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Recipe for Dirac Phonon States with a Quantized Valley Berry Phase in Two-Dimensional Hexagonal Lattices
摘要： The topological quantum states in two-dimensional (2D) materials are fascinating subjects of research, which usually highlight electron-related systems. In this work, we present a recipe that leads to Dirac phonon states with quantized valley Berry phase in 2D hexagonal lattices by first-principles calculations. We show that candidates possessing the three-fold rotational symmetry at the corners of the hexagonal Brillouin zone host valley Dirac phonons, which are guaranteed to remain intact with respect to perturbations. We identify that such special topological features populated by Dirac phonons can be realized in various 2D materials. In particular, the monolayer CrI3, an attractive 2D magnetic semiconductor with exotic applications in spintronics, is an ideal platform to investigate nontrivial phonons in experiments. We further confirm that the phonon Berry phase is quantized to ±π at two inequivalent valleys. The phonon edge states terminated at the projection of phonon Dirac cones are clearly visible. This work demonstrates that 2D hexagonal lattices with attractive valley Dirac phonons will extend the knowledge of valley physics, providing wide applications of topological phonons.
关键词： Dirac phonons,first-principles calculations,valley Berry phase,phonon edge states,2D hexagonal lattices
更新于2025-09-23 15:21:21
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Effective Hamiltonian of Topological Nodal Line Semimetal in Single-Component Molecular Conductor [Pd(dddt) <sub/>2</sub> ] from First-Principles
摘要： Using ?rst-principles density-functional theory calculations, we obtain the non-coplanar nodal loop for a single-component molecular conductor [Pd(dddt)2] consisting of HOMO and LUMO with di?erent parity. Focusing on two typical Dirac points, we present a model of an e?ective 2 × 2 matrix Hamiltonian in terms of two kinds of velocities associated with the nodal line. The base of the model is taken as HOMO and LUMO on each Dirac point, where two band energies degenerate and the o? diagonal matrix element vanishes. The present model, which reasonably describes the Dirac cone in accordance with the ?rst-principles calculation, provides a new method of analyzing electronic states of a topological nodal line semimetal.
关键词： topological nodal line semimetal,effective Hamiltonian,Dirac cone,single-component molecular conductor,first-principles calculations
更新于2025-09-23 15:21:21
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[NanoScience and Technology] Silicene (Prediction, Synthesis, Application) || Optical Properties of Silicene and Related Materials from First Principles
摘要： Slightly buckled, graphene-like honeycomb crystals made by silicon, silicene, or by other group-IV elements such as germanene and stanene represent atomically thin films, i.e., two-dimensional (2D) systems. The theoretical description of their optical properties suffers from three difficulties, (i) a thickness much smaller than the wavelength of light, (ii) their common modeling by superlattice arrangements with sufficiently large layer distances, and (iii) the inclusion of many-body effects. Here, the solutions of all problems are discussed. (i) The optical response of an individual honeycomb crystal is described by a tensor of 2D optical conductivities or dielectric functions, which are related to the optical response of the corresponding superlattice. (ii) The influence of such a sheet crystal on the transmittance, reflectance and absorbance of a layer system is described. (iii) Excitonic and quasiparticle effects are demonstrated to widely cancel each other. Silicene sheets are investigated in detail. As a consequence of the linear bands and Dirac cones the low-frequency absorbance is defined by the Sommerfeld finestructure constant. Van Hove singularities represented by critical points in the interband structure are identified at higher photon energies. Clear chemical trends along the row C → Si → Ge → Sn are derived. The influence of multiple layers is studied for the cases of bilayer silicene and graphene.
关键词： silicene,optical properties,many-body effects,two-dimensional materials,first principles,Van Hove singularities,stanene,Dirac cones,germanene
更新于2025-09-23 15:21:21
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Effects of carbon doping and vacancy defect on the magnetism of anatase TiO2
摘要： The magnetism origin and mechanism of anatase TiO2 with C doping and vacancy remain controversial. In this study, the effects of C doping and vacancy defect on the magnetism of TiO2 were investigated by generalized gradient approximation and Hubbard U method based on density functional theory. Systems with coexisting interstitial C and O vacancies achieved half-metallic ferromagnetism, and the Curie temperature was above the room temperature. The system used as electron injection source reached 100% electron spin polarization. The Ti32CO2O62 system exhibited ferromagnetism, and the Curie temperature was above the room temperature. The magnetism source of Ti16CiO31 and Ti32CO2O62 demonstrated strong hybrid coupling electron exchange effects among Ti-3d, O-2p, and C-2p orbits which is consistent with the RKKY model of d-p orbital electron exchange.
关键词： Carbon doping and vacancy defect,Magnetism,Anatase TiO2,First-principles
更新于2025-09-23 15:21:21
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Structural determination of phosphosilicate glass based on first-principles molecular dynamics calculation
摘要： We report static and dynamic first-principles calculations determining the structure and electronic properties of phosphosilicate glass (PSG). A stable structure of PSG with composition of (SiO2)x(P2O5)y was calculated by simulated annealing based on Car–Parrinello molecular dynamics (CPMD) with a real-space density functional theory (RSDFT) code. As a result, phosphorus (P) atoms in PSG were four-fold coordinated with oxygen (O) atoms and one of the four PO bonds was dangling toward the interstitial region (an –O3PO configuration). We also compared the structures having a different number of O atoms to discuss the stable configuration of PSG in the realistic situation of phosphoryl chloride (POCl3) annealing. We found that –O3PO is most stable for O2-rich conditions, whereas three-coordinate P (an –O3P configuration) becomes stable in O2-poor conditions. The candidates of the states which create levels inside the bandgap of PSG were also calculated.
关键词： phosphosilicate glass,molecular dynamics,first-principles calculation,electronic properties,POCl3 annealing
更新于2025-09-23 15:21:21
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Mixed-Cation Mixed-Metal Halide Perovskites for Photovoltaic Applications: A Theoretical Study
摘要： Perovskite solar cells based on multiple cations have shown excellent optoelectronic properties with high power conversion efficiency. Herein, the structural, electronic, and optical properties of mixed-cation mixed-metal perovskites MA1?xCsxPb0.25Sn0.75I3 were studied by employing the first-principles calculations for the first time. Our calculated results reveal that these perovskite materials possess direct band gaps in the range of 1.0?1.3 eV. Moreover, these compounds show excellent photovoltaic performance in terms of strong optical absorption coefficients compared with MAPbI3. Particularly, they also exhibit good structural stability and decrement of lead content. These results demonstrated that mixed-cation mixed-metal perovskites may be potential candidates for high-efficiency light-absorbing materials.
关键词： mixed-cation mixed-metal perovskites,electronic properties,optical properties,first-principles calculations,photovoltaic applications,structural stability
更新于2025-09-23 15:21:01
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Hybrid functional calculations of electro-optical properties of novel Ga1a??xInxTe ternary chalcogenides
摘要： Doping GaTe semiconductor with indium can be beneficial in the realization of high-quality radiation detectors. In this study, first-principle calculations based on the density functional theory have been used to investigate the structural and electro-optical properties of Ga1?xInxTe alloys. For the electronic structure calculations, the exchange–correlation functionals are treated with an accurate PBE0 hybrid functional giving improved agreement with experimental band energies. The influence of the In concentration on the energy band gaps is analyzed. Furthermore, the direction for the highest mobility of Ga1?xInxTe is estimated by calculating the effective mass of carriers respecting to the crystallographic directions from calculated electronic band structures. Optical spectra of Ga1?xInxTe are evaluated for all compositions (x = 0, 0.25, 0.5 and 0.75) and for different polarization directions in the range of 0–14 eV. The calculated optical spectra of Ga1–xInxTe are found to have a remarkable redshift as the alloying composition increases. The calculated static dielectric constant for the entire concentration shows that the considered alloys are a high-dielectric constant materials. Our study shows that Ga1?xInxTe alloys exhibit metallic properties in some energy ranges. Our results suggest that the new Ga1?xInxTe alloys are a promising material for radiation detectors, microelectronics and optoelectronic devices.
关键词： First-principles calculations,Band structure,Effective mass,Ga1–xinxte,Dielectric function,PBE0 hybrid functional
更新于2025-09-23 15:21:01
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Photogenerated-Carrier Separation and Transfer in Two-Dimensional Janus Transition Metal Dichalcogenides and Graphene van der Waals Sandwich Heterojunction Photovoltaic Cells
摘要： Two-dimensional (2D) Janus transition metal dichalcogenides (JTMDs) show direct band gaps and strong visible-light absorption with promising applications in photovoltaic (PV) cells. Here, we investigate the electronic structures and dynamics of photogenerated carriers in 2D JTMDs and graphene van der Waals sandwich heterojunction (G/JTMDs/G) photovoltaic cells by using first-principles calculations. We find that the intrinsic built-in electric field in JTMDs results in an asymmetry potential, which can be used to effectively enhance the separation and transfer of photogenerated carriers from JTMDs to different graphene layers with a preferred direction within hundreds of femtoseconds in the G/JTMDs/G heterostructures. Furthermore, the photogenerated electrons (holes) can transfer from monolayer MoSSe (MoSeTe) to the graphene sheets by the Se side with lower (higher) potential, while the transfer of the photogenerated holes (electrons) is prohibited due to the large separation between donor and acceptor states.
关键词： First-principles calculations,van der Waals heterostructures,Janus transition metal dichalcogenides,Photogenerated carriers,Two-dimensional materials,Graphene,Charge transfer,Photovoltaic cells
更新于2025-09-23 15:21:01
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First-principles study on XV2S4 (X = Ni, Cr, and Mo) counter electrode for dye-sensitized solar cells
摘要： Structural, electronic, and optical properties of ternary vanadium sulfides, XV2S4 (X = Ni, Cr, and Mo), are investigated using first-principles calculations based on density functional theory (DFT) method. The structural parameters, band structures, density of states (DOS), absorption coefficient, and conductivity are reported to discuss the properties of XV2S4 (X = Ni, Cr, and Mo) as potential Pt-free counter electrode for dye-sensitized solar cell (DSSC). Generally, nickel vanadium sulfide (NiV2S4) is predicted to be a good candidate as a counter electrode compares with chromium vanadium sulfide (CrV2S4) and molybdenum vanadium sulfide (MoV2S4) due to its ability to provide high electron density around Fermi level and high conductivity.
关键词： Electronic properties,First-principles study,Counter electrode,Optical properties,Pt-free
更新于2025-09-23 15:21:01
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Improvement of the Laser-Welded Lap Joint of Dissimilar Mg Alloy and Cu by Incorporation of a Zn Interlayer
摘要： During pulsed laser welding of AZ 31B magnesium (Mg) alloy and T2 pure copper (Cu), Cu2Mg and Mg2Cu are generated, but the bonding ability of the two compounds is usually weak, resulting in low strength. In order to improve the joint of two dissimilar metals, a zinc interlayer was inserted between the Mg alloy and Cu, and the effects of the thickness of the Zn interlayer on the microstructure and properties of the joint were studied. The fused zone consisted of Cu2Mg and MgZn, and, according to first-principles calculation, in the same energy range, the area enclosed by the density of the state curve of MgZn was larger than that of Cu2Mg. Hence, the bonding ability of MgZn was better than that of Cu2Mg, and MgZn improved the strength of the welded joint. The most advantageous thickness of the Zn interlayer was 0.1 mm, and the shear strength was 48.15 MPa that was 161% higher than that of the directly welded Mg/Cu joint.
关键词： interlayer,first-principles calculation,magnesium alloy,laser welding,dissimilar metals
更新于2025-09-23 15:21:01