across the GBs. In this review, we systematically demonstrate the fundamental properties of valley degree of freedom across mostly studied and experimentally feasible polycrystalline structureâ€"the line defect, and the manipulation strategies with electrical, magnetic and mechanical methods to realize the valley polarization. We also introduce an effective numerical method, the non-equilibrium Green’s function technique, to tackle the valley transport issues in the line defect systems. The present challenges and the perspective on the further investigations of the line defect in valleytronics are also summarized. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) transition metal dichalcogenide (TMDC) materials, such as MoS2, WS2, MoSe2, and WSe2, have received extensive attention in the past decade due to their extraordinary electronic, optical and thermal properties. They evolve from indirect bandgap semiconductors to direct bandgap semiconductors while their layer number is reduced from a few layers to a monolayer limit. Consequently, there is strong photoluminescence in a monolayer (1L) TMDC due to the large quantum yield. Moreover, such monolayer semiconductors have two other exciting properties: large binding energy of excitons and valley polarization. These properties make them become ideal materials for various electronic, photonic and optoelectronic devices. However, their performance is limited by the relatively weak lightâ€"matter interactions due to their atomically thin form factor. Resonant nanophotonic structures provide a viable way to address this issue and enhance lightâ€"matter interactions in 2D TMDCs. Here, we provide an overview of this research area, showcasing relevant applications, including exotic light emission, absorption and scattering features. We start by overviewing the concept of excitons in 1L-TMDC and the fundamental theory of cavity-enhanced emission, followed by a discussion on the recent progress of enhanced light emission, strong coupling and valleytronics. The atomically thin nature of 1L-TMDC enables a broad range of ways to tune its electric and optical properties. Thus, we continue by reviewing advances in TMDC-based tunable photonic devices. Next, we survey the recent progress in enhanced light absorption over narrow and broad bandwidths using 1L or few-layer TMDCs, and their applications for photovoltaics and photodetectors. We also review recent efforts of engineering light scattering, e.g., inducing Fano resonances, wavefront engineering in 1L or few-layer TMDCs by either integrating resonant structures, such as plasmonic/Mie resonant metasurfaces, or directly patterning monolayer/few layers TMDCs. We then overview the intriguing physical properties of different van der Waals heterostructures, and their applications in optoelectronic and photonic devices. Finally, we draw our opinion on potential opportunities and challenges in this rapidly developing field of research. [ABSTRACT FROM AUTHOR]

Ferrovalley materials with intrinsic valley polarization present a novel space for materials exploration suitable for valleytronic applications. Here, two Ferrovalley materials, LaI2 and PrI2 monolayers, exhibiting intrinsic valley polarizations of 28 and 35 meV, respectively, mediated by strong magnetic exchange interaction and spin‐orbit coupling are computationally discovered. The unconstrained crystal structure prediction algorithm is used to predict the structures of bulk LaI2 and PrI2, which are layered and analogous to the 2H phase of transition metal dichalcogenides. The monolayers have small exfoliation energy (smaller than MoS2) and exhibit dynamical, mechanical, and thermodynamical stability, advocating their experimental realization. Valley polarization under the effect of bi‐axial strain is preserved and can be used to enhance valley splitting. Finally, the anomalous hall properties of both the monolayers under the effect of in‐plane electric field are discussed. Both the materials exhibit significant anomalous Hall conductivity with proper tuning of the Fermi level. [ABSTRACT FROM AUTHOR]

Valleytronic properties of monolayer transition metal dichalcogenides have attracted extensive attention owing to its intriguing fundamental physics and potential application for future electronics. Here, we investigate the spin-valley coupling and valley polarization of monolayer chromium dichalcogenides induced by the magnetic proximity effect using first-principle calculations. It is found that monolayer CrX2 (X = S, Se, and Te) have inherent spin-valley coupling and a sizable Zeeman-type spin splitting near the maximum of valance band. The valley-contrasting characteristics are further clarified by the Berry curvature, which induces a novel spin/valley Hall effect. Finally, CrSe2/MnO2 heterostructure is proposed to introduce valley polarization via a magnetic proximity effect. A primitive valley splitting of 31 meV is induced at the maximum of valance band, corresponding to a Zeeman field of 185 T. More interestingly, the valley splitting could be effectively enhanced over a wide range by an electric field or out-of-plane strain. [ABSTRACT FROM AUTHOR]

We study the magnetoconductance of small-bandgap carbon nanotube quantum dots in the presence of spin–orbit coupling in the strong-correlations regime. A finite-U slave-boson mean-field approach is used to study many-body effects. Different degeneracies are restored in a magnetic field and Kondo effects of different symmetries arise, including SU(3) effects of different types. Full spin–orbital degeneracy might be recovered at zero field and, correspondingly, the SU(4) Kondo effect sets in. We point out the possibility of the occurrence of electron–hole Kondo effects in slanting magnetic fields, which we predict to occur in magnetic fields with an orientation close to perpendicular. When the field approaches a transverse orientation a crossover from SU(2) or SU(3) symmetry into SU(4) is observed.

Layered transition metal chalcogenides with large spin orbit coupling have recently sparked much interest due to their potential applications for electronic, optoelectronic, spintronics, and valleytronics. However, most current understanding of the electronic structure near band valleys in momentum space is based on either theoretical investigations or optical measurements, leaving the detailed band structure elusive. For example, the exact position of the conduction band valley of bulk MoS2 remains controversial. Here, using angle-resolved photoemission spectroscopy with submicron spatial resolution (micro-ARPES), we systematically imaged the conduction/valence band structure evolution across representative chalcogenides MoS2, WS2, and WSe2, as well as the thickness dependent electronic structure from bulk to the monolayer limit. These results establish a solid basis to understand the underlying valley physics of these materials, and also provide a link between chalcogenide electronic band structure and their physical properties for potential valleytronics applications.

We propose a nanospaser made of an achiral plasmonic–metal nanodisk and a two-dimensional chiral gain medium – a monolayer nanoflake of a transition-metal dichalcogenide (TMDC). When one valley of the TMDC is selectively pumped (e.g. by a circular-polarized radiation), the spaser (surface plasmon amplification by stimulated emission of radiation) generates a mode carrying a topological chiral charge that matches that of the gain valley. There is another, chirally mismatched, time-reversed mode with exactly the same frequency but the opposite topological charge; it is actively suppressed by the gain saturation and never generates, leading to a strong topological protection for the generating matched mode. This topological spaser is promising for use in nano-optics and nanospectroscopy in the near field especially in applications to biomolecules that are typically chiral. Another potential application is a chiral nanolabel for biomedical applications emitting in the far field an intense circularly polarized coherent radiation.

Using the state of valley-polarization of electrons in solids is a promising new paradigm for information storage and processing. The central challenge in utilizing valley-polarization for this purpose is to develop methods for manipulating and reading out the final valley state. Here, we demonstrate optical detection of valley-polarized electrons in diamond. It is achieved by capturing images of electroluminescence from nitrogen-vacancy centers at the surface of a diamond sample that are excited by electrons drifting and diffusing through the sample. Monte Carlo simulations are performed to interpret the resulting experimental diffusion patterns. Our results give insight into the drift-diffusion of valley-polarized electrons in diamond and yield a way of analyzing the valley-polarization of ensembles of electrons.

We investigate the formation of inner edge states and their transport properties in hybrid nanoribbons. Some new inner edge states, such as spin-polarized, spin-valley-polarized and valley-polarized antichiral inner edge states, are obtained, different from the current existence of valley- and spin-valley-momentum locked inner edge states. We also obtain general formula of local bond current with the wave-function matching technique and use it to discover three interesting transport phenomena of the intravalley and intervalley scatterings that depend on the propagating direction, propagating path, spin mode and wave-vector mismatches between inner edge states. In particular, these transport phenomena are further used to design topological spin, spin–valley and valley filters and be representative for graphene, silicene, germanene and stanene, supporting a potential application of inner edge states, which are robust against random vacancies.

Analogous to real spins, valleys as carriers of information can play significant roles in physical properties of two-dimensional Dirac materials. On the other hand, utilizing external periodic potential is an efficient method to manipulate their band structures and transport properties. In this work, we investigate the valley dependent optics-like behaviors based on an 8- Pmmn borophene superlattice with the transfer matrix method and effective band approach. Firstly, it is found that the band structure is renormalized, more tilted Dirac cones are generated, and the group velocities are modified by the periodic potentials. Secondly, due to the exotic tilted Dirac cones in 8- Pmmn borophene, a perfect valley selected angle filter can be realized. The electrons with a specific incident angle can transmit completely in an energy window, which is flexibly tunable by changing the periodic potential. Thirdly, by using the Green’s function to simulate the time evolution of wave packets, electrons can be shown to propagate without any diffraction, valley electron beam supercollimation happens by modulating the potential parameters. Different from the graphene superlattice, the electron supercollimation here is valley dependent and can be used as a valley electron beam collimator. Fourthly, we can tune the polarization and supercollimation angles by changing the superlattice direction. These intriguing results in an 8- Pmmn borophene-based superlattice offer more opportunities in diverse electronic transport phenomena and may facilitate the devices applications in valleytronics and electron-optics.

Monolayer ${\mathrm{MoS}}_2$ is a typical valleytronic material with valley-spin locked valence bands. We numerically investigate the valley-spin polarized current in monolayer ${\mathrm{MoS}}_2$ via adiabatic electron pumping. By introducing an exchange field to break the energy degeneracy of monolayer ${\mathrm{MoS}}_2$ , the top of its valence bands is valley-spin polarized and tunable by the exchange field. A device with spin-up polarized left lead, spin-down polarized right lead, and untuned central region is constructed through applying different exchange fields in the corresponding regions. Then, equal amount of pumped currents with opposite valley-spin polarization are simultaneously generated in the left and right leads when periodically varying two pumping potentials. Numerical results show that the phase difference between the pumping potentials can change the direction and hence polarization of the pumped currents. It is found that the pumped current exhibits resonant behavior in the valley-spin locked energy window, which depends strongly on the system size and is enhanced to resonant current peaks at certain system lengths. More importantly, the pumped current periodically oscillates as a function of the system length, which is closely related to the oscillation of transmission. The effects of other system parameters, such as the pumping amplitude and the static potential, are also thoroughly discussed.