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Manipulation of spin-valleys plays a key role in spintronics and valleytronics. The magnetoelectric coupling effect in the two-dimensional (2D) ferroelectricity/ferromagnetism heterostructures offers a feasible way to achieve the modulation of magnetic properties and valley degrees of freedom. In this work, the electronic structure, magnetism and valley polarization of YMnO 3 /2H-VS 2 multiferroic heterostructures are investigated in detail by first-principles calculations. The results show that the ferromagnetism, valley polarization and conductivity of 2H-VS 2 layer can be largely altered by the YMnO 3 substrate with different terminations. The valley polarization can achieve a maximum modulation of 112.5 meV for the YMnO 3 /2H-VS 2 heterostructure with MnO-termination when the ferroelectric polarization is reversed. On the other hand, the direction of magnetic moment of V atom in the 2H-VS 2 layer can be effectively adjusted in the O2- and Y-terminated configurations by altering the ferroelectric polarization, which can be attributed to the magnetic interaction between the ferromagnetic 2H-VS 2 and ferroelectric YMnO 3. Compared with the case of isolated 2H-VS 2 monolayer, 2H-VS 2 exhibits p-type doping character in O- and Y-terminated configurations. The valley and direct bandgap semiconductor properties of 2H-VS 2 monolayer are preserved for all configurations. Moreover, the O–S distance can be considered as a main factor for regulating the valley polarization by affecting orbital hybridization. These results exhibit new coupling effects between ferromagnetism, ferroelectronics and ferrovalley by the electrical control of spin-valleys behaviors, which provides a new platform for developing nonvolatile spintronic and valleytronic devices. • Magnetism and valley polarization of YMnO 3 /2H-VS 2 multiferroic heterostructures are investigated. • Valley polarization of 2H-VS 2 is largely altered by YMnO 3 substrate with different terminations. • Valley polarization achieves a maximum modulation of 112.5 meV with MnO-termination. • The O–S distance can be considered as a main factor for regulating the valley polarization. [ABSTRACT FROM AUTHOR]

Hybrid layered materials assembled from atomically thin crystals and small molecules bring great promises in pushing the current information and quantum technologies beyond the frontiers. We demonstrate here a class of layered valley-spin hybrid (VSH) materials composed of a monolayer two-dimensional (2D) semiconductor and double-decker single molecule magnets (SMMs). We have materialized a VSH prototype by thermal evaporation of terbium bis-phthalocyanine onto a MoS 2 monolayer and revealed its composition and stability by both microscopic and spectroscopic probes. The interaction of the VSH components gives rise to the intersystem crossing of the photogenerated carriers and moderate p-doping of the MoS 2 monolayer, as corroborated by the density functional theory calculations. We further explored the valley contrast by helicity-resolved photoluminescence (PL) microspectroscopy carried out down to liquid helium temperatures and in the presence of the external magnetic field. The most striking feature of the VSH is the enhanced A exciton-related valley emission observed at the out-of-resonance condition at room temperature, which we elucidated by the proposed nonradiative energy drain transfer mechanism. Our study thus demonstrates the experimental feasibility and great promises of the ultrathin VSH materials with chiral light emission, operable by physical fields for emerging opto-spintronic, valleytronic, and quantum information concepts.

Valleytronics in transition metal dichalcogenides has been intensively investigated for potential applications in next-generation information storage, data processing, and signal transmission devices. Here a ferroelectric gating approach is engaged in achieving nonvolatile electrical tuning of the valley-excitonic properties of monolayer and bilayer WS 2 . The gating effects include carrier doping and ferroelectric coupling, which are further distinguished by comparing two geometries where the gate electrodes are in direct contact with or insulated from the WS 2 crystal. The results show that the carrier doping from gate electrodes acts on WS 2 through carrier screening, which only moderately alters the valley polarization. In contrast, the ferroelectric gating promotes electron-phonon interaction, introduces a strong surface polarization field, and controls the interfacial charge trapping/detrapping, causing a Stark shift in exciton energy and strongly enhancing room-temperature valley polarization. In bilayer WS 2 , the intralayer-interlayer exciton transition is further induced, contributing to even higher valley polarization. The ferroelectric coupling effect can still be maintained after the removal of gate voltage, showing its nonvolatile nature. The role of ferroelectricity is further verified by the anomalous temperature dependence in valley polarization. This work has revealed effective electrical control over valley excitons in semiconductors through interaction with ferroelectric materials. The reported high room-temperature valley polarization in WS 2 will boost the development of valleytronics devices.

Synthesizing a material with the desired polymorphic phase in a chemical vapor deposition (CVD) process requires a delicate balance among various thermodynamic variables. Here, we present a methodology to synthesize rhombohedral (3 R )-phase MoS 2 in a well-defined sword-like geometry having lengths up to 120 μm, uniform width of 2-3 μm and thickness of 3-7 nm by controlling the carrier gas flow dynamics from continuous mode to pulsed mode during the CVD growth process. Characteristic signatures such as high degree of circular dichroism (∼58% at 100 K), distinct evolution of low-frequency Raman peaks and increasing intensity of second harmonic signals with increasing number of layers conclusively establish the 3 R -phase of the material. A high value (∼844 pm/V) of second-order susceptibility for few-layer-thick MoS 2 swords signifies the potential of MoS 2 to serve as an atomically thin nonlinear medium. A field effect mobility of 40 cm 2 /V-s and I on / I off ratio of ∼10 6 further confirm the electronic-grade standard of this 3 R -phase MoS 2 . These findings are significant for the development of emerging quantum electronic devices utilizing valley-based physics and nonlinear optical phenomena in layered materials.

We numerically investigate the electronic transport properties of graphene nanoribbons and carbon nanotubes with inter-valley coupling, e.g., in $\sqrt{3}N\times \sqrt{3}N$ and $3N\times 3N$ superlattices. By taking the $\sqrt{3}\times \sqrt{3}$ graphene superlattice as an example, we show that tailoring the bulk graphene superlattice results in rich structural configurations of nanoribbons and nanotubes. After studying the electronic characteristics of the corresponding armchair and zigzag nanoribbon geometries, we find that the linear bands of carbon nanotubes can lead to the Klein tunnelling-like phenomenon, i.e., electrons propagate along tubes without backscattering even in the presence of a barrier. Due to the coupling between K and ${K}^{\prime }$ valleys of pristine graphene by $\sqrt{3}\times \sqrt{3}$ supercells, we propose a valley-field-effect transistor based on the armchair carbon nanotube, where the valley polarization of the current can be tuned by applying a gate voltage or varying the length of the armchair carbon nanotubes.

We investigate the electronic transport in a silicene-based ferromagnetic metal/ferromagnetic insulator/ferromagnetic metal tunnel junction. The results show that the valley and spin transports are strongly dependent on local application of a vertical electric field and effective magnetization configurations of the ferromagnetic layers. In particular, it is found that the fully valley and spin polarized currents can be realized by tuning the external electric field. Furthermore, we also demonstrate that the tunneling magnetoresistance ratio in such a full magnetic junction of silicene is very sensitive to the electric field modulation. [ABSTRACT FROM AUTHOR]

In topological materials, interacting with short and strong optical pulses, electrons can accumulate a topological phase during the pulse. Such phase can compensate the dynamic phase resulting in topological resonance, which is visible as a large inter-band transfer of electron population. We study theoretically the topological resonance in materials of the gapped multilayer graphene type. We show that the resonance can be observed only in the systems with finite bandgap. For graphene monolayer the topological resonance can occur only in the field of an elliptically polarized pulse, while for graphene systems with many layers the topological resonance can be also realized in a linearly polarized pulse., (© 2022 IOP Publishing Ltd.)

Abstract: We develop a tight-binding theory to study the electronic transport through an extended line defect in monolayer graphene. After establishing an analytical expression of the transmission probability, we clarify the following issues concerning the valley polarization in the electronic transport process. Firstly, we find that the valley polarization is robust in the total linear dispersion region. More interestingly, we find that the lattice deformation around the line defect play an important role in tuning the incident angle for complete transmission. Finally, we indicate that next nearest neighbor interaction only causes a small suppression to the valley polarization. [Copyright &y& Elsevier]

Enhancing and expanding the manipulated range of room-temperature valley polarization at off-resonance wavelength is extremely crucial to developing various functional valleytronic devices. Although these have been realized through the double-resonance strategy or twist-angle engineering, the demand for electrical control over the concepts remains elusive. Here, we fabricate a gate-tunable double-resonance chiral microstructure using a molybdenum disulfides (MoS 2 ) monolayer. On the basis of the varied interface charge density, we demonstrate the huge photoluminescence (PL) tuning ability of this configuration. Furthermore, benefiting predominately from the screening of long-range e-h exchange interactions and the chiral Purcell effect, the electrical switching of the room-temperature valley polarization at off-resonance wavelength is also realized. Our work enriches the functions of TMDs-based optoelectronic devices and may create important applications in future valley-polarized encode and information processing devices.

Graphene moiré superlattice formed by rotating two graphene sheets can host strongly correlated and topological states when flat bands form at so-called magic angles. Here, we report that, for a twisting angle far away from the magic angle, the heterostrain induced during stacking heterostructures can also create flat bands. Combining a direct visualization of strain effect in twisted bilayer graphene moiré superlattices and transport measurements, features of correlated states appear at "non-magic" angles in twisted bilayer graphene under the heterostrain. Observing correlated states in these "non-standard" conditions can enrich the understanding of the possible origins of the correlated states and widen the freedom in tuning the moiré heterostructures and the scope of exploring the correlated physics in moiré superlattices.

Monolayer transition metal dichalcogenides have attracted great attention for potential applications in valleytronics. However, the valley polarization degree is usually not high because of the intervalley scattering. Here, a largely enhanced valley polarization up to 80% in monolayer WS 2 under nonresonant excitation at 4.2 K is demonstrated using WS 2 /LaMnO 3 thin film heterostructure, which is much higher than that for monolayer WS 2 on SiO 2 /Si substrate with a valley polarization of 15%. Furthermore, the greatly enhanced valley polarization can be maintained to a high temperature of about 160 K with a valley polarization of 53%. The temperature dependence of valley polarization is strongly correlated with the thermomagnetic curve of LaMnO 3 , indicating an exciton-magnon coupling between WS 2 and LaMnO 3 . A simple model is introduced to illustrate the underlying mechanisms. The coupling of WS 2 and LaMnO 3 is further confirmed with an observation of two interlayer excitons with opposite valley polarizations in the heterostructure, resulting from the spin-orbit coupling induced splitting of the conduction bands in monolayer transition metal dichalcogenides. The results provide a pathway to control the valleytronic properties of transition metal dichalcogenides by means of ferromagnetic van der Waals engineering, paving a way to practical valleytronic applications., (© 2022 Wiley-VCH GmbH.)

Moiré patterns of two-dimensional (2D) van der Waals (vdW) heterostructures can induce novel electronic structures and physical properties. However, the effects of moiré pattern on magnetic properties of 2D materials are not clear yet. Herein, the electronic structures and magnetic properties of 2D graphene/Janus 2H–VSeTe (Gr/VSeTe) heterostructures with moiré patterns are investigated by density functional theory. It is found that twist angles (θ) can induce a band gap at Dirac cone of graphene. With further considering spin-orbit coupling and V magnetic moments of VSeTe monolayer along + z axis, valley polarization of graphene at K and K′ points appears and is found to be sensitive to the twist angle. Additionally, flat band occurs at θ = 5.2° and 10.9° due to the quasi-periodic potential induced by moiré patterns. A high spin polarization also appears in Gr/VSeTe heterostructures with certain twist angles of θ = 0°, 7.6°, 10.9° and 13.9°. Due to the changed occupation of interfacial Te- p orbitals, the magnetic anisotropy and Dzyaloshinskii-Moriya interaction of Gr/VSeTe heterostructures can be changed by twist angles. Therefore, twist angle provides a new degree of freedom for inducing the novel electronic structures and magnetic properties in the 2D magnetic vdW heterostructures. [Display omitted] • Moiré pattern has a significant impact on the physical properties of Gr/VSeTe heterostructures. • Valley splitting is found to be sensitive to the twist angles in Gr/VSeTe heterostructures. • Flat band can be induced by twist angles at θ = 5.2° and 10.9° in Gr/VSeTe heterostructures. • Magnetic anisotropy and Dzyaloshinskii-Moriya interaction can also be tuned by twist angles. [ABSTRACT FROM AUTHOR]

[Display omitted] • The valley polarization (VP) of the H-Tl 2 O/CrI 3 heterostructure is observed in both the conduction and the valence band. • A large valley spin splitting (VSS) of 0.57 (0.58) eV at the K (K') points are shown in the conduction band. • The VP and the VSS can be manipulated by adjusting the interlayer spacing and the in-plane biaxial strains. We report the two-dimensional H-Tl 2 O/CrI 3 van der Waals (vdW) heterostructure as a promising valleytronic material and investigate its valley polarization and valley spin splitting manipulation using first-principles calculations. The H-Tl2O/CrI 3 heterostructure displays valley polarization both in the conduction and valence band at two valleys. Particularly, valley polarization can attain 14.95 meV at the conduction band. In addition, a large valley spin splitting of 0.57 (0.58) eV at the K (K') point is shown in the conduction band. With the manipulation of the interlayer spacing and in-plane biaxial strain, the valley polarization and valley spin splitting of the H-Tl 2 O/CrI 3 heterostructure follow the short-range effects of interfacial orbital hybridization, thus a nearly linear relationship is shown. Our work provides not only application prospects of the H-Tl 2 O/CrI 3 heterostructure nanodevices but also theoretical effective support for research and development of valleytronics. [ABSTRACT FROM AUTHOR]

In this work, we presented a workable scheme to induce valley polarized current in a graphene junction with circularly polarized light and carrier mass. A non-linear effect of the circularly polarized light opens a gap in the Dirac cone. The mass term corresponding to a symmetry-breaking perturbation admits a band gap. The result shows that a valley-dependent energy gap can be opened in the presence of both circularly polarized light and carrier mass. We found that pure valley-polarized current is achieved in the valley-dependent energy gap. However, only one of the two modulations is present, no valley polarization can be generated. So the combination of the two modulations provides a new way to producing the pure valley current in the graphene system. • The combined effect of circularly polarized light and carrier mass can induce a valley-related energy gap in the graphene. • Pure valley-polarized current is achieved in the gap and can be modulated by circularly polarized light and carrier mass. • Perfect valley switch effect in graphene junction can be achieved by adjusting circularly polarized light and carrier mass. [ABSTRACT FROM AUTHOR]

Inspired by the new progress in the research field of two-dimensional valleytronics materials, we propose a new class of transition metal halides, i.e. H-ZrX 2 (X = Cl, Br, I), and investigated their valleytronics properties under the first-principles calculations. It harbors the spin-valley coupling at K and K' points in the top of valence band, in which the valley spin splitting of ZrI 2 can reach up to 115 meV. By carrying out the strain engineering, the valley spin splitting and Berry curvature can be effectively tuned. The long-sought valley polarization reaches up to 108 meV by doping Cr atom, which corresponds to the large Zeeman magnetic field of 778 T. Furthermore, the valley polarization in ZrX 2 can be lineally adjusted or flipped by manipulating the magnetization orientation of the doped magnetic atoms. All the results demonstrate the well-founded application prospects of single-layer ZrX 2 , which can be considered as great candidate for the development of valleytronics and spintronics., (© 2021 IOP Publishing Ltd.)

Valley-dependent excitation and emission in transition metal dichalcogenides (TMDCs) have recently emerged as a new avenue for optical data manipulation, quantum optical technologies, and chiral photonics. The valley-polarized electronic states can be optically addressed through photonic spin-orbit interaction of excitonic emission, typically with plasmonic nanostructures, but their performance is limited by the low quantum yield of neutral excitons in TMDC multilayers and the large Ohmic loss of plasmonic systems. Here, we demonstrate a valleytronic system based on the trion emission in high-quantum-yield WS 2 monolayers chirally coupled to a low-loss microfiber. The integrated system uses the spin properties of the waveguided modes to achieve long-range directional routing of valley excitations and also provides an approach to selectively address valley-dependent emission from different spatial locations around the microfiber. This valleytronic interface can be integrated with fiber communication devices, allowing for merging valley polarization and chiral photonics as an alternative mechanism for optical information transport and manipulation in classical and quantum regimes.

Effects of strain, interlayer coupling and electric field on the valley polarization, magnetocrystalline anisotropy and dipole moment of layered Janus 2H–VSSe are systematically investigated by first-principles calculations. A unique V 3 d1 electronic configuration and strong electron coupling interaction guarantee the stable in-plane ferromagnetic ground states with Tc of 512 K. By considering spin-orbit coupling (SOC) and exchange interaction based on the effective Hamiltonian k•p model, a large valley splitting of 94.8 meV is observed, which is mainly contributed by the spontaneous magnetic exchange field and not SOC effect. Under various bi-axis strains, Janus 2H–VSSe monolayer still maintains intrinsic in-plane ferromagnetism, but exhibits tunable valley polarization and magnetocrystalline anisotropy, which are associated with the in-plane d x 2 -y 2/d xy orbitals of V atoms. Importantly, stacking patterns can influence the spatial inversion symmetry and further alter the valley polarization for the layered Janus 2H–VSSe. Applied external electric-field can effectively control intrinsic built-in electric-field by charge redistribution in the vertical direction, which can remarkably change the dipole moments and slightly adjust the valley polarization. The layered Janus 2H–VSSe possesses intrinsic ferromagnetism, robust valley-contrasting physics and outfield-dependent dipole moment, making them competitive materials for practical applications in spin-valleytronics, optoelectronics and piezoelectric sensor. • Electric field and strain effects on layered Janus 2H–VSSe are studied. • 2H–VSSe monolayer shows in-plane ferromagnetic ground states with Tc of 512 K. • Tunable valley splitting, magnetic anisotropy and dipole moment are observed. [ABSTRACT FROM AUTHOR]

We investigated the spin- and valley-dependent energy band and transport properties in ferromagnetic silicene superlattice with an off-resonant circularly polarized light. It is obviously found that the spin- and valley-dependent energy gap can be well tuned via the off-resonant circularly polarized light. Fully spin and valley-polarized can be realized in certain ranges of light intensity. What is particularly interesting is that by slightly adjusting the energy of the light field, the valley polarization can be suddenly reversed from 100% to -100%. Thus, our results are expected to be informative for switching devices related to valleytronics. • Spin and valley energy gaps can be tuned via the off-resonant polarized light. • Energy range of full spin polarization and full valley polarization is enhanced. • Perfect valley switch effect is achieved by only modeling circularly polarized light. [ABSTRACT FROM AUTHOR]

Transition-metal dichalcogenides with intrinsic spin-valley degree of freedom have enabled great potentials for valleytronic and optoelectronic applications. However, the degree of valley polarization is usually low under nonresonant excitation at room temperature due to the phonon-assisted intervalley scattering. Here, achiral and chiral Au arrays are designed to enhance the optical response and valley polarization in monolayer and bilayer WS 2 . A considerable band edge emission with 7 times increment is realized under the resonant coupling with Au dimer-prism arrays. Valley polarization enhancement is quantitatively predicted by the inherent mechanisms from elevated electromagnetic field intensity and radiation efficiency and further realized in polarized photoluminescence. A tunable valley polarization up to 30.0% is achieved in bilayer WS 2 under a nonresonant excitation at room temperature. All of these results provide a promising route toward the development of room-temperature valley-dependent optoelectronic devices.

Magnetic proximity effect can be used to tailor the magnetic ground state and valley polarization in the monolayer of transition metal dichalcogenides. Thus, we explore the effect of biaxial tensile and compressive strain on valley polarization in the WSe 2 /CrSnSe 3 heterostructures with different stacking orders systematically. The indirect band gaps in the two most stable stackings; hollow (0.27 eV) and top (0.33 eV) were further enhanced to 0.35 eV under tensile strain while suppressed to almost 0.13 eV under compressive strain. The heterostructures had a FM ground state with a total magnetic moment per unit cell of 6 μ B in pristine as well as strained structures. In hollow stacking and compressively strained structures, we obtained a perpendicular magnetocrystalline anisotropy, while the top stacking and tensile strain structures had small in-plane anisotropy. An enhancement was found in Curie temperature from 73 K in pristine to 128 K in a 6% tensile strained structure. The valley splittings found in pristine hollow (4 meV) and top (9 meV) stacked heterostructures were further enhanced to 29 meV and 22 meV at 5% compressive strain respectively. This enhancement was attributed to the increased spatial dependence of the charge density along K + and K - directions of the Brillouin zone, which give rise to the different local dipolar fields at these valleys. Our results suggest that strain could be an effective way to control or tune the valley splitting in WSe 2 /CrSnSe 3 heterostructures., (© 2021 IOP Publishing Ltd.)

• We study spin- and valley-dependent transport properties in a MoS 2 superlattice. • The conductance without and with spin-flip can be controlled by a gate voltage. • The amplitude of the spin polarization is very sensitive to potential height. • In MoS 2 superlattice the perfect valley and spin polarized currents can be observed. • Direction of the spin and valley polarization can be easily switched. Using the transfer matrix method, the valley- and spin-dependent transport properties of a monolayer MoS 2 superlattice in the presence of the extrinsic Rashba spin–orbit interaction (ERSOI) are theoretically investigated. The presence of the extrinsic Rashba and intrinsic spin-orbit coupling in this superlattice enhanced the valley and spin polarizations. By adjusting the superlattice parameters or the ERSOI strength, the perfect valley and spin-polarized currents can be observed. The valley- and spin-dependent conductance can be manipulated by the ERSOI strength and the barrier height. Due to the presence of the direct band gap and ERSOI, the conductance has a gap with respect to the barrier height. Furthermore, it was found that the efficiency of the spin-inversion can be modulated via the ERSOI strength, the number of superlattice barriers, and the gate voltage. [ABSTRACT FROM AUTHOR]

• Co-doping generates big valley polarization in WTe 2. • Strain force enhances valley polarization at conduction/valence band in Co-doped WTe 2. • Novel spin valley Hall device with high efficiency and low energy consumption is designed. Valleytronics has gained significantly numerous research interests, relative to consideration of the valley index in particular materials. The effects of the strain force on the valley polarization properties in transition metal doped WTe 2 are thoroughly investigated in this paper through first principles calculations. The results demonstrate that significant valley polarization can be exclusively observed in Co-doped WTe 2 via consideration of the spin-orbit coupling. Additionally, the valley polarization can be enhanced by the strain force. Moreover, a giant valley polarization can be observed both in the valence band and the conduction band in the Co-doped WTe 2 under appropriate tension strain. Electrons and holes with opposite spins would occupy these valley-polarized energy levels, thus, a novel spin valley Hall device with high efficiency and low energy consumption can be designed based on these special valley polarization properties. In general, this work would open up a new insight into the detection and application of valleytronics. [ABSTRACT FROM AUTHOR]

We have theoretically investigated the spin- and valley-dependent transport properties in a silicene superlattice under strain, with extrinsic Rashba spin–orbit coupling. It is demonstrated that due to the extrinsic Rashba spin–orbit coupling, spin-inversion can be achieved. It is shown that the valley and spin polarizations increase with increasing the number of superlattice barriers, and the magnitude and sign of valley and spin polarizations can be tuned by strain and extrinsic Rashba spin–orbit coupling. It is also found that the spin polarization through this superlattice reaches a maximum value at 2% strain. In particular, the fully valley polarized current can be achieved by adjusting the strain strength. Image 10 • We study spin- and valley-dependent transport properties in a silicene superlattice. • Fully valley polarized current can be achieved by adjusting the strain strength. • The spin-inversion can be efficiently controlled mechanically. • The valley and spin polarizations increase with increasing the number of barriers. • The spin polarization reaches the highest value at ε = − 2 % or ε = 2 % ac. [ABSTRACT FROM AUTHOR]