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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]

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.

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.

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.

Structural imperfections can cause both beneficial and detrimental consequences on the excitonic characteristics of transition metal dichalcogenides (TMDs). Regarding valley selection, structural defects typically promote valley depolarization in monolayer TMDs, but defect healing via an additional growth process can restore valley polarization in vertical heterobilayers (VHs). In this study, we analyzed the valley polarization of center-nucleated and edge-nucleated VHs (WS 2 /MoS 2 ) grown using a controlled growth process and discovered that defect-related photoluminescence (PL) is strongly suppressed in the center-nucleated VHs due to defect healing. Additionally, we demonstrated that the valley polarization of lower-lying intralayer excitons is more sensitive to the defect density of the sample than to higher-lying intralayer excitons. Despite defect healing in the center-nucleated VHs, the temperature-dependent PL study indicated that valley depolarization of the lower-lying intralayer excitons becomes significant below 100 K because of stronger hybridization of defect states. Also, we conducted a comprehensive study on the excitation intensity dependence to investigate the electron-doping-induced Auger recombination mechanism, which also contributes to valley depolarization of intralayer excitons via regeneration of intervalley trions. Our findings provide valuable insight into the development of VH-based valleytronic devices.

• The double-layered RuClF is divided into three types of interfaces based on the different contact atoms in the upper and lower layers. • The most stable structures under different interface conditions have various sizes of spontaneous valley splittings. • A detailed study on the effect of an external electric field on the valley split values of the double-layer iron-valley material, RuClF, was conducted. • The Berry curvature of the double molecular layer was studied as well. • The anomalous Valley Hall effect is realized in the anti-ferromagnetic double molecular layer. Valleytronic devices are increasingly gaining attention for their unique capability to encode and process information by leveraging electronic valley degrees of freedom. In this study, we performed first-principle calculations to analyze the band dispersions of double-layer antiferromagnetic valley material RuClF subjected to an external electric field. Our findings show that the double-layered structure exhibits interlayer antiferromagnetic order and that the bilayer bands display opposite valley splitting. We further categorized the atomic contact surfaces of the top and bottom layers into three interface conditions: F/F, Cl/Cl, and Cl/F. Under F/F and Cl/Cl interface conditions, when the electric field changes by 0.1 V/nm, the valley splitting values change by 12.2 meV and 9.5 meV, respectively. However, under the Cl/F interface condition, the structure exhibited metallic characteristics and was as such excluded from our analysis. These findings substantiate the practicality of employing an electric field to regulate the valley splitting of the bilayer structure, offering a new control strategy for valleytronic devices. [ABSTRACT FROM AUTHOR]

Materials with spontaneous polarization is very promising in valley electronics because of its potential application in encoding and storing information. In this paper, we proposed a stable two-dimensional ferromagnetic valley material H -ScAt 2 which can spontaneously generate valley splitting and exhibit intrinsic anomalous Hall effect. The valley polarization is 0.245 eV, which is large than most of the reported materials. The structural stability is verified by our phonon and molecular dynamic calculation. The Curie temperature of H -ScAt 2 monolayer is about 840 K, which is above room temperature. [Display omitted] • H-ScAt 2 exhibits anomalous valley Hall effect with huge valley polarization. • The spin direction is out of plane, and the Curie temperature is about 840 K. • The band gap could be significantly tuned by strain with steady valley polarization. [ABSTRACT FROM AUTHOR]

The spin–valley transport is studied in a Fermi velocity-modulated monolayer WSe 2 magnetic superlattice. Based on these results, a scheme that can achieve an enhancing and switching spin–valley filter is proposed in the superlattice system. We find the Fermi velocity enhanced and switched spin-valley dependent conductance transport gaps by increasing the number of Fermi velocity barriers, which are verified by the transmission probabilities and Bloch theorem. When compared to a mono-periodic superlattice, the maximum spin ↓ and valley K′ polarized conductance in a well-constructed superlattice structure are greatly improved to a detectable degree. More importantly, the crucial bias voltage or exchange field can dynamically flip and control the valley polarized conductance, something that is not possible in a mono-periodic superlattice. Our findings indicate that a Fermi velocity modulated superlattice structure can be a great way to improve the spin-valley filtered polarizations of devices based on transition metal dichalcogenides. • A Fermi velocity combined with superlattice-modulated WSe 2 junction was proposed. • The spin ↓ and valley K′ polarized conductance is improved to a measurable amount. • These polarized conductance can be switched and tuned by bias voltage/exchange field. • These polarizations are supported by transmission probabilities and Bloch theorem. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) valleytronics materials have garnered significant attention due to great potential for information encoding and processing. Here, we investigate the strain effect on the valley and topological properties of a novel septuple-layer FeO 2 Si 2 N 2 monolayer by employing the first-principles calculations. The monolayer is affirmed to be a stable ferrovalley semiconductor with in-plane magnetization and high Curie transition temperature (156 K). Further, by applying the compressive strain from −3.24 to −3.4%, the magnetization of the monolayer can be tuned from in-plane to out-of-plane direction, and consequently, the monolayer exhibits topological nontrivial valley state with valley polarization due to the breaking of inversion and time-reversal symmetry, and higher Curie temperature (213 K) due to the enhanced superexchange interaction between Fe and O atoms. Thus, the long-sought valley-polarized quantum anomalous Hall (VQAH) effect can be realized in higher Curie temperature in FeO 2 Si 2 N 2 monolayer by applying the appropriate compressive strain. [Display omitted] • FeO 2 Si 2 N 2 monolayer is a FM semiconductor with high T c of 156 K and MAE of 0.95 meV. • The MAE can be flipped from in-plane to out-of-plane under −3.24% to −3.4% strain. • The reversal of magnetization direction indicates the emergence of valley polarization. • The valley-polarized quantum anomalous Hall effect can be observed under strain. [ABSTRACT FROM AUTHOR]

Magnetic proximity interaction provides a promising route to manipulate the spin and valley degrees of freedom in van der Waals heterostructures. Here, we report a control of valley pseudospin in the WS 2 /MoSe 2 heterostructure by utilizing the magnetic proximity effect of few-layered CrBr 3 and, for the first time, observe a substantial difference in valley polarization of intra/interlayer excitons under different circularly polarized laser excitations, referred to as chirality-dependent valley polarization. Theoretical and experimental results reveal that the spin-selective charge transfer between MoSe 2 and CrBr 3 , as well as between MoSe 2 and WS 2 , is mostly responsible for the chiral feature of valley polarization in comparison with the proximity exchange field. This means that a long-distance manipulation of exciton behaviors in multilayer heterostructures can be achieved through spin-selective charge transfer. This work marks a significant advancement in the control of spin and valley pseudospin in multilayer structures.

Valleytronics, i.e., the manipulation of the valley degree of freedom, offers a promising path for energy-efficient electronics. One of the key milestones in this field is the room-temperature manipulation of the valley information in thick-layered material. Using scanning photocurrent microscopy, we achieve this milestone by observing a geometrically dependent circular photocurrent in a few-layer molybdenum disulfide (MoS 2 ) under normal incidence. Such an observation shows that the system symmetry is lower than that of bulk MoS 2 material, preserving the optical chirality-valley correspondence. Moreover, the circular photocurrent polarity can be reversed by applying electrical bias. We propose a model where the observed photocurrent results from the symmetry breaking and the built-in field at the electrode-sample interface. Our results show that the valley information is still retained even in thick-layered MoS 2 at room temperature and opens up new opportunities for exploiting the valley index through interface engineering in multilayer valleytronics devices.

[Display omitted] • We utilize the intrinsic magnetism of CrI 3 to break the time-reversal symmetry of WTe 2 , thus inducing a giant valley splitting of up to 34.01 meV. • By modulating the interlayer exchange coupling and applying perpendicular external electric fields, a wide range of valley splitting manipulation can be achieved. • The method of magnetic ion intercalation, particularly the self-intercalated Cr ion, significantly enhances the valley splitting by an order of magnitude. • The method of magnetic ion intercalation, particularly the self-intercalated Cr ion, can significantly enhance the valley splitting of WTe 2 /CrI 3 heterostructure by an order of magnitude. Lifting the valley degeneracy is an essential condition for exploiting the valley degrees of freedom to process and store information in valleytronics. The magnetic proximity effect (MPE) has been proven to be an effective method for introducing a magnetic field via interlayer exchange interaction, which can be conveniently achieved by two-dimensional (2D) van der Waals (vdW) heterostructure engineering. We have investigated the electronic properties and valley physics of 2D WTe 2 /CrI 3 heterostructure using first-principles calculations. The results show that a significant valley splitting of 34.01 meV can be obtained in the WTe 2 /CrI 3 heterostructure due to the simultaneous breaking of inversion and time-reversal symmetry, which sensitively depends on the atomic overlap of the projected position in the specific stacking model. In addition, we also show that the valley splitting can be enhanced by applying a vertical external electric field (E-field) or reducing the interlayer distance. More than that, it can be boosted by an order of magnitude (∼115.25 meV) through Cr ion intercalation. This is attributed to that the self-intercalated Cr ion has ferromagnetic (FM) ordering and perpendicular magnetic anisotropy due to the exchange coupling of the CrI 3 layer. This result provides fundamental insights and valuable guidance for the application of 2D WTe 2 /CrI 3 heterostructure in novel spintronic and valleytronic devices. [ABSTRACT FROM AUTHOR]

We investigate the optical conductivity, along with longitudinal and transverse conductivities, in buckled hexagonal lattice such as silicene subjected to both an in-plane magnetic field and a perpendicular electric field. In this model, we neglect the effect of the spin-orbit interaction, which is of a smaller order compared to the strong staggered potential and the next-nearest hoping energy. The orientations of the in-plane magnetic field and the perpendicular electric field give rise to a non-uniform, tunable gap. The Chern number for each valley degree of freedom deviates from being constant but remains steady when summed over the entire Brillouin zone. The longitudinal and transverse currents, in the case of a specific valley, can be selected by adjusting the direction of the electric field in the semimetal phase. Furthermore, the defining characteristics of topological phases induces the rapid change in longitudinal conductivity when varying the angle of orientation of the in-plane magnetic field under monochromatic light, and perfect valley filtering in transverse conductivity. The transverse current associated with a specific valley can be selected when the angle of orientation satisfies the specific conditions. This investigation paves the way for materials design with valley-locked current, using a specific orientation of the in-plane magnetic field. • We studied the optical conductivity in honeycomb lattices under an applied in-plane magnetic field. • The next-nearest hopping energy has been taken into account. • We predict a switching effect of longitudinal and transverse currents, controlled by the orientation of the magnetic field. • Our study shows controllable direction of pure valley Hall current by manipulating the orientation of the magnetic field. • This paves the way for material design with spin-valley momentum locking. [ABSTRACT FROM AUTHOR]

Schematic illustration of valley properties based on the H-HfI 2 /T-VBrCl heterostructure. It could be a potential candidate for valleytronic devices. [Display omitted] The electronic band structure and valley splitting of monolayer H-HfI 2 and H-HfI 2 /T-VBrCl heterostructure have been investigated by first principles calculations. The monolayer H-HfI 2 exhibits valleytronic feature at K and K' points of the valence bands. The valley polarization in monolayer H-HfI 2 arises from the magnetic proximity effect induced by T-VBrCl monolayer. The stacking configurations have a significant influence on the valley polarization of the H-HfI 2 /T-VBrCl heterostructure. The maximum valley polarization can be as high as −20 meV. It is possible to manipulate the magnitude of valley polarization by adjusting the layer spacing and applying biaxial strain. The Berry curvature between two valleys indicates that the H-HfI 2 /T-VBrCl heterostructure should be a potential candidate for valleytronic devices. [ABSTRACT FROM AUTHOR]

Two-dimensional ferrovalley magnetic materials have attracted much attention due to the applications in valley-based nonvolatile random access memories and valley filters. In this work, using first-principles calculations, we predict a promising class of bipolar magnetic semiconductors, namely non-Janus GdXY(X≠Y=Cl,Br,I) monolayers, which exhibit excellent mechanical and thermal stability, large magnetic moment (8 μ B /Gd), and high Curie temperature (above 450 K). When magnetized along the ± z direction, a spontaneous valley polarization can be observed in non-Janus GdXY. Due to the non-zero Berry curvature, the anomalous Hall effect will be able to be observed in non-Janus GdXY. In addition, the system transforms into a semi-semiconductor from a bipolar magnetic semiconductor with increasing biaxial tensile strain. Under the strain of -4%∼+4%, the ferrovalley characteristics can be well maintained. Our findings not only reveal that non-Janus GdXY is a novel room-temperature ferrovalley semiconductor material, but also provide a new platform for designing spintronics and valley electronics devices. • Non-Janus GdXY monolayers are novel two-dimensional rare-earth ferro-valley materials. • Non-Janus GdXY monolayers exhibit high Curie temperatures above 450 K. • Non-Janus GdXY monolayers have large valley splitting. • Non-Janus GdXY monolayers provide a new platform for realizing the anomalous Hall effect. • The ferro-valley characteristics of non-Janus GdXY monolayers can be well maintained under the biaxial strain of -4%∼+4%. [ABSTRACT FROM AUTHOR]

We study theoretically the electron dynamics of transition metal dichalcogenide (TMDC) quantum dots (QDs) in the field of an ultrashort and ultrafast circularly polarized optical pulse. The QDs have the shape of a disk and their electron systems are described within an effective model with infinite mass boundary conditions. Similar to TMDC monolayers, a circularly polarized pulse generates ultrafast valley polarization of such QDs. The dependence of the valley polarization on the size of the dot is sensitive to the dot material and, for different materials, show both monotonic increase with the dot radius and nonmonotonic behavior with a local maximum at a finite dot radius., (© 2024 IOP Publishing Ltd.)

Over the past decade, significant advancements have been made in phase engineering of two-dimensional transition metal dichalcogenides (TMDCs), thereby allowing controlled synthesis of various phases of TMDCs and facile conversion between them. Recently, there has been emerging interest in TMDC coexisting phases, which contain multiple phases within one nanostructured TMDC. By taking advantage of the merits from the component phases, the coexisting phases offer enhanced performance in many aspects compared with single-phase TMDCs. Herein, this review article thoroughly expounds the latest progress and ongoing efforts on the syntheses, properties, and applications of TMDC coexisting phases. The introduction section overviews the main phases of TMDCs (2H, 3R, 1T, 1T', 1T d ), along with the advantages of phase coexistence. The subsequent section focuses on the synthesis methods for coexisting phases of TMDCs, with particular attention to local patterning and random formations. Furthermore, on the basis of the versatile properties of TMDC coexisting phases, their applications in magnetism, valleytronics, field-effect transistors, memristors, and catalysis are discussed. Lastly, a perspective is presented on the future development, challenges, and potential opportunities of TMDC coexisting phases. This review aims to provide insights into the phase engineering of 2D materials for both scientific and engineering communities and contribute to further advancements in this emerging field.