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GeTe wins the renewed research interest due to its giant bulk Rashba spin orbit coupling (SOC), and becomes the father of a new multifunctional material, i.e., ferroelectric Rashba semiconductor. In the present work, we investigate Rashba SOC at the interface of the ferroelectric semiconductor superlattice GeTe(111)/InP(111) by using the first principles calculation. Contribution of the interface electric field and the ferroelectric field to Rashba SOC is revealed. A large modulation to Rashba SOC and a reversal of the spin polarization is obtained by switching the ferroelectric polarization. Our investigation about GeTe(111)/InP(111) superlattice is of great importance in the application of ferroelectric Rashba semiconductor in the spin field effect transistor.

The concept of ferrovalley materials has been proposed very recently. The existence of spontaneous valley polarization, resulting from ferromagnetism, in such hexagonal two-dimensional 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 two-dimensional 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.

Transition metal dichalcogenide (TMD) monolayers MXY (M=Mo, W, X(not equal to)Y=S, Se, Te) are two-dimensional polar semiconductors. Setting 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 Gamma point, which, however, can be easily tuned by applying the in-plane biaxial strain. Through a relatively small strain (from -2% to 2%), a large tunability (from around -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 W-dz2 and Se-pz 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. 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

Multiferroics are materials where two or more ferroic orders coexist owing to the interplay between spin, charge, lattice and orbital degrees of freedom. The explosive expansion of multiferroics literature in recent years demon-strates the fast growing interest in this field. In these studies, the first-principles calculation has played a pioneer role in the experiment explanation, mechanism discovery and prediction of novel multiferroics or magnetoelectric materials. In this review, we discuss, by no means comprehensively, the extensive applications and successful achievements of first-principles approach in the study of multiferroicity, magnetoelectric effect and tunnel junc-tions. In particular, we introduce some our recently developed methods, e.g., the orbital selective external potential (OSEP) method, which prove to be powerful tools in the finding of mechanisms responsible for the intriguing phe-nomena occurred in multiferroics or magnetoelectric materials. We also summarize first-principles studies on three types of electric control of magnetism, which is the common goal of both spintronics and multiferroics. Our review offers in depth understanding on the origin of ferroelectricity in transition metal oxides, and the coexistence of fer-roelectricity and ordered magnetism, and might be helpful to explore novel multiferroic or magnetoelectric materi-als in the future., Comment: 44 pages, 30 figures. Review paper

Atomic-scale magnetic nanostructures are promising candidates for future information processing devices. Utilizing external electric field to manipulate their magnetic properties is an especially thrilling project. Here, by careful identifying different contributions of each atomic orbital to the magnetic anisotropy energy (MAE) of the ferromagnetic metal films, we argue that it is possible to engineer both the MAE and the magnetic response to the electric field of atomic-scale magnetic nanostructures. Taking the iron monolayer as a matrix, we propose several interesting iron nanostructures with dramatically different magnetic properties. Such nanostructures could exhibit strong magnetoelectric effect. Our work may open a new avenue to the artificial design of electrically controlled magnetic devices.

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.

Using first-principles density functional calculations, electronic and optical properties of ferromagnetic semiconductor EuO are investigated. In particular, we have developed a way to obtain the spin-dependent optical response of the magnetic materials, which is helpful to verify the spin-dependent band structure of EuO. Significantly different optical responses from spin-up and spin-down channels are obtained in both linear and nonlinear cases, making it possible to distinguish contributions from different spin-channels in the optical absorption spectra if spin-flip process can be neglected. In addition, the red-shift of the absorption edge from paramagnetic to ferromagnetic ordering is explained by exchange interactions. Using such method, we have also compared the optical properties of multiferroic EuO which is induced by strong epitaxial strain. Our results show that from tensile to compressive strain, the blue-shift of the leading absorption peaks in the optical spectra, the red-shift of the optical band gap in spin-up state can be observed, consistent to the energy difference between spin-splitting orbits. The spin-dependent nonlinear optical properties reveal that in the infrared and visible light region, the contributions to second-harmonic generation (SHG) susceptibilities are mainly from spin-majority channels. In addition, the strain effect is also discussed. With the increase of epitaxial strain, the larger energy shift of the leading absorption peaks, and the more remarkable nonlinear optical response can be obtained., Comment: 8 pages, 6 figures

Using density-functional theory calculations, we investigate the magnetic as well as the dynamical properties of tetragonal SrRuO3 (SRO) under the influence of epitaxial strain. It is found that both the tensile and compressive strain in the xy-plane could induce the abrupt change in the magnetic moment of Ru atom. In particular, under the in-plane ~4% compressive strain, a ferromagnetic to nonmagnetic transition is induced. Whereas for the tensile strain larger than 3%, the Ru magnetic moment drops gradually with the increase of the strain, exhibiting a weak ferromagnetic state. We find that such magnetic transitions could be qualitatively explained by the Stoner model. In addition, frozen phonon calculations at {\Gamma} point reveal structural instabilities could occur under both compressive and tensile strains. Such instabilities are very similar to those of the ferroelectric perovskite oxides, even though SRO remains to be metallic in the range we studied. These might have influence on the physical properties of oxide supercells taking SRO as constituent., Comment: 5 pages