Wen-Yi Tong

Concepts of ferrovalley material and anomalous valley Hall effect

Valleytronics rooted in the valley degree of freedom is of both theoretical and technological importance as it offers additional opportunities for information storage, as well as electronic, magnetic and optical switches. In analogy to ferroelectric materials with spontaneous charge polarization, or ferromagnetic materials with spontaneous spin polarization, here we introduce a new member of ferroic family, that is, a ferrovalley material with spontaneous valley polarization. Combining a two-band k·p 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, functional devices based on ferrovalley materials, such as valley-based nonvolatile random access memory and valley filter, are contemplated for valleytronic applications.

Manipulation of the large Rashba spin splitting in polar two-dimensional transition-metal dichalcogenides

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.

Spin-dependent optical response of multiferroic EuO: First-principles DFT calculations

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 the 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 a 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 the 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 regions, 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.

Facile preparation of rare-earth semiconductor nanocrystals and tuning of their dimensionalities

EuS and Gd2O2S nanocrystals with narrow size distribution are synthesized in high yields by the thermal decomposition of Eu(oleate)3 or Gd(oleate)3 in oleylamine using CS2 as the sulfur source. The dimensionalities of these nanocrystals can be facilely tuned by the addition of 1-dodecanethiol. The morphologies and crystal structures of EuS and Gd2O2S nanocrystals are characterized by TEM and XRD. The magnetic properties of the obtained nanocrystals are also investigated. The experimental results illustrate that CS2 is an effective sulfur source for the preparation of metal sulfide and lanthanide oxysulfide semiconductor nanocrystals.

First-principles study of enhanced magnetic anisotropies in transition-metal atoms doped WS2 monolayer

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.

Electric field control of magnetism in nickel with coaxial cylinder structure at room temperature by electric double layer gating

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.

Electrically tunable polarizer based on 2D orthorhombic ferrovalley materials

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.