Materials with large intrinsic valley splitting and high Curie temperature are a huge advantage for studying valleytronics and practical applications. In this work, using first-principles ...calculations, a new Janus TaNF monolayer is predicted to exhibit excellent piezoelectric properties and intrinsic valley splitting, resulting from the spontaneous spin polarization, the spatial inversion symmetry breaking and strong spin−orbit coupling (SOC). TaNF is also a potential two-dimensional (2D) magnetic material due to its high Curie temperature and large magnetic anisotropy energy. The effective control of the band gap of TaNF can be achieved by biaxial strain, which can transform TaNF monolayer from semiconductor to semi-metal. The magnitude of valley splitting at the CBM can be effectively tuned by biaxial strain due to the changes of orbital composition at the valleys. The magnetic anisotropy energy (MAE) can be manipulated by changing the energy and occupation (unoccupation) states of d orbital compositions through biaxial strain. In addition, Curie temperature reaches 373 K under only −3% biaxial strain, indicating that Janus TaNF monolayer can be used at high temperatures for spintronic and valleytronic devices.
Due to the symmetry breaking, Janus two-dimensional materials possess novel electronic properties such as energy valley and spin polarization. In this research, we predicted the Zeeman-type spin ...splitting and Rashba splitting of monolayer Janus ZrNX (X = Br, I). The Rashba spin splitting and valley splitting can be greatly tuned by the magnetic proximity effect of monolayer H–VTe2, which are dependent on the stacking configurations. There is strong interlayer coupling between Janus ZrNX and VTe2, leading to a sizable valley splitting of 87 meV. The valley splitting can be significantly enlarged by the vertical compression strain, because of the increases of effective Zeeman field and in-plane orbital component. In addition, a helical gap is opened and the dispersion relation of Rashba spin-orbit coupling is obviously modified by in-plane biaxial strain. The Rashba constant and helical gap increase with the in-plane tensile strain, which leads from the larger potential energy difference and magnetic moment of surface V atoms. The greatly tunable spin/valley splittings in ZrNBr/VTe2 heterostructure are of some signification for the development of spintronics/valleytronics and the realization of new functional devices.
•The valley polarization and Rashba SOC of monolayer Janus ZrNX (X = Br, I) are demonstrated.•The valley splitting and Rashba splitting of ZrNBr/VTe2 heterostructure are effectively regulated by stacking configurations.•The factor of greater valley splitting is larger in-plane orbital composition and strong effective Zeeman field.•The helical gap is found and can be manipulated by biaxial strain.
We investigate the effects of biaxial strain and spin-orbit coupling on the electronic structures of Janus monolayer group-III monochalcogenides based on first-principle calculations. Due to the ...spin-orbit coupling and mirror symmetry breaking, Rashba band splitting appears at Γ point of the bottom conduction band and the band gap decreases significantly. The spin-orbit coupling strength and electronic structures can be greatly tuned by biaxial strain, which originate from the change of interaction between pz orbitals. As the lattice constant decreases, the interaction between pz orbitals is enhanced, leading to the decrease (increase) of the energy of corresponding bonding (anti-bonding) state, which is the origin of the indirect-direct-indirect transition of the band gap. In addition, when tensile strain is applied, the pz orbital component of Te atoms increases, leading to the enhancement of the Rashba effect. These results are of great significance for the band engineering and spin-orbitronics of two-dimensional materials.
•The mirror symmetry breaking and the intrinsic and anisotropic Rashba splitting are introduced in Janus monolayer M2XY.•The electronic structures including orbital compositions are deeply analyzed.•Due to the interaction between the pz orbitals, strain can effectively affect the electronic structures of monolayer M2XY.•The Rashba SOC of Janus monolayer Ga2SeTe is studied, which is enough for measurement in experiment and applications.•The tensile strain enhances the strength of SOC, which leads from the change of the pz orbital component of Te atoms.
Cobalt-based sulfides with variable valence states and unique physical and chemical properties have shown great potential as oxygen evolution reaction (OER) catalysts for electrochemical ...water-splitting reactions. However, poor morphological characteristics and a small specific surface area limit its further application. Here, hexagonal single-crystal two-dimensional (2D) CoS nanosheets with different thicknesses are successfully prepared by an atmospheric-pressure chemical vapor deposition method. Because of the advantages of the 2D structure, more exposed catalytic active sites, better reactant adsorption ability, accelerated electron transfer, and enhanced electrical conductivities can be achieved from the thinnest 5 nm CoS nanosheets (CoS-5), significantly improving OER performance. The electrochemical tests manifest that CoS-5 show an overpotential of 290 mV at 10 mA cm–2 and a Tafel slope of 65.6 mV dec–1 in the OER in an alkaline solution, superior to those for other thicknesses of CoS, bulk CoS, and RuO2. For the mechanistic investigation, the lowest charge transfer resistance (R ct) and the highest double-layer capacitance (C dl) were obtained for CoS-5, demonstrating the faster OER kinetics and the larger active area. Density functional theory calculations further reveal the enhanced density of states around the Fermi level and higher H2O molecule adsorption energy for thinner CoS nanosheets, promoting its intrinsic catalytic activity. Moreover, the two-electrode system with CoS-5 as the anode and Pt/C as the cathode requires only 1.56 V to attain 10 mA cm–2 in the overall water-splitting reaction. We believe that this study will provide a fresh view for thickness-dependent catalytic performance and offers a new material for the study of electronic and energy devices.
Materials with large intrinsic valley splitting and high Curie temperature are a huge advantage for studying valleytronics and practical applications. In this work, using first-principles ...calculations, a new Janus TaNF monolayer is predicted to exhibit excellent piezoelectric properties and intrinsic valley splitting, resulting from the spontaneous spin polarization, the spatial inversion symmetry breaking and strong spin-orbit coupling (SOC). TaNF is also a potential two-dimensional (2D) magnetic material due to its high Curie temperature and huge magnetic anisotropy energy. The effective control of the band gap of TaNF can be achieved by biaxial strain, which can transform TaNF monolayer from semiconductor to semi-metal. The magnitude of valley splitting at the CBM can be effectively tuned by biaxial strain due to the changes of orbital composition at the valleys. The magnetic anisotropy energy (MAE) can be manipulated by changing the energy and occupation (unoccupation) states of d orbital compositions through biaxial strain. In addition, Curie temperature reaches 373 K under only -3% biaxial strain, indicating that Janus TaNF monolayer can be used at high temperatures for spintronic and valleytronic devices.
In order to study oil-gas-liquid three-phase flow regularity in oil well, the vertical upward three-phase flow dynamic experiment with low velocity was carried out in a large diameter (125 mm i.d.) ...flow loop using the combination sensors of VMEA and mini-conductance probe array and six three-phase flow patterns were defined. VMEA sensor conductance fluctuating signals of five flow patterns with water as the continuous liquid were acquired. And then the multi-scale entropy processing and analysis of them are conducted to describe the dynamics characteristics of the different flow patterns. The study result indicates that the multi-scale entropy reveals the nonlinear dynamics characteristics of flow patterns in different scales and Rate of MSE as a characteristic representing approximately constant increasing rate of sample entropy at low scales sensitively indicates the variation of flow patterns and can act as the efficient criterion of three-phase flow patterns identification. And it is helpful to understand the dynamics mechanism of three-phase flow pattern evolution.
In order to investigate the flow patterns of gas/liquid two phase flow existing widely in Daqing oilfield of China, conductance signals of 104 gas/liquid two phase flow conditions has been acquired ...in concentrating channel on the three phase flow experimental facility of Tianjin University. Firstly, the fluctuating conductance signals are decomposed into seven scale signals and then recurrence plot is employed to analysis the original signal and each scale signal. The results indicate the recurrence plot has perfect identification ability in analyzing the flow patterns of complex gas/liquid flow. Simultaneously, two dimensional attractor morphologic characteristics of each order have been extracted, the effective feature combination used to identify the flow pattern is found through the analysis of attractor morphologic characteristics. The result shows that attractor morphologic characteristics can well reflect the flow pattern transition and it is an effective analysis method of flow pattern identification.
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