Nickel Dimers Adsorbed on Graphene: First-Principles Study

The structural stabilities, electronic, and magnetic properties of Ni dimers adsorption on pure and single-vacancy graphene were studied using a first-principle method. It was found that structures of the Ni dimer adsorbed on single-vacancy graphene are more stable than those structures of the Ni dimer adsorbed on pure graphene. Total energy calculation results demonstrated that for Ni dimer adsorption on single-vacancy graphene and pure graphene, the most stable structure is PV2 where the Ni dimer is positioned perpendicular to the vacancy site and PH1 where Ni dimer standing perpendicular to hollow site. Furthermore, it indicated covalent bond was formed between Ni atoms and their nearest C atoms, suggestive of chemisorption. The structures where Ni dimer lay horizontal to the pure graphene layer (H(T1–T3), H(B1–B2), H(B1–B3)) showed strong magnetism, while the structures of dimer-adsorption structures on single-vacancy graphene do not exhibit magnetism. The electronic origin of magnetic states was located on Ni atoms, specifically the d orbit. The Ni dimer, positioned parallel to the graphene plane, offers the possibility for the formation of Ni atom chain or wire on graphene, which might have potential nanoelectronic applications.     

First-Principles Study on Half-Metallic Full-Heusler Compound Mn2ZnSi

By first-principle calculation, the electronic structure and magnetic properties of the Mn₂ZnSi full-Heusler alloy with CuHg₂Ti-type structure are investigated. Calculations show that Mn₂ZnSi compound presents half-metallic ferrimagnetic properties under the equilibrium lattice constant. The influence of spin-orbit interaction for the magnetic moments is investigated. The result shows spin-orbit interaction has little influence on magnetic moment. The bulk modulus of Mn₂ZnSi obtained by a fit of the Murnaghan equation of state is 134.3 GPa, which is more compressible than some other Heusler alloys. At the pressure range of 0 to 17.7 GPa, Mn₂ZnSi presents half-metallic character. Mn₂ZnSi would be a promising material for future spintronic applications.     

First-Principles Study of Ferromagnetism in Mn-Doped GaN

We investigate the magnetic properties of Mn-doped GaN through first-principles pseudopotential calculations within the spin-density-functional approximation. We examine the nature of magnetic interactions between Mn ions, and find that the ferromagnetic coupling has a short-range nature, effective for Mn–Mn distances up to about 7~{Å}. For Mn concentrations of about 6%, we find that the ferromagnetic solution is more stable than the antiferromagnetic state, while the stability of the ferromagnetic state is weakened by electron doping. Based on the calculated exchange coupling and the percolation approach, we estimate the Curie temperature lying above room temperature. Analyzing the Mn d levels, we suggest that the d–d hybridization between Mn ions is the main reason for stabilizing the ferromagnetic state. We also find that the formation of small Mn nanoclusters consisting of a few Mn atoms is energetically favorable. Since these small clusters are stable in the ferromagetic state, offering large magnetic moments, we do not rule out a possibility that small Mn nanoclusters are responsible for the ferromagnetism observed in Mn-doped GaN.     

Two-Gap Superconductivity in Niobium Carbide-Coated Single-Walled Carbon Nanotubes: A First-Principles Study

Superconductivity in carbon nanotubes is attracting worldwide attention because of the reported high superconducting transition temperature in small-diameter single-walled carbon nanotubes (SWCNTs). However, it is well known that superconductivity in low-dimensional (quasi-1D) systems is not so common due to low density of states (DOS), strong quantum fluctuations and other phenomena in such systems. In this paper, we present theoretical investigations of the proximity effect of superconducting niobium carbide on single-walled carbon nanotube using density functional theory (DFT). The relaxed structure shows that Nb atoms are held around the SWCNT, forming a layer through weak van der Waals’ forces. The stability of the structure has been confirmed by Hirshfeld analysis and Mullikan population analysis as well. The study of the electronic band structure of the pristine and modified SWCNT shows a fascinating condensation of electronic states and a striking shift in the Fermi level. Further, two additional band gaps have appeared below the valence band suggesting some kind of pairing mechanism being operational. This indicates the possibility of superconducting behaviour in SWCNT in proximity of niobium carbide. The relaxed structure thus envisions the feasibility and stability of NbC-coated SWCNTs which will have superconducting properties as well as the remarkable mechanical and optical properties of SWCNTs. This prediction seeks interest of the researchers to try and develop such a novel nanomaterial which, if realized, will prove to be highly significant for many technological applications.     

First-Principles Study on the (111) Surface of Half-Metallic CsN

Using the full-potential local-orbital minimum-basis method based on electronic structure calculations, we have probed the structural, electronic, and magnetic properties of the (111) surface of CsCl-type CsN. Our results indicate that bulk CsN is found to be a half-metallic ferromagnet at equilibrium lattice constant with a total spin magnetic moment of 2.0 μB per formula unit. At the same equilibrium lattice constant, the Cs-terminated (111) and N-terminated (111) surfaces preserve the half-metallic characteristics of the bulk CsN. In addition, we also show that the Cs-terminated (111) surface is energetically more stable than the N-terminated (111) surface.     

Half-metallic Ferromagnetism in the Ti2FeGe Heusler Compound: A First-Principles Study

In this paper, the structural, electronic and magnetic properties of the Ti₂FeGe Heusler alloy with CuHg₂Ti-type structure have been investigated using first-principles calculations based on density functional theory (DFT). It was predicted that the Ti₂FeGe Heusler alloy is a half-metal with a magnetic moment of 2μ _B per formula unit for the equilibrium lattice parameter. The minority-spin and spin-flip gaps were calculated to be 0.84 and 0.34 eV, respectively. In addition, the band structure and density of states (DOS) were studied. The magnetic moments of Ti(1) and Ti(2) atoms are antiparallel to that of Fe atoms showing ferrimagnetic arrangement. It was noted that the half-metallicity exists within a wide range of the lattice constant (5.54–6.55 Å) which makes the Ti₂FeGe Heusler alloy an interesting material in the field of spintronics.     

First-Principles Study on the Electronic and Magnetic Properties of Zigzag AlN-SiC Nanoribbons

The electronic and magnetic properties of zigzag AlN-SiC nanoribbons are investigated by using the first-principles calculations. The band structures reveal that all the investigated AlN-SiC systems are the magnetic semiconductors, the band gaps of which decrease with the increasing width of the ribbon. The majority spin density is mostly contributed by the edge C atoms with dangling bonds. The total magnetic moments increase with the increasing width of the ribbon and decrease with the increase of the strain. These studies are helpful to the potential applications of the AlN-SiC ribbon in spintronics.     

First-Principles Study of the Electronic Structure of Heavy Fermion YbNi2

We investigate the electronic properties of YbNi₂ by means of band structure calculations based on the density functional theory within LDA (local density approximation), fully relativistic, and LDA+U schemes. The 4f derived bands are studied within a relativistic framework which yields flat and spin-orbit split bands, and a correlated band method (LDA+U) that includes correlation corrections. In both cases, the 4f bands, which is located roughly 200 meV below the Fermi level (E _F ), hybridize weakly with the dispersive Ni-3d bands. When the fully relativistic scheme is applied, the 4f derived bands split into lower and higher bands due to spin-orbit coupling effects. The 4f electrons are delocalized through the hybridization with conduction electrons, and the hybridization between f and conduction d electrons also plays a important role in YbNi₂. The on-site Coulomb potential is added to the Yb-derived 4f orbitals, the degeneracy between the 4f orbitals would be lifted partially and they are split into three manifolds bands. The Fermi surface splits into three different sheets which are from main the Yb-4f derived bands and Ni-3d bands. Band structure calculations reveal a saddle points existence at the L point in the energy dispersion curve closed to E _F , whereby, we think YbNi₂ might have a superconducting properties. In addition, the quasiparticle mass enhancement inferred by comparing γ to the density of states (DOS) at the Fermi level indicates the effective mass of YbNi₂ enhanced with the fully relativistic results.     

First-Principles Study of Lithium and Sodium Atoms Intercalation in Fluorinated Graphite

The structure evolution of fluorinated graphite(CFx) upon the Li/Na intercalation has been studied by firstprinciples calculations. The Li/Na adsorption on single CF layer and intercalated into bulk CF have been calculated. The better cycling performance of Na intercalation into the CF cathode, comparing to that of Li intercalation, is attributed to the different strength and characteristics of the Li-F and Na-F interactions. The interactions between Li and F are stronger and more localized than those between Na and F. The strong and localized Coulomb attraction between Li and F atoms breaks the C—F bonds and pulls the F atoms away, and graphene sheets are formed upon Li intercalation.     

碳原子环对水分子吸附的第一性原理研究

采用基于密度泛函理论的第一性原理,研究了碳原子环对水分子的吸附行为,比较了不同吸附位置下体系能隙、费米能以及吸收光谱的区别.研究结果表明,碳原子环上的水分子吸附只是物理吸附,且水分子吸附位置不同对体系的电学与光学性能产生显著影响.特别是水分子的吸附为放热过程,由于水分子吸附,使体系的能隙以及费米能级发生改变,将影响体系的物理化学性质.此基础上研究了体系的吸收光谱,计算结果发现,体系的吸收峰位置会随水分子吸附位置的改变而发生振荡移动.     

A Comparative First-Principles Study of Fe-, Co- and FeCo-Doped ZnO with Wurtzite and Zinc Blende Structures

First-principles study of the electronic and magnetic properties of zinc-blende and wurtzite structures of Fe-, Co-, and FeCo-doped ZnO is presented. It is found that after doping, this diamagnetic material becomes ferromagnetic and half-metallic. It is also shown that the half-metallicity may be obtained for ZnFeO, ZnCoO, and ZnFeCoO. The analysis of the spin density reveals that the ferromagnetic phase is due to the ferromagnetic coupling between the p?Cd states. The effects of Fe on the magnetic properties of ZB and WZ Fe-doped ZnO compound have been investigated with the GGA calculations. In order to understand the role of Fe atom in the ferromagnetism, the density of states both in the presence and absence of Co doping, were calculated. The obtained results show the presence of coupling between Co and Fe atoms through the spin-split impurity band exchange mechanism. More importantly, the calculations show that the magnetic moment changes sensitively with the type of structure of ZnO, zinc-blende, or wurtzite. A discussion by comparing the results obtained in this study and the experimental results reported in the literature of similar systems show a very good agreement.     

A First-Principles Investigation on Electronic Structure,Optical and Thermoelectric Properties of Janus In2SeTe Monolayer

Journal of Superconductivity and Novel Magnetism - The Janus two-dimensional (2D) materials have recently demonstrated excellent physical and chemical properties. The electronic, thermoelectric,...     

First-Principles Study on Structural,Electronic, and Magnetic Properties of 3d Transition-Metal Nanowires Encapsulated Inside GaN Nanotubes

We have performed the first-principles calculations on the structural, electronic, and magnetic properties of 3d transition-metal nanowires (TMNW₄s), Fe₄, Co₄, and Ni₄, encapsulated inside GaN nanotubes (GaNNTs). The results show that, all three types of the TMNW₄ encapsulated inside the narrower (6,6) and broader (8,8) GaNNTs are both exothermic. The spin polarization and magnetic moment of the TMNW₄@(8,8) systems are larger than those of the TMNW₄@(6,6) systems due to more weak restriction of the broader (8,8) GaNNT, but those of these two combined systems are smaller than those of the corresponding freestanding TMNW₄. Our results reveal that either the TMNW₄@(6,6) or TMNW₄@(8,8) systems have high spin polarization and magnetic moment and stably exist in atmosphere for long time and thus can be expected to have potential applications in building nanodevices.     

有限温度第一原理热力学:关于处理有序相和无序相的观点

第一原理计算一个悬而未决的难题是预测无序相在有限温度下的热力学性能.作者团队指出该难题的最新解决思路是采用可以处理微观组态的配分函数方法,该方法已成为处理只有一种主要微观组态构成的有序相以及有多种明显的微观组态构成的无序相的关键.结合第一原理声子计算和准简谐近似可以有效地预测任意一个给定微观组态的热力学性质.总结了作者团队在第一原理热力学方面的最新研究进展并具体给出了有序相方面的例子:Li2S,hcp Mg和fcc Ni,以及无序相方面的例子:Cu2 ZnSnS4 (CZTS)和fcc Ce.同时指出:①从常用的“相”扩展到“微观组态”开辟了一条定量研究材料相变、热膨胀等异常性能的新途径,而这些异常性能的起源可以追溯到“微观组态构型熵”;②这些微观组态也可以认为是材料基因组的基本组成模块.     

First-Principles Study of Pressure-Induced Structural and Magnetic Phase Transitions of Binary Ferromagnets: MnSn and MnSb

First-principles calculations within density functional theory have been performed to study the structural and magnetic phase transitions of two 3d compounds (MnSn and MnSb) under pressure. We consider the rocksalt (B1), cesium chloride (B2), zinc-blende (B3), nickel arsenide (B8₁) modifications of MnSn and MnSb. Both spin-polarized and nonspin-polarized calculations are carried out for ferromagnetic (FM) and paramagnetic (PM) states, which can be used to distinguish the ground-state magnetic configuration with increasing pressure. The crystal structure preferences and the possible phase transitions among them have been studied. We report FM B8₁?^14GPaFM B2?^145GPaPM B2 phase transition for MnSn, FM B8₁?^23GPaPM B2 for MnSb. Phonon band dispersions, density of phonon states, and elastic stiffness constants are given and used to analyze the lattice dynamical and mechanical stability for the pressure-induced phase. Electronic band structures and density of states at different pressures are given to discuss the detailed electronic and magnetic properties.