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Because the electron transport mechanism in graphene is heavily impacted by the strain and the ferromagnetic metal stripe as well as several other avenues, in this paper we investigate the effects of the strained barrier induced by the strain and the magnetic field generated by the ferromagnetic metal stripe on the valley polarization through numerical calculation. When the strength and the width of the strained barrier as well as the magnitude of the magnetic field are changed, the rapid variation of the valley polarization is observed. This study will be helpful for devising and manufacturing new-style valleytronic devices. [ABSTRACT FROM AUTHOR]

Two-dimensional heterostructures based on transition metal dichalcogenides (TMDs) exhibit broad application prospects in valleytronics due to the space-reversal symmetry breaking and strong spin–orbit coupling. In this work, the electronic structure, magnetic anisotropy and valley polarization of 2H–WSSe/VN van der Waals heterostructure under various interlayer spacings, magnetic angle and in-plane strain are investigated in detail by first-principles calculations. The stacked configuration of Se-C2-1 with most stable structure shows the largest valley polarization of 386.5meV. By adjusting the interlayer spacing of heterostructure, the largest valley polarization of 702.7meV appears in Se-C2-1 stacked configuration with interlayer spacing of 2.24 Å. The magnetic angle θ exhibits significant effects on valley polarization and magnetic anisotropy of 2H–WSSe/VN heterostructures. The stability and valley polarization of 2H–WSSe/VN heterostructure decrease after the in-plane biaxial strain is applied. The large and tunable valley polarization as well as magnetic anisotropy in the 2H–WSSe/VN heterostructures make it potential applications in valleytronic devices. [ABSTRACT FROM AUTHOR]

Valley degeneracy can be broken owing to the strong spin-orbit coupling in two-dimensional transition metal dichalcogenides (2D-TMDCs). Valley-dependent interaction of carriers in TMDCs with different circular polarizations of light offers valley degree-of-freedom besides charge and spin to carry information. Thus, bandgap engineering of 2D-TMDCs plays a critical role in developing practical valleytronic devices. Hereby, we demonstrate a great enhancement in quantum yield as well as polarization of monolayer MoS 2 achieved by gradually alloying W atoms in MoS 2. By appropriately setting a time offset between the evaporation of MoO 3 and WO 3 precursors during chemical vapor deposition, a compositionally graded heterostructure of MoS 2 -WS 2 monolayer can be readily grown at large scale. Raman and transmission electron microscopy measurements demonstrate that the interface possesses a steep gradient in composition, spanning from MoS 2 to WS 2 over a length ∼2 ​μm. Compared to pure monolayer MoS 2 , the photoluminescence intensity at the compositionally graded interface of Mo 1-x W x S 2 was observed to increase by a factor of 16 owing to the effective separation of photogenerated carriers by the built-in electric field. Particularly, a remarkably high polarization of 70% at 16 ​K is demonstrated for the compositionally graded interface of Mo 1-x W x S 2 , which is ∼1.4 times larger than that in MoS 2 and is attributed to the combined effect of the alloyed structure and graded bandgap. Such an engineering scheme with a graded bandgap offers new approach for the development of high-efficiency valleytronics devices. [ABSTRACT FROM AUTHOR]

Inner edge state with spin and valley degrees of freedom is a promising candidate for designing a dissipationless device due to the topological protection. The central challenge for the application of the inner edge state is to generate and modulate the polarized currents. In this work, we discover a new mechanism to generate fully valley- and spin–valley-polarized current caused by the Bloch wavevector mismatch (BWM). Based on this mechanism, we design some serial-typed inner-edge filters. By using once of the BWM, the coincident states could be divided into transmitted and reflected modes, which can serve as a valley or spin–valley filter. In particular, while with twice of the BWM, the incident current is absolutely reflected to support an off state with a specified valley and spin, which is different from the gap effect. These findings give rise to a new platform for designing valleytronics and spin-valleytronics. [ABSTRACT FROM AUTHOR]

In recent years, valleytronics researches based on 2D semiconducting transition metal dichalcogenides have attracted considerable attention. On the one hand, strong spin–orbit interaction allows the presence of spin–valley coupling in this system, which provides spin addressable valley degrees of freedom for information storage and processing. On the other hand, large exciton binding energy up to hundreds of meV enables excitons to be stable carriers of valley information. Valley polarization, marked by an imbalanced exciton population in two inequivalent valleys (+ K and − K), is the core of valleytronics as it can be utilized to store binary information. Motivated by the potential applications, we present a thorough overview of the recent advancements in the generation, relaxation, manipulation, and transport of the valley polarization in nonmagnetic transition metal dichalcogenide layered semiconductors. We also discuss the development of valleytronic devices and future challenges in this field. [ABSTRACT FROM AUTHOR]

Inspired by potential application prospects of spintronics and valleytronics, a novel heterobilayer Janus structure is designed by replacing the chalcogenide atomic layers in the original bilayer MoS2. Based on first-principles calculations, it is found that the SMoS/SeMoS structure exhibits a direct band-gap semiconductor and a typical type-II band alignment with longer carrier lifetime. The transition metal (TM) atom represented by V/Cr/Mn can be stably adsorbed on the heterobilayer Janus SMoS/SeMoS sheet and effectively introduce magnetic moments (m). The calculation results demonstrate that the most stable adsorption site of the TM atom is CX(A), and the TM (V/Cr/Mn) adatom modified SMoS/SeMoS system is converted into metal (V-) or half-metal (Cr/Mn-), respectively. Under the coupling of different indirect exchange interactions, the structure exhibits stable intrinsic anti-ferromagnetic interactions for V-SMoS/SeMoS and ferromagnetic ground state for Cr/Mn-SMoS/SeMoS, respectively, and the magnetic transition temperature (T c) reaches a high temperature or even room temperature. Moreover, the robust out-of-plane magnetocrystalline anisotropy energy ensures stable long-range magnetic order. Specifically, the combination of spin injection and strong spin–orbit coupling interaction effectively breaks the time-reversal symmetry, which leads to valley polarization of the system. Based on this, the biaxial strain can effectively regulate the electronic structure, magnetic properties and valley polarization of TM-SMoS/SeMoS nanosheets with double breaking of spatial-inversion and time-reversal symmetry. [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 zeroth Landau level (0LL) in graphene has emerged as a flat band platform in which distinct many-body phases can be explored with unprecedented control by simply tuning the strength and/or direction of the magnetic field. A rich set of quantum Hall ferromagnetic phases with different lattice-scale symmetry-breaking orders are predicted to be realized in high magnetic fields when the 0LL in graphene is half-filled. Here we report on a field-tuned continuous phase transition of different valley orderings in a quantum Hall ferromagnetic phase of charge-neutral graphene on insulating tungsten diselenide (WSe2). The phase transition is clearly revealed by an anomalous field-dependent energy gap in the half-filled 0LL. Using atomic resolution imaging of electronic wavefunctions during the phase transition, we unexpectedly observe the microscopic signatures of field-tuned continuous-varied valley polarization and valley inversion, which are beyond current theoretical predictions. Moreover, the quantum Hall conducting channel of the graphene is directly imaged when the substrate (WSe2) introduces band bending of the 0LL. [ABSTRACT FROM AUTHOR]

Owing to the inherent characteristics of collective excitations in graphene, electrical control of edge plasmons is highly desirable for nanoplasmonic applications. This study investigates valley-polarized edge pseudomagnetoplasmons in a graphene p–n junction subjected to a strain-induced pseudomagnetic field. A four-component hydrodynamic model is employed and solved via the Wiener–Hopf method, revealing the coexistence of three plasmon modes, including counterpropagating acoustic edge modes, gapless topological edge states, and zero modes. The valley polarization, as determined from the numerically exact solution, is stronger than that predicted by the approximate models. Notably, the confinement of edge plasmons at the graphene p–n junction significantly exceeds that at the graphene/vacuum interface, possibly because of the electron–hole attraction. Furthermore, gate-controlled subwavelength confinement is successfully achieved by applying an appropriate gate voltage, thereby highlighting a unique and promising attribute of edge pseudomagnetoplasmons in graphene p–n junctions.

Monolayer transition metal dichalcogenides (ML‐TMDCs) possess degenerate levels with antiparallel spins in K and K′ valleys, providing the intrinsic valley polarization, which attracts great interest for potential applications on quantum information technology and on‐chip nanophotonics. Unfortunately, it is difficult to distinguish the degree of valley polarization (DoP) near room temperature due to the intensive phonon‐assisted intervalley scattering and the long‐range electron‐hole exchange interaction in ML‐TMDCs, limiting their practical applications. In this study, a novel design is proposed for great promotion of DoP in ML‐WSe2 with a microsphere cavity array, introducing Purcell effect and nanofocusing effect into the system. The radiative decay rate is dramatically enhanced owing to Purcell effect in microcavity in weak coupling regime, thus locking more polarized excitons in the corresponding valley under certain circularly‐polarized pumping. In addition, the nanofocusing effect contributes to increasing the number of charged excitons by suppressing the bright to dark exciton conversion. The present work achieves a great DoP of ML‐WSe2 with a simple configuration and promises broad applications from valleytronic devices to chiral optics in the future. [ABSTRACT FROM AUTHOR]

We numerically study the general valley polarization and anomalous Hall effect in van der Waals (vdW) heterostructures based on monolayer jacutingaite family materials Pt2AX3 (A = Hg, Cd, Zn; X = S, Se, Te). We perform a systematic study on the atomic, electronic, and topological properties of vdW heterostructures composed of monolayer Pt2AX3 and two-dimensional ferromagnetic insulators. We show that four kinds of vdW heterostructures exhibit valley-polarized quantum anomalous Hall phase, i.e., Pt2HgS3/NiBr2, Pt2HgSe3/CoBr2, Pt2HgSe3/NiBr2, and Pt2ZnS3/CoBr2, with a maximum valley splitting of 134.2 meV in Pt2HgSe3/NiBr2 and sizable global band gap of 58.8 meV in Pt2HgS3/NiBr2. Our findings demonstrate an ideal platform to implement applications on topological valleytronics. [ABSTRACT FROM AUTHOR]

Layered transition metal dichalcogenides (TMDs) provide a favorable research platform for the advancement of spintronics and valleytronics because of their unique spin-valley coupling effect, which is attributed to the absence of inversion symmetry coupled with the presence of time-reversal symmetry. To maneuver the valley pseudospin efficiently is of great importance for the fabrication of conceptual devices in microelectronics. Here, we propose a straightforward way to modulate valley pseudospin with interface engineering. An underlying negative correlation between the quantum yield of photoluminescence and the degree of valley polarization was discovered. Enhanced luminous intensities were observed in the MoS2/hBN heterostructure but with a low value of valley polarization, which was in stark contrast to those observed in the MoS2/SiO2 heterostructure. Based on the steady-state and time-resolved optical measurements, we reveal the correlation between exciton lifetime, luminous efficiency, and valley polarization. Our results emphasize the significance of interface engineering for tailoring valley pseudospin in two-dimensional systems and probably advance the progression of the conceptual devices based on TMDs in spintronics and valleytronics. [ABSTRACT FROM AUTHOR]

Two unequal K and K' valleys in transition metal dichalcogenides (TMDC) enable large and controllable polarization, which is the cornerstone of emerging valleytronic applications. Here, a phase engineering strategy aided by resonant plasmonic coupling is proposed to manipulate the valley degree of freedom. Compared with the pristine WSe2 monolayer, the hybrid H/T phase WSe2 exhibits an enhanced degrees of circular polarization (DCP) and valley polarization (DVP). As further aided by the designed Au plasmonic array, the T phase facilitates the excitons process and promotes the charge transfer in WSe2/Au interface under the plasmonic‐enhanced electromagnetic field. Consequently, both the DCP and DVP values are considerably enhanced to 38.5% (15.6%) and 15.1% (7.6%) at 13 K (room temperature), respectively. Through finite difference time domain simulations (FDTD), the near‐field excitation, exciton decay, and far‐field detection processes are systematically analyzed, and highly consistent polarizations are quantitatively achieved between the theoretical and the experimental results. Accordingly, the high polarizations are revealed to be contributed by the increased exciton generation and radiation efficiency, chiral electromagnetic field, and non‐equilibrium spin distribution in the hybrid phase. The research presented here illustrates a promising route to control the spin and valley degrees of freedom in TMDC materials. [ABSTRACT FROM AUTHOR]

Two unequal K and K' valleys in transition metal dichalcogenides (TMDC) enable large and controllable polarization, which is the cornerstone of emerging valleytronic applications. Here, a phase engineering strategy aided by resonant plasmonic coupling is proposed to manipulate the valley degree of freedom. Compared with the pristine WSe2 monolayer, the hybrid H/T phase WSe2 exhibits an enhanced degrees of circular polarization (DCP) and valley polarization (DVP). As further aided by the designed Au plasmonic array, the T phase facilitates the excitons process and promotes the charge transfer in WSe2/Au interface under the plasmonic‐enhanced electromagnetic field. Consequently, both the DCP and DVP values are considerably enhanced to 38.5% (15.6%) and 15.1% (7.6%) at 13 K (room temperature), respectively. Through finite difference time domain simulations (FDTD), the near‐field excitation, exciton decay, and far‐field detection processes are systematically analyzed, and highly consistent polarizations are quantitatively achieved between the theoretical and the experimental results. Accordingly, the high polarizations are revealed to be contributed by the increased exciton generation and radiation efficiency, chiral electromagnetic field, and non‐equilibrium spin distribution in the hybrid phase. The research presented here illustrates a promising route to control the spin and valley degrees of freedom in TMDC materials. [ABSTRACT FROM AUTHOR]

In this paper, the effect of ferromagnetic metal stripe and the strained barrier on the valley-dependent transport characteristics of electrons is studied in a graphene nanostructure. The numerical results show that a large valley polarization can be obtained in such a graphene, and the valley-dependent transport characteristics can be well controlled by changing the strength of the magnetic field induced by the ferromagnetic metal stripe, the width of the ferromagnetic metal stripe and the position of the strained barrier. Therefore, the valley polarization can be modulated by controlling the ferromagnetic metal stripe and the strained barrier. This work can promote the research and development of the new valleytronic devices, and then meet the practical application needs. [ABSTRACT FROM AUTHOR]

The graphene nanostructure model under the joint modulation of ferromagnetic stripes and hard barriers is established. The effects of the magnitude of magnetic field generated by ferromagnetic stripes and the width of ferromagnetic stripes on the valley-dependent electron transport properties in graphene are calculated, and the electron conductance and the valley polarization in the graphene nanostructure are studied. The numerical results show that the significant valley polarization effect can be realized in such a nanostructure, and the strength of the magnetic field and the width of the ferromagnetic stripes will have a great influence on the electron conductance and valley polarization. Therefore, the valley polarization intensity actually required can be obtained by controlling the width of the ferromagnetic stripes and the strength of the magnetic field generated by it. This study is very helpful for understanding and designing valleytronic devices. [ABSTRACT FROM AUTHOR]

Valley, as a new degree of freedom for electrons, has drawn considerable attention due to its significant potential for encoding and storing information. Lifting the energy degeneracy to achieve valley polarization is necessary for realizing valleytronic devices. Here, on the basis of first-principles calculations, we show that single-layer FeCl2 exhibits a large spontaneous valley polarization (∼101 meV) arising from the broken time-reversal symmetry and spin-orbital coupling, which can be continuously tuned by varying the direction of magnetic crystalline. By employing the perturbation theory, the underlying physical mechanism is unveiled. Moreover, the coupling between valley degree of freedom and ferromagnetic order could generate a spin- and valley-polarized anomalous Hall current in the presence of the in-plane electric field, facilitating its experimental exploration and practical applications.

The emergence of atomically thin valleytronic semiconductors and 2D ferromagnetic materials is opening up new technological avenues for future information storage and processing. A key fundamental challenge is to identify physical knobs that may effectively manipulate the spin‐valley polarization, preferably in the device context. Here, a novel spin functional device that exhibits both electrical and magnetic tunability is fabricated, by contacting a monolayer MoSe2 with a 2D ferromagnetic semiconductor Cr2Ge2Te6. Remarkably, the valley‐polarization of MoSe2 is found to be controlled by a back‐gate voltage with an appreciably enlarged valley splitting rate. At fixed gate voltages, the valley‐polarization exhibits magnetic‐field and temperature dependence that corroborates well with the intrinsic magnetic properties of Cr2Ge2Te6, pointing to the impact of magnetic exchange interactions. Due to the interfacial arrangement, the charge‐carrying trion photoemission predominates in the devices, which may be exploited to enable drift‐based spin‐optoelectronic devices. These results provide new insights into valley‐polarization manipulation in transition metal dichalcogenides by means of ferromagnetic semiconductor proximitizing and represent an important step forward in devising field‐controlled 2D magneto‐optoelectronic devices. [ABSTRACT FROM AUTHOR]

Two–dimensional (2D) transition metal dichalcogenides endow individually addressable valleys in momentum space at the K and K' points in the first Brillouin zone due to the breaking of inversion symmetry and the effect of spin–orbit coupling. However, the application of 2H–VS2 monolayer in valleytronics is limited due to the valence band maximum (VBM) located at the Γ point. Here, by involving the 2D ferroelectric (FE) CuInP2Se6 (CIPSe), the ferrovalley polarization, electronic structure, and magnetic properties of 2D 2H–VS2/CIPSe heterostructures with different stacking patterns and FE polarizations have been investigated by using first–principles calculations. It is found that, for the energetically favorable AB–stacking pattern, the valley polarization is preserved when the FE polarization of CIPSe is upwards (CIPSe↑) or downwards (CIPSe↓) with the splitting energies slightly larger or smaller compared with that of the pure 2H–VS2. It is intriguing that, for the FE CIPSe↑ case, the VBM is expected to pass through the Fermi energy level, which can be eventually achieved by applying biaxial strain and thus the valleytronic nature is turned off; however, for the CIPSe↓ situation, the heterostructure basically remains semiconducting even under biaxial strains. Therefore, with the influence of proper strains, the FE polar reversal of CIPSe can be used as a switchable on/off to regulate the valley polarization in VS2. These results not only demonstrate that 2H–VS2/CIPSe heterostructures are promising potential candidates in valleytronics, but also shed some light on developing practical applications of valleytronic technology. [ABSTRACT FROM AUTHOR]

Interlayer excitons in transition-metal dichalcogenide heterobilayers combine high binding energy and valley-contrasting physics with a long optical lifetime and strong dipolar character. Their permanent electric dipole enables electric-field control of the emission energy, lifetime, and location. Device material and geometry impact the nature of the interlayer excitons via their real- and momentum-space configurations. Here, we show that interlayer excitons in MoS 2 /MoSe 2 heterobilayers are formed by charge carriers residing at the Brillouin zone edges, with negligible interlayer hybridization. We find that the moiré superlattice leads to the reversal of the valley-dependent optical selection rules, yielding a positively valued g-factor and cross-polarized photoluminescence. Time-resolved photoluminescence measurements reveal that the interlayer exciton population retains the optically induced valley polarization throughout its microsecond-long lifetime. The combination of a long optical lifetime and valley polarization retention makes MoS 2 /MoSe 2 heterobilayers a promising platform for studying fundamental bosonic interactions and developing excitonic circuits for optical information processing.

Owing to their strong exciton effects and valley polarization properties, monolayer transition-metal dichalcogenides (1L TMDs) have unfolded the prospects of spin-polarized light-emitting devices. However, the wavefront control of exciton emission, which is critical to generate structured optical fields, remains elusive. In this work, the experimental demonstration of spin-locked vortex emission from monolayer Tungsten Disulfide (1L WS 2 ) integrated with Silicon Nitride (SiN x ) PhC slabs is presented. The symmetry-protected bound states in the continuum (BIC) in the SiN x PhC slabs engender azimuthal polarization field distribution in the momentum space with a topological singularity in the center of the Brillouin zone, which imposes the resonantly enhanced WS 2 exciton emission with a spin-correlated spiral phase front by taking advantage of the winding topologies of resonances with the assistance of geometric phase scheme. As a result, exciton emission from 1L WS 2 exhibits helical wavefront and doughnut-shaped intensity beam profile in the momentum space with topological charges locked to the spins of light. This strategy on spin-dependent excitonic vortex emission may offer the unparalleled capability of valley-polarized structured light generation for 1L TMDs., (© 2024 Wiley‐VCH GmbH.)

Layer-engineered interlayer excitons from heterostructures of transition-metal dichalcogenides (TMDCs) exhibit a rich variety of emissive states and intriguing valley spin-selection rules, the effective modulation of which is crucial for excitonic physics and related device applications. Strain or high pressure provides the possibility to tune the energy of the interlayer excitons; however, the reported emission intensity is substantially quenched, which greatly limits their practical application in optoelectronic devices. Here, via applying uniaxial strain based on polyvinyl alcohol (PVA) encapsulation technique, we report enhanced layer-engineered interlayer exciton emission intensity with largely modulated emission energy in WSe 2 /WS 2 heterobilayer and heterotrilayer. Both momentum-direct and momentum-indirect interlayer excitons were observed, and their emission energies show an opposite shift tendency upon applied strain, which agrees with our DFT calculations. We further demonstrate that intralayer and interlayer exciton states with low phonon interactions can be modulated through the mechanical strain applied to the PVA substrate at low temperatures. Due to strain-induced breaking of the 3-fold rotational symmetry, we observe the enhanced valley polarization of interlayer excitons. Our study contributes to the understanding and modulation of the optical properties of interlayer excitons, which could be exploited for optoelectronic device applications.

As the rapid development of valleytronics and its huge-potential applications in valleytronic devices, to well understand the valley-dependent transport mechanism of electrons in graphene is a very urgent work. Thus, in this paper, we use transfer matrix method to investigate the valley-dependent electron transport in a magnetic-strained superlattice. We analyze the dependence of the electron transmission and the conductance as well as the valley polarization on the periodic number of the magnetic-strained barriers. Through careful analysis of numerical results, we find that the periodic number of the magnetic-strained barriers plays an important role in the electron transmission, and so strongly affects the conductance and the valley polarization. This provides theoretical guidance for controlling the valley-dependent transport properties of electrons. [ABSTRACT FROM AUTHOR]

Twist angle provides a new degree of freedom for 2D material modifications. In principle, the intrinsic properties of twisted multilayers can be regulated by twist angle between each adjacent layer and thus have greater tunability than widely studied bilayer structures. Considering its complexity, it is important to first investigate the simplest twisted multilayers with only one interface twisted. In this work, multilayer Moiré superlattices with only one twisted interface via paraffin‐assisted folding of non‐twisted stacked (highly symmetrically stacked) multilayer MoS2 are successfully fabricated, and their twist‐angle dependent optical properties are systematically studied. Compared to non‐twisted stacked multilayer MoS2, the one‐interface‐twisted multilayers show a 2–3.5 times higher PL intensity, and their interlayer coupling, indirect bandgap, and degree of circular polarization (DOCP) are tunable by twist angle. Notably, the DOCP for the one‐interface‐twisted four‐layer (folded bilayer) can reach 86%, which is the highest value ever reported for transition metal dichalcogenide homostructures above liquid nitrogen temperature. This work provides a solid base for understanding twist‐angle dependent properties of twisted multilayer 2D‐materials. [ABSTRACT FROM AUTHOR]

An investigation is carried on a quantum system featuring a MoS 2 zigzag nanoribbon employing double magnetic barriers. This study aims to overcome a valley polarization limitation observed in systems using a uniform magnetic field or a single magnetic barrier. A magnetic field breaks the degeneracy of the K and K ′ valleys owing to the valley Zeeman effect, but reversing the direction of the magnetic field is required to invert the valley polarization. In a double magnetic barrier system, interplay between the Fano resonance (arising from the quantum interference between the continuum state of the magnetic barrier and the Landau level) and the resonant tunneling (arising from the quantum states between two barriers) indicates the potential for achieving selective spin and valley polarization by adjusting the strength and width of the magnetic barrier. This can enhance the degree of valley polarization and enable the existence of four distinct spin and valley currents. Obtaining full spin and valley polarization is shown with the optimized parameters of this system. The experimental realization of this system should enable the control of carrier valley and spin degrees of freedom in future devices. [Display omitted] • Three-band tight-binding model using three d orbitals (dx 2 , dx2−y2, and dz2) of Mo atoms. • Double magnetic barrier is used to enhance valley and spin polarization. • Resonant tunneling effect in the K valley and Fano resonance effect in the K ′ valley. • The MoS 2 zigzag nanoribbon system with double magnetic barriers along the same direction has great advantages compared with a single magnetic barrier. [ABSTRACT FROM AUTHOR]

Layered transition metal dichalcogenides (TMDs) provide a favorable research platform for the advancement of spintronics and valleytronics because of their unique spin-valley coupling effect, which is attributed to the absence of inversion symmetry coupled with the presence of time-reversal symmetry. To maneuver the valley pseudospin efficiently is of great importance for the fabrication of conceptual devices in microelectronics. Here, we propose a straightforward way to modulate valley pseudospin with interface engineering. An underlying negative correlation between the quantum yield of photoluminescence and the degree of valley polarization was discovered. Enhanced luminous intensities were observed in the MoS2/hBN heterostructure but with a low value of valley polarization, which was in stark contrast to those observed in the MoS2/SiO2 heterostructure. Based on the steady-state and time-resolved optical measurements, we reveal the correlation between exciton lifetime, luminous efficiency, and valley polarization. Our results emphasize the significance of interface engineering for tailoring valley pseudospin in two-dimensional systems and probably advance the progression of the conceptual devices based on TMDs in spintronics and valleytronics.

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.

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]

Comprehensive understanding of the interactions between excited states with spin‐valley polarizations is crucial for the applications of monolayer 2D transition metal dichalcogenides (TMDs) in spin‐valleytronics. Here, by broadband femtosecond transient absorption spectroscopy with circularly polarized pump/probe lights, a systematic investigation on the many‐body interactions of band‐edge valley excitons in monolayer MoS2 is performed, where intravalley coherent coupling processes are directly observed at room temperature. Namely, when the intravalley mixing states in the superposition region of A‐exciton and B‐exciton states are excited, there is an intravalley coherent coupling between A‐exciton and B‐exciton in K valley (or A′‐exciton and B′‐exciton in K′ valley) with an oscillation frequency of ≈7–8 THz, rather than the intervalley excitonic coupling in general. The findings elucidate intravalley mixing and coherent oscillations in valley excitons of monolayer TMDs at room temperature, which can give insight into the many‐body coupling mechanisms of excited valley excitons in 2D TMDs. [ABSTRACT FROM AUTHOR]

Comprehensive understanding of the interactions between excited states with spin‐valley polarizations is crucial for the applications of monolayer 2D transition metal dichalcogenides (TMDs) in spin‐valleytronics. Here, by broadband femtosecond transient absorption spectroscopy with circularly polarized pump/probe lights, a systematic investigation on the many‐body interactions of band‐edge valley excitons in monolayer MoS2 is performed, where intravalley coherent coupling processes are directly observed at room temperature. Namely, when the intravalley mixing states in the superposition region of A‐exciton and B‐exciton states are excited, there is an intravalley coherent coupling between A‐exciton and B‐exciton in K valley (or A′‐exciton and B′‐exciton in K′ valley) with an oscillation frequency of ≈7–8 THz, rather than the intervalley excitonic coupling in general. The findings elucidate intravalley mixing and coherent oscillations in valley excitons of monolayer TMDs at room temperature, which can give insight into the many‐body coupling mechanisms of excited valley excitons in 2D TMDs. [ABSTRACT FROM AUTHOR]

Valleytronic device based on graphene is recently attracting growing interest because of its potential applications such as in quantum information storage and transmission. However, one of the major challenges in designing valleytronic devices is to realize the large valley polarization. Therefore, in the present work, we investigate the effect of the electrostatic barrier on the valley polarization in a graphene modulated by two ferromagnetic metal stripes and one strain, where the large valley polarization can be achieved due to the effect of the electrostatic barrier. With the increase in the height or the width of the electrostatic barrier, the degree of the valley polarization will greatly increase. These interesting finds are very helpful for understanding the valley-dependent transport mechanism of electrons in graphene and designing the valleytronic devices based on graphene under the modulation of the electrostatic barrier. [ABSTRACT FROM AUTHOR]

Benefiting from strong photon–exciton and phonon–exciton interactions in atomic thickness, transition metal dichalcogenides (TMDCs) are viewed as one promising platform for exploring elementary excitonic photoluminescence (PL) and intrinsic spin–valley properties at the monolayer limit. Despite well-studied Stokes downconversion (DC) PL, the anti-Stokes upconversion (UC) PL has been recently reported in TMDC monolayers, which mainly focus on UC mechanisms while detailed valley-related dynamical processes are unwittingly less concerned. Here, we carry out an in-depth investigation on both DC and UC emission features of monolayer WS2 at room temperature, where UC PL persists with energy gain up to 190 meV. The PL excitation and power-dependent experiments clearly distinguish the origins of DC PL and UC PL, which refer to saturated absorption and phonon-assisted transition from charged trions to neutral A-excitons. And contrast valley properties are observed in DC and UC scenarios with polarization-resolved PL and pump–probe measurements. According to the experimental facts, phenomenological dynamical DC and UC scenarios are modeled with intervalley depolarization taken into consideration, in which intermediates from spontaneous intervalley depolarization account for the observed emission and valley properties. This work can help understand the light–matter interactions and valley properties in monolayer TMDCs.

The key to manipulating valley degrees of freedom is to achieve valley polarization. Intrinsic valley polarization materials provide a new platform for the development of valleytronics. Herein, it is found that single‐layer (SL) GdX2 (X = Br, Cl) are ferrovalley materials. Their valley polarization values are 79 and 35 meV by first‐principles calculations. In addition, SL GdBr2 is taken as an example and its valley physical properties are studied. Its valley polarization can be effectively tuned by the external strains. When tensile strain 4% is applied, the valley polarization value of a SL of GdBr2 can reach 107 meV. Moreover, by reversing the spin orientation of SL GdBr2, the valley polarization of the SL GdBr2 can be reversed. This is very necessary for using the electron′s valley degree of freedom to encode and process information. In addition, the doping of electrons and holes has a significant effect on the valley polarization of SL GdBr2. The results provide a new family of ferrovalley materials GdX2 (X = Br, Cl). [ABSTRACT FROM AUTHOR]

Valley polarization is one basic concept in valleytronic device applications. However, the coexistence of this scenario in graphene under the joint modulation of a realistic magnetic field and a strain has not been reported. Therefore, in this paper, we use the transfer-matrix method to illustrate the effect of a strain on the valley-dependent transport properties of electrons in a graphene with a realistic magnetic field. We find that the valley polarization strongly depends on the strength and the width of the strained barrier rather than on its position. We also find that the 100% valley polarization can be continuously achieved in a broad Fermi energy region by controlling the strain. This point makes it easier to design the new kinds of the valleytronic devices based on graphene modulated by the realistic magnetic field and the strain. [ABSTRACT FROM AUTHOR]

Due to its inherent size and surface effect, the ferroelectric properties of two-dimensional ferroelectrics are very different from those of traditional bulk ferroelectrics. So far, two-dimensional ferroelectrics have been reported to have many properties, including bulk photoelectric effect, piezoelectric/pyroelectric effect, valley polarization and spin polarization. These properties either depend on ferroelectric polarization or couple with it for electrical control, which makes 2 D ferroelectrics suitable for multi-functional nano devices. At present, the research of two-dimensional ferroelectrics in theory, experiment and application is accumulating. This study briefly introduces these theories and methods. Then, the intrinsic and extrinsic origins of two-dimensional ferroelectrics are summarized respectively. Finally, the existing problems and development prospects of two-dimensional ferroelectrics are discussed. [ABSTRACT FROM AUTHOR]

Abstract We theoretically investigate the valley polarization in silicene with two parallel line defects due to Rashba spin-orbit coupling (RSOC). It is found that as long as RSOC exceeds the intrinsic spin-orbit coupling (SOC), the transmission coefficients of the two valleys oscillate with the same periodicity and intensity, which consists of wide transmission peaks and zero-transmission plateaus. However, in the presence of a perpendicular electric field, the oscillation periodicity of the first valley increases, whereas that of the second valley shortens, generating the corresponding wide peak-zero plateau regions, where perfect valley polarization can be achieved. Moreover, the valley polarizability can be changed from 1 to −1 by controlling the strength of the electric field. Our findings establish a different route for generating valley-polarized current by purely electrical means and open the door for interesting applications of semiconductor valleytronics.

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.

Proximate-induced magnetic interactions present a promising strategy for precise manipulation of valley degrees of freedom. Taking advantage of the splendid valleytronic platform of transition metal dichalcogenides, magnetic two-dimensional VSe 2 with different phases are introduced to intervene in the spin of electrons and modulate their valleytronic properties. When constructing the heterostructures, 1T-VSe 2 /WX 2 (X = S and Se) showcases significant improvement in the valley polarizations at room temperature, while 2H-VSe 2 /WX 2 exhibits superior performance at low temperatures and demonstrates heightened sensitivity to the external magnetic field. Simultaneously, considerable valley splitting with a large g eff factor up to -29.0 is observed in 2H-VSe 2 /WS 2 , while it is negligible in 1T-VSe 2 /WX 2 . First-principles calculations reveal a phase-dependent magnetic proximity mechanism on the valleytronic modulations, which is dominated by interfacial charge transfer in 1T-VSe 2 /WX 2 and the proximity exchange field in 2H-VSe 2 /WX 2 heterostructures. The effective control over valley degrees of freedom will bridge the valleytronic physics and devices, rendering enormous potential in the field of valley quantum applications.

Two–dimensional (2D) transition metal dichalcogenides endow individually addressable valleys in momentum space at the K and K’ points in the first Brillouin zone due to the breaking of inversion symmetry and the effect of spin–orbit coupling. However, the application of 2H–VS2 monolayer in valleytronics is limited due to the valence band maximum (VBM) located at the Γ point. Here, by involving the 2D ferroelectric (FE) CuInP2Se6 (CIPSe), the ferrovalley polarization, electronic structure, and magnetic properties of 2D 2H–VS2/CIPSe heterostructures with different stacking patterns and FE polarizations have been investigated by using first–principles calculations. It is found that, for the energetically favorable AB–stacking pattern, the valley polarization is preserved when the FE polarization of CIPSe is upwards (CIPSe↑) or downwards (CIPSe↓) with the splitting energies slightly larger or smaller compared with that of the pure 2H–VS2. It is intriguing that, for the FE CIPSe↑ case, the VBM is expected to pass through the Fermi energy level, which can be eventually achieved by applying biaxial strain and thus the valleytronic nature is turned off; however, for the CIPSe↓ situation, the heterostructure basically remains semiconducting even under biaxial strains. Therefore, with the influence of proper strains, the FE polar reversal of CIPSe can be used as a switchable on/off to regulate the valley polarization in VS2. These results not only demonstrate that 2H–VS2/CIPSe heterostructures are promising potential candidates in valleytronics, but also shed some light on developing practical applications of valleytronic technology.

Two-dimensional (2D) heterostructures based on graphene (Gr) and 2D magnetic materials have attracted much attention due to the unique properties, which have the potential applications in nanoelectronics, optoelectronics and spintronics. Here, the electronic structure and magnetic properties of 2D Gr/Janus 2H-VSe X (X = S, Te) (Gr/VSe X) heterostructures are investigated systematically by density functional theory. The Dirac cones of graphene in Gr/VSe X heterostructures open a band gap, where the valley splitting appears at K and K′ points by considering spin-orbit coupling effect. By comparing to Janus VSeTe monolayer, the valley splitting and Dzyaloshinskii-Moriya interaction (DMI) of Gr/VSeTe heterostructure are significantly enhanced. Meanwhile, the electronic structure and magnetic properties of Gr/VSeTe heterostructure can also be modulated by external electric field, interlayer distance and in-plane biaxial strain, where the in-plane biaxial strain effects on DMI is more pronounced. Moreover, the valley splitting of graphene (VSeTe) can be effectively enhanced by decreasing the interlayer distance of Gr/VSeTe heterostructure. These results show the potential applications of 2D magnetic heterostructures in nonoelectronics, spintronics and valleytronics. Image 1 • Dirac cones of graphene in graphene/Janus 2H-VSe X heterostructures open a band gap. • Valley splitting and Dzyaloshinskii-Moriya interaction in graphene/VSeTe are enhanced. • The in-plane biaxial strain can effectively tailor Dzyaloshinskii-Moriya interaction. • The valley splitting of graphene is effectively enhanced by decreasing interlayer distance. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) van der Waals transition metal dichalcogenides (TMDs) are highly attractive due to their novel phenomena and potential device applications, but most of the studies are focused on binary TMDs except a few reports on ternary TMDs. Here, we report thickness-dependent mono-to-multi-layer transition behaviors of mechanically-exfoliated ternary TMD Mo 0.55 W 0.45 S 2. Dependences of Raman modes and their peak intervals on thickness (d) are divided into three regions (1.5 ≤ d ≤ 9.2, 9.2 < d ≤ 34, and > 34 nm), mostly resulting from the increase of van der Waals force and long-range Coulomb force in interlayer interactions at larger d. Photoluminescence (PL) peak and its intensity/full width at half maximum show similar three-region behaviors, which are associated with direct-to-indirect-bandgap transition as d increases. In particular, critical changes in the Raman scattering and PL occur at d = 9.2 nm (8–13 layers), meaning a 2D-to-3D transition. The effect of the valley polarization is observed up to d = 8.1 nm (7–11 layers), resulting from random mixing-induced suppression of the inversion symmetry in the multilayer. These results suggest that the critical layer number for the 2D-to-3D dimensional crossover in ternary Mo 0.55 W 0.45 S 2 is considerably larger than in binary TMDs, thereby exhibiting the direct-bandgap nature in an extended range of layer number, more useful for their optoelectronic device applications. [Display omitted] • First thickness-dependent characterization of ternary transition metal dichalcogenide (TMDs). • Unique 2D-to-3D-transition-crossover behaviors of ternary TMDs. • Extended layer number of ternary TMDs for the preservation of the direct-bandgap nature. [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]

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]

Exploring valley-contrasting physics in scarce intrinsic high-temperature ferromagnetic semiconductors with feasible synthesis route is of great significance for ever-evolving moden information technology. By first-principles calculations, the TiInTe 3 monolayer is predicted to demonstrate excellent structural stability and easy-plane semiconducting ferromagnetism arising from indirect superexchange mechanism. Monte Carlo simulations based on the Heisenberg model reveal a magnetic phase transition at 583 K well above room temperature. Due to the inversion and time-reversal asymmetries, tunable valley polarization in the conduction band up to 110 meV can be achieved by varying the magnetization orientation, which is further validated by a first-order perturbation theory. More excitingly, electron doping for realizing the anomalous valley Hall effect gives rise to an out-of-plane preference for magnetization in the TiInTe 3 monolayer and thus generates spontaneous and larger valley polarizations. After doping, the ferromagnetism above room temperature remains robust in the experimentally attainable electron density regime. Our findings highlight that the TiInTe 3 monolayer is a promising ferrovalley material for developing high-performance spintronic and valleytronic nanodevices. [Display omitted] [ABSTRACT FROM AUTHOR]

Atomically thin, two-dimensional, transition-metal dichalcogenide (TMD) monolayers have recently emerged as a versatile platform for optoelectronics. Their appeal stems from a tunable direct bandgap in the visible and near-infrared regions, the ability to enable strong coupling to light, and the unique opportunity to address the valley degree of freedom over atomically thin layers. Additionally, monolayer TMDs can host defect-bound localized excitons that behave as single-photon emitters, opening exciting avenues for highly integrated 2D quantum photonic circuitry. By introducing plasmonic nanostructures and metasurfaces, one may effectively enhance light harvesting, direct valley-polarized emission, and route valley index. This review article focuses on these critical aspects to develop integrated photonic and valleytronic applications by exploiting exciton–plasmon coupling over a new hybrid material platform.