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Valley, the energy extrema in the electronic band structure, is regarded as a new degree of freedom of electrons, in addition to charge and spin. Valleytronics focuses on the valley degree of freedom and has attracted considerable attention due to its potential applications in information storage. In this study, to use the binary valley polarization in a controllable and nonvolatile manner is still the core issue. We propose that the 2D CuInP2S6/MnPS3 system could be used to electrically tune the valley polarization. In these ferroelectric/antiferromagnetic heterostructures, the ferroelectric CuInP2S6 layer, which is originally in the paravalley state, turns into the ferrovalley state due to the magnetic proximity effect of the antiferromagnetic MnPS3 substrate. Interestingly, when the ferroelectric state of the CuInP2S6 layer is changed, the valley polarization of the system is significantly reversed, providing the possibilities to manipulate the valley degree of freedom via electrical means. On account of the above-mentioned findings, all electrical reading and writing memory devices based on such heterostructures are proposed to utilize the valley degree of freedom. Our findings open an appealing route toward the design of functional 2D materials for valleytronics.

Functional oxides on silicon have been the subject of in-depth research for more than 20 years. Much of this research has been focused on the quality of the integration of materials due to their intrinsic thermodynamic incompatibility, which has hindered the flourishing of the field of research. Nevertheless, growth of epitaxial transition metal oxides on silicon with a sharp interface has been achieved by elaborated kinetically controlled sequential deposition while the crystalline quality of different functional oxides has been considerably improved. In this Research Update, we focus on three applications in which epitaxial ferroelectric oxides on silicon are at the forefront, and in each of these applications, other aspects of the integration of materials play an important role. These are the fields of piezoelectric microelectromechanical system devices, electro-optical components, and catalysis. The overview is supported by a brief analysis of the synthesis processes that enable epitaxial growth of oxides on silicon. This Research Update concludes with a theoretical description of the interfaces and the possibility of manipulating their electronic structure to achieve the desired coupling between (ferroelectric) oxides and semiconductors, which opens up a remarkable perspective for many advanced applications. © 2021 Author(s).

Recently, the concept of ferrovalley materials has been proposed. These systems, in which ferromagnetism or ferroelectricity triggers spontaneous valley polarization, can be applied to nonvolatile valleytronic devices. In addition, a fabricated Janus transition metal dichalcogenide has been attracting immense attention due to its intrinsic valley properties and piezoelectricity. Inspired by these works, we predict a ferrovalley material, Janus VClBr monolayer, by performing first-principles calculations. The Janus VClBr monolayer possesses a large valley polarization due to the spin-orbit coupling coexisting with the intrinsic exchange interaction of vanadium $3d$ electrons, which can be illustrated by a $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ model. Combining the valley band structures with group theory analysis, we also find the chirality-dependent optical band gap that could be used to detect the valley state of the system. An anomalous valley Hall effect is confirmed by calculating the Berry curvatures in such a material system. Meanwhile, mirror symmetry breaking along the perpendicular direction not only results in an emergent internal electric dipole and a giant in-plane piezoelectricity, but also generates an out-of-plane second-harmonic-generation coefficient. Our findings of these distinctive properties, particularly the coupling of piezoelectricity and valley, could pave the way to design the Janus ferrovalley material as multifunctional valleytronics and optoelectronics devices.

Valley, the energy extrema in the electronic band structure at momentum space, is regarded as a new degree of freedom of electrons, in addition to charge and spin. The studies focused on valley degree of freedom now form an emerging field of condensed-matter physics, i.e., valleytronics, whose development is exactly following that of spintronics, which focuses on the spin degree of freedom. Here, in analogy to half-metals in spintronics where one spin channel is conducting, whereas the other is insulating, we propose the concept of half-valley metal, in which conduction electrons are intrinsically 100% valley polarized, as well as 100% spin polarized even when spin–orbit interactions are considered. Combining first-principle calculations with a two-band k·p model, the physical mechanism to form the half-valley metal is illuminated. Taking the ferrovalley H-FeCl2 monolayer with strong exchange interaction as an example, we find that the strong electron correlation effect can induce the ferrovalley to half-valley-metal transition. Due to the valley-dependent optical selection rules, such a system could be transparent to, e.g., left-circularly polarized light, yet the right-circularly polarized light will be reflected, which can in turn be used as a crucial method to detect the half-valley-metal state. Interestingly, with the increase of the correlation effect, the system becomes insulating again with all valleys following the same optical selection rule. We confirm that in this specific case, the valence bands, which consist of single spin, possess nonzero Chern number and consequently an intrinsic quantum anomalous valley Hall effect emerges. Our findings open an appealing route toward functional 2D materials design of valleytronics.

Based on the electronic band structure obtained from first-principles Hubbard corrected $U$ density functional theory $(\mathrm{DFT}+U)$ calculations, the optical spectra of yttrium and neodymium nickelates are computed. We show that the results are in fair agreement with available experimental data. We clarify the electronic transitions at the origin of the first two peaks, highlighting the important role of transitions from ${t}_{2g}$ states neglected in previous models. We discuss the evolution of the optical spectra from small to large rare-earth cations and relate the changes to the electronic band structure.

In analogy to all-electric spintronics, all-electric valleytronics, i.e., valley manipulation via electric means, becomes an exciting new frontier as it may bring revolutions in the field of data storage with ultra-high speed and ultra-low power consumption. The existence of the anomalous valley Hall effect in ferrovalley materials demonstrates the possibility of electrical detection for valley polarization. However, in previously proposed valley-polarized monolayers, the anomalous valley Hall effect is controlled by external magnetic fields. Here, through elaborate structural design, we propose the antiferrovally bilayer as an ideal candidate for realizing all-electric valleytronic devices. Using the minimal k·p model, we show that the energy degeneracy between valley indexes in such system can be lifted by electric approaches. Subsequently, the anomalous valley Hall effect strongly depends on the electric field as well. Taking the bilayer VSe2 as an example, all-electric tuning and detecting of anomalous valley Hall effect is confirmed by density-functional theory calculations, indicating that the valley information in such antiferrovalley bilayer can be reversed by an electric field perpendicular to the plane of the system and easily probed through the sign of the Hall voltage. First-principles calculations reveal that the anomalous valley Hall effect can be detected and tuned through an external electric field in bilayer VSe2. In transition-metal dichalcogenides, the local maxima or minima of the energy bands (so called valleys) are degenerate; in analogy to spintronics, manipulation of the electronic valley degree of freedom is promising for valleytronics applications. Monolayer 2H-VSe2 has already been put forth as a ferrovalley material, capable to exhibit spontaneous valley polarization and control its polarity under external magnetic fields. Now, Wen-Yi Tong and Chun-Gang Duan report that the valley Hall effect can be present in VSe2 bilayers without external stimuli. A perpendicular electric field enhances the accumulation of the majority carriers in the two layers in opposite directions, induces the anomalous valley Hall effect, and controls the sign of the Hall voltage. Such a way to access and control the valley information, is significant for valleytronics applications.

We report the lattice dynamics of 0.8Pb(Zr0.52Ti0.48)O3−(0.2−x)Pb(Zn1/3Nb2/3)O3−xPb(Al1/2Nb1/2)O3 (0.8PZT−(0.2−x)PZN−xPAN, 0.02 ≤ x ≤ 0.08 ) ceramics around morphotropic phase boundary (MPB) by infrared and Raman spectra. The dielectric functions in the wavenumbers range between 50 and 1000 cm−1 were extracted from the factorized oscillator model. The addition of PAN to PZT-PZN system introduces Al3+ ions to the B-site and all of these Raman-active modes in the measured range are related to B-site atoms vibration. The effect of PAN addition leads to infrared and Raman modes shifting to higher wavenumbers, because the atomic weight of Al is smaller than that of Zn. Therefore, the substitution of B-site atom in PZT-PZN system is the dominant reason to influence the frequency shift of infrared and Raman modes.

Spin capacitors, which not only store charges but also spins, have been recently proposed based on the surface/interface magnetoelectric effect. The realization of spin capacitors is, however, not straightforward due to the small change of magnetization under normal electric fields in an ordinary capacitor structure. Here we demonstrate electric-field control of the magnetism in Ni/Cu coaxial cylinders in an electric double layer capacitor (EDLC) configuration. The huge electric field effect generated by EDLCs results in a relative magnetization change as large as 18% after charging at a voltage of 2.0 V. Furthermore, the total magnetization change of the device can be enhanced by increasing the number of Ni/Cu wires and surface/bulk ratio. Our first-principles calculations, together with the results from X-ray photoelectron spectroscopy, X-ray diffraction and transmission electron microscopy, confirm that the magnetization change in this coaxial cylinder structure is due to the surface magnetoelectric effect, indicating that the proposed electric double layer capacitors with Ni/Cu coaxial cylinders have potential applications in spin capacitors at room temperature.

Engineering the electronic band structure of material systems enables the unprecedented exploration of new physical properties that are absent in natural or as-synthetic materials. Half metallicity, an intriguing physical property arising from the metallic nature of electrons with singular spin polarization and insulating for oppositely polarized electrons, holds a great potential for a 100% spin-polarized current for high-efficiency spintronics. Conventionally synthesized thin films hardly sustain half metallicity inherited from their 3D counterparts. A fundamental challenge, in systems of reduced dimensions, is the almost inevitable spin-mixed edge or surface states in proximity to the Fermi level. Here, we predict electric field-induced half metallicity in bilayer A-type antiferromagnetic van der Waals crystals (i.e., intralayer ferromagnetism and interlayer antiferromagnetism), by employing density functional theory calculations on vanadium diselenide. Electric fields lift energy levels of the constituent layers in opposite directions, leading to the gradual closure of the gap of singular spin-polarized states and the opening of the gap of the others. We show that a vertical electrical field is a generic and effective way to achieve half metallicity in A-type antiferromagnetic bilayers and realize the spin field effect transistor. The electric field-induced half metallicity represents an appealing route to realize 2D half metals and opens opportunities for nanoscale highly efficient antiferromagnetic spintronics for information processing and storage.

Multiferroic and magnetoelectric materials are considered as major candidates for next-generation information storage technologies due to their simultaneous presence and interplay of two or more ferroic orders. In this paper, we briefly review our theoretical progress relating to the all-electric spintronics, i.e., spin manipulation via an electric means rather than a magnetic field. Special focus is given to interface/surface magnetoelectric effect, electric field control of magnetocrystalline anisotropy, Rashba spin–orbit coupling, spin transport, and other generalized all-electric modulation of magnetism. Our recently developed method, i.e., the orbital selective external potential method, is also expounded. This method might be a powerful tool in finding the mechanisms responsible for the intriguing phenomena occurred in multiferroics or magnetoelectric materials.

Transition-metal dichalcogenide (TMD) monolayers $MXY\phantom{\rule{0.16em}{0ex}}(M=\mathrm{Mo},\phantom{\rule{0.16em}{0ex}}\mathrm{W};X\phantom{\rule{0.16em}{0ex}}\ensuremath{\ne}\phantom{\rule{0.16em}{0ex}}Y=\mathrm{S},\phantom{\rule{0.16em}{0ex}}\mathrm{Se},\phantom{\rule{0.16em}{0ex}}\mathrm{Te})$ are two-dimensional polar semiconductors. Setting the WSeTe monolayer as an example and using density functional theory calculations, we investigate the manipulation of Rashba spin-orbit coupling (SOC) in the MXY monolayer. It is found that the intrinsic out-of-plane electric field due to the mirror symmetry breaking induces the large Rashba spin splitting around the $\mathrm{\ensuremath{\Gamma}}$ point, which, however, can be easily tuned by applying the in-plane biaxial strain. Through a relatively small strain (from $\ensuremath{-}2%$ to 2%), a large tunability (from around $\ensuremath{-}50%$ to 50%) of Rashba SOC can be obtained due to the modified orbital overlap, which can in turn modulate the intrinsic electric field. The orbital selective external potential method further confirms the significance of the orbital overlap between $\mathrm{W}\text{\ensuremath{-}}{d}_{{z}^{2}}$ and $\mathrm{Se}\text{\ensuremath{-}}{p}_{z}$ in Rashba SOC. In addition, we also explore the influence of the external electric field on Rashba SOC in the WSeTe monolayer, which is less effective than strain. By calculating the electric-field-induced Rashba SOC in all six $M{X}_{2}$ monolayers, the rule of the electric-field influence on Rashba SOC in TMD monolayers is demonstrated. The large Rashba spin splitting, together with the valley spin splitting in MXY monolayers, may make a special contribution to semiconductor spintronics and valleytronics.

Valleytronics rooted in the valley degree of freedom is of both theoretical and technological importance as it offers additional opportunities for information storage and electronic, magnetic and optical switches. In analogy to ferroelectric materials with spontaneous charge polarization in electronics, as well as ferromagnetic materials with spontaneous spin polarization in spintronics, here we introduce a new member of ferroic-family, i.e. a ferrovalley material with spontaneous valley polarization. Combining a two-band kp model with first-principles calculations, we show that 2H-VSe2 monolayer, where the spin-orbit coupling coexists with the intrinsic exchange interaction of transition-metal-d electrons, is such a room-temperature ferrovalley material. We further predict that such system could demonstrate many distinctive properties, for example, chirality-dependent optical band gap and more interestingly, anomalous valley Hall effect. On account of the latter, a series of functional devices based on ferrovalley materials, such as valley-based nonvolatile random access memory, valley filter, are contemplated for valleytronic applications., Comment: 6 pages,5 figures

Magnetic ordering, as one of the most important characteristics in magnetic materials, could have significant influence on the band structure, spin dependent transport, and other important properties of materials. Its measurement, especially for the case of antiferromagnetic ordering, however, is generally difficult to be achieved. Here we demonstrate the feasibility of magnetic ordering detection using a noncontact and nondestructive optical method. Taking the compressive strained tetragonal BiFeO3 (BFO) as an example and combining density functional theory calculations with the minimal one-band tight-binding models, we find that when BFO changes from C1-type antiferromagnetic (AFM) phase to G-type AFM phase, the top of valance band shifts from the Z point to {\Gamma} point, which makes the original direct band gap become indirect. This can be explained by the two-center Slater-Koster parameters using the Harrison approach. The impact of magnetic ordering on energy band dispersion dramatically changes the optical properties of tetragonal BFO. For the linear ones, the energy shift of the optical band gap could be as large as 0.4 eV. As for the nonlinear ones, the change is even larger. The second-harmonic generation coefficient d33 of G-AFM becomes more than 13 times smaller than that of C1-type AFM case. Finally, we propose a practical way to distin-guish the C1- and G-type AFM of BFO using the optical method, which might be of great importance in next-generation information storage technologies and widens the potential application of BFO to optical switch.

Considerable progress in contemporary spintronics has been made in recent years for developing nanoscale data memory and quantum information processing. It is, however, still a great challenge to achieve the ultimate limit of storage bit. 2D materials, fortunately, provide an alternative solution for designing materials with the expected miniaturizing scale, chemical stability as well as giant magnetic anisotropy energy. By performing first-principles calculations, we have examined two possible doping sites on a WS2 monolayer using three kinds of transition metal (TM) atoms (Mn, Fe and Co). It is found that the TM atoms prefer to stay on the W atom site. Additionally, differently from the case of Mn, doping Co and Fe atoms on the W vacancy can achieve perpendicular magnetic anisotropy with a much larger magnitude, which provides a bright prospect for generating atomic-scale magnets of storage devices.

The concept of ferrovalley materials has been proposed very recently. The existence of spontaneous valley polarization, resulting from ferromagnetism, in such hexagonal 2D materials makes nonvolatile valleytronic applications realizable. Here, we introduce a new member of ferrovalley family with orthorhombic lattice, i.e. monolayer group-IV monochalcogenides (GIVMs), in which the intrinsic valley polarization originates from ferroelectricity, instead of ferromagnetism. Combining the group theory analysis and first-principles calculations, we demonstrate that, different from the valley-selective circular dichroism in hexagonal lattice, linearly polarized optical selectivity for valleys exists in the new type of ferrovalley materials. On account of the distinctive property, a prototype of electrically tunable polarizer is realized. In the ferrovalley-based polarizer, a laser beam can be optionally polarized in x- or y-direction, depending on the ferrovalley state controlled by external electric fields. Such a device can be further optimized to emit circularly polarized radiation with specific chirality and to realize the tunability for operating wavelength. Therefore, we show that 2D orthorhombic ferrovalley materials are the promising candidates to provide an advantageous platform to realize the polarizer driven by electric means, which is of great importance in extending the practical applications of valleytronics.

Rashba spin orbit coupling (SOC) in noble metal surfaces is of great importance for the application of metal films in spintronic devices. By combining the density-functional theory calculations with our recently developed orbital selective external potential method, we investigate the Rashba SOC in the Shockley surface states of Au(111) and Ag(111). We find that the large Rashba SOC in the sp-character surface states of Au(111) is mainly contributed by the minor d-orbitals in the surface states. While for the sd-character surface states, although they are dominated by the d-orbitals, Rashba splitting is found to be rather small. Band structure analysis reveals that this is mainly because the sd-character surface states are well below the Fermi level and can be less influenced by the asymmetric surface potential. We demonstrate that the Rashba SOC in noble metal surfaces can be effectively manipulated by shifting the d-orbitals in the surface states, which can be physically implemented through surface decoration. Our investigation provides a deep understanding on Rashba SOC in noble metal surfaces and could be helpful to their applications in spintronic devices.