Valley splitting of monolayer Hf3C2O2 by the spin-orbit coupling effect: first principles calculations using the HSE06 methods.

Valleytronics is an emerging field of electronics that aims to utilize valley degrees of freedom in materials for information processing and storage. Nowadays, the valley splitting of 2D materials is not particularly large, therefore, the search for large valley splitting materials is very important for the development of valleytronics. This work theoretically predicts that MXene Hf3N2O2 is a 2D material with large valley splitting. It is an indirect bandgap semiconductor with a bandgap of 0.32 eV at the PBE level and increases to 0.55 eV at the HSE06 level. Since Hf3N2O2 breaks the symmetry of spatial inversion, when we consider spin–orbit coupling (SOC), there is a valley splitting at K/K′ of the valence band with a valley splitting value of 98.76 meV. The valley splitting value slightly decreases to 88.96 meV at the HSE06 level. In addition, The phonon spectrum and elastic constants indicate that it is both dynamically and mechanically stable. According to the maximum localization of the Wannier function, it is obtained that the Berry curvature is not zero at K/K′. When a biaxial strain is applied, Hf3N2O2 transitions from metal to semiconductor. With increasing biaxial strain, the valley splitting value increased from 70.13 meV to 109.11 meV. Our research shows that Hf3N2O2 is a promising material for valleytronics.

In this study, we discuss the tunability of valley splitting using first-principles calculations with a monolayer MoTe2 and layered ferromagnetic MnS2 heterostructure as an example. We observe that, due to the magnetic proximity effect (MPE) at the interface, a monolayer of MoTe2 can exhibit a significant valley splitting of 55.2 meV. The production of the interlayer dipoles with spin-adapted configuration could be the origin of MPE at the interface. Furthermore, the valley splitting can be regulated continuously by the perpendicular electric field and biaxial strain. Interestingly, the valley splitting increases with the increasing induced magnetic moments in MoTe2 by applying an electric field while the inverse laws are presented by applying biaxial strains, which indicates that the mechanisms of valley splitting manipulating in these two ways are quite different. The calculation results suggest that the electric field influences the electric dipole distributions at the interface, which determines the induced magnetic moments in monolayer MoTe2, and results in valley splitting variations. However, biaxial strains not only affect MPE at the interface but also the intrinsic spin splitting caused by spin-orbital coupling (SOC) effects of monolayer MoTe2 itself and the latter is even the dominating mechanism of valley splitting variations.

Valleytronic as a hot topic in recent years focuses on electrons’ valley degree of freedom as a quantum information carrier. Here, by combining two-band k.p model with high-throughput density functional theory (DFT) calculations, the valley states of Janus 2H-VSSe monolayer are studied which have spontaneous polarization. Nonvolatile valley polarization state is mainly arises from intrinsic ferromagnetism contributed by V-3d electronic configuration and not the spontaneous out-of-plane dipole moment of VSSe monolayer. The effective Hamiltonian model and DFT calculations both showed that the valley splitting mainly originates from the smaller spin splitting coming from the spin–orbit coupling effect rather than the spin splitting of magnetic exchange field. By using the effective Dirac Hamiltonian and Kubo formula, we further calculated the longitudinal and transversal conductivities and absorption spectra of VSSe monolayer which exhibits an anomalous valley Hall effect and clear valley-selective circular dichroism. Our calculations indicate that the modification of valley and spin splitting related to Berry curvature by applying an external strain is more noticeable than by the change of the magnetic moment orientation and electric field. We found that carriers accumulation with particular spin and valley label can be manipulated by tuning effective Hamiltonian parameters. The coexistence of robust in-plane magnetic ordering and spontaneous valley polarization of 2H-VSSe monolayer supports the possibility of applications in spintronics, valleytronics and optoelectronics devices.

Tuning electronic, magnetic and optical properties of Cr-doped antimonene via biaxial strain engineering

First-principles calculations to investigate the elastic, electronic, dynamical, and optical properties of cubic ZrCoAs half-Heusler semiconductor for photovoltaic applications

Valley polarization, magnetic anisotropy and Dzyaloshinskii-Moriya interaction of two-dimensional graphene/Janus 2H-VSeX (X = S, Te) heterostructures

Rare-earth nickelates ${R}^{3+}{\mathrm{Ni}}^{3+}{\mathrm{O}}_{3}$ ($R=\mathrm{Lu}\text{\ensuremath{-}}\mathrm{Pr}$, Y) show a striking metal-insulator transition in their bulk phase whose temperature can be tuned by the rare-earth radius. These compounds are also the parent phases of the newly identified infinite layer $R\mathrm{Ni}{\mathrm{O}}_{2}$ superconductors. Although intensive theoretical works have been devoted to understand the origin of the metal-insulator transition in the bulk, there have only been a few studies on the role of hole and electron doping by rare-earth substitutions in $R\mathrm{Ni}{\mathrm{O}}_{3}$ materials. Using first-principles calculations based on density functional theory (DFT) we study the effect of hole and electron doping in a prototypical nickelate $\mathrm{SmNi}{\mathrm{O}}_{3}$. We perform calculations without Hubbard-like $U$ potential on Ni $3d$ levels but with a meta--generalized gradient approximation better amending self-interaction errors. We find that at low doping, polarons form with intermediate localized states in the band gap resulting in a semiconducting behavior. At larger doping, the intermediate states spread more and more in the band gap until they merge either with the valence (hole doping) or the conduction (electron doping) band, ultimately resulting in a metallic state at 25% of $R$ cation substitution. These results are reminiscent of experimental data available in the literature and demonstrate that DFT simulations without any empirical parameter are qualified for studying doping effects in correlated oxides and exploring the mechanisms underlying the superconducting phase of rare-earth nickelates.

Two-dimensional (2D) valley materials are promising materials for writing and storing information. The search for 2D materials with large valley splitting is essential for the development of spintronics and valley electronics. In this study, we theoretically design 2D W2NSCl MXenes with large valley splitting based on first-principle calculations. Due to the strong spin-orbit coupling (SOC) and the broken inversion symmetry, the W2NSCl monolayer exhibits valley splitting values of 491 meV and 83 meV at K/K' of the valence and conduction bands, respectively. The valley splitting of W2NSCl is robust to biaxial strain. Because of the broken mirror symmetry of W2NSCl, there is a Rashba effect at Γ with a Rashba parameter of 1.019 V Å. Based on the maximum localization of the Wannier function, we found the non-zero Berry curvature at K/K'. Furthermore, the non-zero Berry curvature at the K/K' valley increases monotonically with an external strain from -4% to 4%. Our finding shows that W2NSCl is a candidate material for valley electronics and spintronics applications.

Two-dimensional van der Waals heterostructures (vdWHs) have been extensively studied for their excellent physical characteristics. In this paper, two twisted $\mathrm{Sb}/\mathrm{W}{\mathrm{Te}}_{2}$ vdWHs are respectively constructed by stacking of Sb and $\mathrm{W}{\mathrm{Te}}_{2}$ monolayers with different interlayer rotation angles, and their electronic properties are studied by first-principles calculation. Firstly, the effects of spin-orbit coupling on the electronic structure of $\mathrm{Sb}/\mathrm{W}{\mathrm{Te}}_{2}$ vdWHs, such as band gap and band alignment, are addressed in detail. Furthermore, for the $\mathrm{Sb}/\mathrm{W}{\mathrm{Te}}_{2}$ vdWH with an interlayer rotation angle of ${30}^{\ensuremath{\circ}}$, its band gap, band alignment, and spin splitting are investigated by adjusting the external electric field, biaxial strain, and interlayer coupling, respectively. Under an applied external electric field, the band structure of the $\mathrm{Sb}/\mathrm{W}{\mathrm{Te}}_{2}$ vdWH undergoes a transition from a direct band gap to an indirect band gap, and a semiconductor-metal transition occurs at $\ifmmode\pm\else\textpm\fi{}0.7\phantom{\rule{0.16em}{0ex}}\mathrm{V}/\AA{}$ along with the transition of types I, II, and III band alignments. Similarly, the band gap and band alignment of the $\mathrm{Sb}/\mathrm{W}{\mathrm{Te}}_{2}$ vdWH can also be modulated by biaxial strain and interlayer coupling. In addition, the calculated electronic structure present that the Rashba- and Zeeman-type spin splitting are dependent on the external electric field, biaxial strain, and interlayer coupling. Thus, the controllable electronic properties of $\mathrm{Sb}/\mathrm{W}{\mathrm{Te}}_{2}$ vdWHs have great application potential for spintronic and optoelectronic devices.

Structural symmetry breaking in two-dimensional materials can lead to superior physical properties and introduce an additional degree of piezoelectricity. In the present paper, we propose three structural phases ($1H, 1T$, and $1{T}^{\ensuremath{'}}$) of Janus $\mathrm{W}X\mathrm{O}$ ($X=\mathrm{S}$, Se, and Te) monolayers and investigate their vibrational, thermal, elastic, piezoelectric, and electronic properties by using first-principles methods. Phonon spectra analysis reveals that while the $1H$ phase is dynamically stable, the $1T$ phase exhibits imaginary frequencies and transforms to the distorted $1{T}^{\ensuremath{'}}$ phase. Ab initio molecular dynamics simulations confirm that $1H$- and $1{T}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{W}X\mathrm{O}$ monolayers are thermally stable even at high temperatures without any significant structural deformations. Different from binary systems, additional Raman active modes appear upon the formation of Janus monolayers. Although the mechanical properties of $1H\text{\ensuremath{-}}\mathrm{W}X\mathrm{O}$ are found to be isotropic, they are orientation dependent for $1{T}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{W}X\mathrm{O}$. It is also shown that $1H\text{\ensuremath{-}}\mathrm{W}X\mathrm{O}$ monolayers are indirect band-gap semiconductors and the band gap narrows down the chalcogen group. Except $1{T}^{\ensuremath{'}}$-WSO, $1{T}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{W}X\mathrm{O}$ monolayers have a narrow band gap correlated with the Peierls distortion. The effect of spin-orbit coupling on the band structure is also examined for both phases and the alteration in the band gap is estimated. The versatile mechanical and electronic properties of Janus $\mathrm{W}X\mathrm{O}$ monolayers together with their large piezoelectric response imply that these systems are interesting for several nanoelectronic applications.

Electron spins in Si/SiGe quantum wells suffer from nearly degenerate conduction band valleys, which compete with the spin degree of freedom in the formation of qubits. Despite attempts to enhance the valley energy splitting deterministically, by engineering a sharp interface, valley splitting fluctuations remain a serious problem for qubit uniformity, needed to scale up to large quantum processors. Here, we elucidate and statistically predict the valley splitting by the holistic integration of 3D atomic-level properties, theory and transport. We find that the concentration fluctuations of Si and Ge atoms within the 3D landscape of Si/SiGe interfaces can explain the observed large spread of valley splitting from measurements on many quantum dot devices. Against the prevailing belief, we propose to boost these random alloy composition fluctuations by incorporating Ge atoms in the Si quantum well to statistically enhance valley splitting.

In this work, we explore the interfacial properties of the C60-Py@MAPbI3 heterojunction of the PbI-terminated MAPbI3(001) surface and pyridine-functionalized C60-Py fullerene derivative through both collinear and noncollinear density functional theory calculations with and without spin-orbit coupling (SOC) effects. C60-Py is bound to the MAPbI3 surface through interfacial Pb-O and Pb-N bonds. Although C60-Py@MAPbI3 is predicted to be the same type II heterojunction at all of the computational levels considered, the SOC effects largely decrease the energy gap of the first conduction bands of C60-Py and MAPbI3, thereby accelerating the interfacial electron transfer. Further dynamics simulations show that the inclusion of the SOC effects induces the transfer of approximately 80% of electrons from MAPbI3 to C60-Py within 1 ps. The work demonstrates that the SOC effects are indispensable for the interfacial properties of C60-Py@MAPbI3 and could also play a non-negligible role in tuning the optoelectronic properties of fullerene-based or similar perovskite devices.

Electronic, magnetic, and thermodynamic properties of rhombohedral Dysprosium Manganite and discussions of effects of uniform strain, spin-orbit coupling, hole and electron doping on its electronic structures

The effects of electron and hole doping on the magnetic properties of hydrogenated and fluorinated graphene structures are theoretically investigated by additional charge mimic. The studied hydrogenated and fluorinated graphene with different electronic structures display different relations between magnetism and charge, in which the spin moment of the former has the maximum value without charge, followed by linearly and symmetrically decreasing with increase of the positive and negative charge (hole and electron doping), while the latter continuously but not linearly increases its spin moment with the charge variation and finally achieves a maximum at certain positive charge doping. Moreover, the phase transition from ferromagnetism to nonmagnetism occurs. With the analysis of the spin-polarized band structures, the electron and hole doping effects on spin moment in the hydrogenated graphene mainly arise from the shifts of the Fermi level, while that in the fluorinated graphene not only results from the shifts of Fermi level, but also from the relative shifts between up- and down-spin band lines. The discovery of the effects of electron and hole doping on magnetism provides fundamental insight on functionalized graphene, rendering new promising potentials for unique spintronics applications.

A systematic density functional theory (DFT)+U study is conducted to investigate the electron correlation and spin-orbit coupling (SOC) effects in US(3) and USe(3). Our calculations reveal that inclusion of the U term is essential to get energy band gaps for them, indicating the strong correlation effects for uranium 5f electrons. Taking consideration of the SOC effect results in small reduction on the electronic band gaps of US(3) and USe(3), but largely changes the energy band shapes around the Fermi energy. As a result, US(3) has a direct band gap while USe(3) has an indirect one. Our calculations predict that both US(3) and USe(3) are antiferromagnetic insulators, in agreement with corresponding experimental results. Based on our DFT+U calculations, we systematically present the ground-state electronic, mechanical, and Raman properties for US(3) and USe(3).