Ti2SiC在高压下结构、弹性和电子性质的第一性原理研究

采用第一性原理方法,研究了Ti_2SiC在高压下的结构、弹性和电子性质。结果表明,随着外压的增大,Ti_2SiC的晶格常数a、c和体积V均减小,且a比c减小幅度更大,表明Ti_2SiC在a轴方向比c轴方向更容易被压缩,体现了该材料的各向异性。计算分析了Ti_2SiC的弹性常数、体模量、剪切模量、杨氏模量、泊松比等弹性性质,这些弹性性质均随着外压的增加而增大,并根据弹性常数证明了Ti_2SiC在0~50GPa范围内均是力学稳定的。此外,还从电子态密度的角度考察了Ti_2SiC的电子性质,认为其具有共价键和金属键的双重性质,并发现在0~50GPa范围内压力对Ti_2SiC的态密度性质影响较小。     

高压强下GaN的结构相变及电子结构的第一性原理研究

基于密度泛函理论的平面波赝势方法,选择广义梯度近似(GGA)下的PBE算法-关联泛函对GaN晶体结构、能带结构以及电子态密度随压强的变化进行了研究,并计算出GaN材料的相变点压强值。研究结果表明:随着压强增加,常见的纤锌矿与闪锌矿GaN会发生结构相变成岩盐矿结构,并且其能带结构均由直接带隙转变成间接带隙。其中,通过焓相等原理得到纤锌矿到岩盐矿结构的相变压强为44.4GPa,而闪锌矿到岩盐矿结构的相变压强为43.6GPa。此外,随着压强增大,GaN纤锌矿、闪锌矿和岩盐矿的价带态密度均向低能方向偏移,而导带态密度向高能方向偏移,从而导致GaN共价性增强及带隙随压强增大而展宽。     

Zinc-Blende MnTe Under Pressure: Structural,Mechanical, and Optical Properties from Ab Initio Calculation

We present first-principles studies of structural, mechanical, and optical properties of zinc blende MnTe using the pseudopotential plane-wave method within the local density approximation. The effect of hydrostatic pressure on investigated properties has been examined and discussed. At zero pressure, our results are found to agree reasonably well with those reported in the literature. The generalized elastic stability criteria showed that the material of interest is mechanically stable in all the studied pressure ranges. Applied pressure is found to shift all optical spectra under consideration, giving new optical parameters.     

高压下Zr2AlC电子结构的第一性原理研究

通过基于密度泛函理论的第一性原理方法,计算了三元层状化合物Zr2AlC在不同压力下的电子结构。在0~40GPa的压力范围内,Zr2AlC沿c轴的压缩性较a轴更容易,晶格的内坐标z随压力的增加而增加。对能带结构和电子态密度分析表明,能带随着压力的增加将会展宽,费米能级处的电子态密度减少,金属性减弱。在Zr2AlC中还表现出一定共价键和离子键特性,其中共价性随压力的增加而增强。     

First Principles Study of Electronic and Magnetic Properties of DyNiPb and YNiPb Compounds Under Hydrostatic Pressure

Density functional theory calculations have been performed to study the electronic and magnetic properties of DyNiPb and YNiPb compounds. The total energy calculations indicate that at zero pressure the ferromagnetic and nonmagnetic phases are the most stable phases of the DyNiPb and YNiPb compounds respectively. The Pb atom has a negligible contribution to the magnetic properties of DyNiPb compared to the Dy and Ni atoms. The effect of hydrostatic pressure on the magnetic moment of these compounds is also studied. The calculated results show that the YNiPb compound has zero magnetic moment over the whole pressure range considered (p<20 GPa), while in the DyNiPb compound the total and local magnetic moments at the Dy and Ni atomic sites increase with increasing pressure. Furthermore, I have investigated the electric field gradient (EFG) at different atomic sites of these compounds. The contribution of different orbitals to the EFG shows that the strongest anisotropy in the charge distribution is due to the electrons in the p orbitals. The variation of EFG at different atomic positions of the DyNiPb and YNiPb compounds with pressure is not linear and increases with pressure.     

Electronic and Structural Properties of Oxygen-Doped BN Nanotubes

The structural and electronic properties of oxygen substitutional doping in a (10,0) BN nanotube were obtained using ab initio calculation based on the density functional theory. For the oxygen replacing a boron atom in the tube$(hboxO_ B)$, the structure is locally deformed. In the case of nitrogen substitution$(hboxO_ N)$, however, the tube structure remains practically the same with negligible deformation observed around the oxygen atom. The electronic band structure for$hboxO_ B$nanotubes is modified by the appearance of three strongly localized states, two of then as gap states. In the case of$hboxO_ N$nanotubes the Fermi level is shifted into the conduction band inducing a metallic character to the doped tube. The analysis of the formation energies shows that the$hboxO_ N$substitution is more favorable, particularly in the case of a boron-rich environment.     

Improved Properties of Polymers Under Hydrostatic Pressure

The role of hydrostatic pressure in strengthening of polymer materials was studied. It was established that the orientation crystallization of fibers and films in a high-pressure chamber leads to significant increase of their mechanical properties. The optimal temperature and pressure for the crystallization of polyethylene were determined, providing a static strength of 490 MPa, that is higher than the strength of a type 3 steel. A strength of 100 MPa was achieved for fluorinoplast-4 oriented under pressure at a stretch ratio of about 6. This opens the possibility of obtaining high strength fibers and fabrics using this material.     

Extremely Anisotropic Thermoelectric Properties of SnSe Under Pressure

SnSe has attracted extensive attention due to its ultralow thermal conductivity and excellent thermoelectric properties.In this work,pressureinduced thermoelectric properties of Pnma SnSe are investigated via firstprinciples calculations.We uncover distinct energy isosurfaces topology transition of conduction band by applying pressure.The newly created conduction band valley caused by pressure has a distinct anisotropic shape compared to the old one.Inducing pressure can greatly enhance the anis...     

Theoretical Investigation of Structural,Electronic, and Magnetic Properties of V-Doped MgSe and MgTe Semiconductors

In this study, we have explored the structural, electronic, and magnetic properties of V-doped zincblende MgSe and MgTe compounds using density functional calculations. The Wu-Cohen generalized gradient approximation is used for optimizing the structural properties, while the modified Becke and Johnson local (spin) density approximation functional has been employed to compute the electronic and magnetic properties. The spin dependent band structures, electronic density of state, and magnetic moments calculated for V-doped MgSe and MgTe semiconductors exhibit occurrence of 100 % spin polarization at the Fermi level which confirms stable half-metallic ferromagnetism in these materials. The spin-down gaps and the half-metallic gaps are analyzed in terms of V-3d and Se-4p (Te-5 p) hybridization, where it is observed that the V-3dstates play a key role in generating spin polarization and the magnetic moment in these compounds. The exchange constants N ₀ αand N ₀ β have been calculated to demonstrate the effects resulting from exchange splitting process. Furthermore, spin-polarized charge density calculation is presented for elucidating the bonding nature, while pressure dependence of total magnetic moment for three concentrations of V-doped MgSe and MgTe are also discussed.     

First-Principles Calculations of Structural,Electronic, Optical,and Thermoelectric Properties of LuNiBi and LuNiSb Half-Heusler

Journal of Superconductivity and Novel Magnetism - The structural, electronic, optical, and thermo-electric properties of LuNiBi and LuNiSb Half-Heusler have been studied using a full potential...     

Ti2GeC电子结构和弹性性质的第一性原理研究

采用第一原理的方法研究了Ti2GeC的电子结构和弹性性质.从电子结构中可以看出,Ti2GeC中存在着共价键、离子键和金属键3种键.在态密度和Mulliken布居分析中有赝能隙和电荷转移.还得到了体模量、杨氏模量、切变模量、泊松比和德拜温度等物理参数.各向异性参数表明了Ti2GeC在压缩和剪切上主要呈现出各向同性.     

High Pressure Effects on Electronic and Magnetic Properties of LaOFeAs Superconductor

The effect of pressure has been studied on structural and electronic properties of LaOFeAs high-T _c superconductor by ab initio density functional theory by using pseudopotential Quantum Espresso code. The lattice parameters and ionic positions in the ambient pressure and some high pressure up to 20 GPa have been calculated. The obtained data versus the simple scaling relation for the ionic positions and distances for mechanical pressures have been discussed. The results of band structure and magnetic moment calculations of this compound versus the applied pressure are presented in this paper. The results are compared with the other experimental and computational data in the literature.     

First-Principles Study of the Structural,Optical, Dynamical and Thermodynamic Properties of BaZn$$\hbox {O}_{2}$$ Under Pressure

The structural, optical, dynamical, and thermodynamic properties of BaZn\(\hbox {O}_{2}\) under pressure are studied based on the density functional theory. The calculated structural parameters are consistent with the available experimental data. In the ground state, the electronic band structure and density of states indicate that BaZn\(\hbox {O}_{2}\) is an insulator with a direct gap of 2.2 eV. The Mulliken charges are also analyzed to characterize the bonding property. After the structural relaxation, the optical properties are studied. It is found that the dielectric function of E \(\Vert x\hbox { and }E\Vert y\) are isotropic, whereas the \(E\Vert x\) and \(E\Vert z\) are anisotropic. The effect of pressure on the energy-loss function in the ultraviolet region becomes more obvious as the pressure increases. Furthermore, the dynamical properties under different pressures are investigated using the finite displacement method. We find that the \(P3_{1}21\) phase of \(\hbox {BaZn}\hbox {O}_{2}\) is dynamically stable under the pressure ranging from 0 GPa to 30 GPa. The phonon dispersion curves, phonon density of states, vibrational modes and atoms that contribute to these vibrations at \({{\varvec{\Gamma }}}\) point under different pressures are also reported in this work. Finally, by employing the quasi-harmonic approximation, the thermodynamic properties such as the temperature dependence of the thermal expansion coefficient, specific heat, entropy and Gibbs free energy under different pressures are investigated. It is found that the influences of the temperature on the heat capacity are much more significant than that of the pressure on it.     

Theoretical Investigation of the Phase Transition,Elastic and Superconductivity Properties of LaS Under Pressure

The first-principles method has been performed to investigate the phase transition, elastic, thermodynamic and superconductivity properties of lanthanum monosulphide (LaS) under pressure. A structural phase transition from the NaCl-type (B₁) to CsCl-type (B₂) structure is found to occur at around 16.8 GPa. The calculated ground state properties such as lattice constants, bulk modulus, and Debye temperatures are in good agreement with experimental data. Finally, the pressure dependence of the theoretical elastic constants and elastic modulus of LaS has been studied. The observations show that LaS is mechanically stable not only in B₁ phase below 8.77 GPa but also in B₂ phase under high pressure. LaS is ductile in B₁ phase while brittle in B₂ phase. The present observation of physical properties in B₂ phase of LaS needs validation by future experimental.     

Heusler合金Co2MnAl1-xGex的电子结构、磁性和半金属性

基于密度泛函理论(DFT),使用广义梯度近似(GGA)和局域密度近似(LDA)研究了Heusler合金Co2MnAl1-xGex(x=0、0.25、0.5、0.75、1)的磁性和电子结构。随着掺杂浓度x的增加,合金的晶格常数和磁矩都线性增加,分别很好地满足Vegard和S-P规律。对于整个合金系列,随x的增加,自旋向下带带隙宽度略有减少,费米面向高能移动。由于Co2MnAl0.5Ge0.5合金的费米面恰好居于其自旋向下带带隙中部,此时合金具有最好的半金属稳定性,预期在磁隧道结器件中能实现高的自旋极化率。     

SrS（Cu,Sm）的红外激励上转换发光及光存储特性

本文首次报道了ＳｒＳ（Ｃｕ，Ｓｍ）的红外激励上转换发光及光存储特生，给出了二种典型样品的光致发光的激光光谱，测定红外激励发光和激励光谱，结果表明ＳｒＳ９Ｃｕ，Ｓｍ）具有极好的可擦除可重写的光存储功能，其最长的写入光波段在３５０ｎｍ附近，读出光波长约在８００－１４００ｎｍ附近，发射兰绿光峰波长在５１５ｎｍ附近。     

Ab Initio,Mean Field and Series Expansions Calculations Study of Structural,Electronic and Magnetic Properties of MnAs

The self-consistent ab initio calculations, based on density functional theory (DFT) approach and using full-potential linear augmented plane wave (FLAPW) method, are performed to investigate both electronic and magnetic properties of the MnAs compounds. Polarized spin and spin-orbit coupling are included in calculations within the framework of the ferromagnetic state between two adjacent Mn atoms. The ferromagnetic and antiferromagnetic energies of MnAs systems are obtained. Magnetic moment considered to lie along (001) axes is computed. The obtained data from ab initio calculations are used as input for the high-temperature series expansion (HTSE) calculations to compute other magnetic parameters. The exchange interactions between the magnetic atoms Mn-Mn in MnAs are given using the mean field theory. The HTSEs of the magnetic susceptibility with the magnetic moments in MnAs (m _Mn) through the Ising model is given up to the tenth order series in x=J ₁(Mn-Mn)/ k _B T. The Néel temperature T _C is obtained by HTSEs of the magnetic susceptibility series combined with the Padé approximant method. The critical exponent γ associated with the magnetic susceptibility is deduced as well.     

First-Principles Investigation of Thermophysical Properties of Cubic ZrC Under High Pressure

With a motivation to understand the effect of pressure on the thermophysical properties of a transition metal carbide, zirconium carbide (ZrC) in rock salt (RS) and CsCl phases, a systematic study of the thermodynamic functions with pressure for ZrC in both phases is performed. First-principles theoretical calculations are used, based on density functional perturbation theory within the generalized gradient approximation and with a quasi-harmonic approximation. The results demonstrate that the free and internal energies are greater while the specific heat at constant volume and entropy is smaller for the RS phase of ZrC than for the CsCl phase. The pressure significantly affects the thermodynamic functions. The results also demonstrate that the effect of increasing pressure on ZrC is the same as that of decreasing temperature. The k-point convergence of the phonon spectrum at zero pressure is also analyzed, which may be useful for investigating the pressure-induced dynamic instabilities in the transition metal carbide, ZrC.     

基于第一性原理研究Heusler合金Fe2CuGa的结构和性能

采用基于密度泛函理论的第一性原理,系统研究了Heusler合金Fe2CuGa的结构、磁性、弹性性能和电子性质.计算结果表明:立方相的基态结构是铁磁态的Hg2CuTi结构.立方到四方的相变几乎是体积不变的,这是形状记忆合金的特性.奥氏体和马氏体的磁矩分别是4.48和4.56μB/f.u..另外,预测了Fe2CuGa的弹性系数.Fe2CuGa的立方结构在力学上是不稳定的而四方结构是稳定的.根据体模量和剪切模量的比值,发现Fe2CuGa在本质上是可延展的.利用态密度的方法解释了Fe2CuGa马氏体相变的来源.