Yu Ben-Hai

Nonlinear optical behaviors in a silver nanoparticle array at different wavelengths

The optical nonlinearities of an Ag nanoparticle array are investigated by performing Z-scan measurements at the selected wavelengths (400, 600, 650, and 800 nm). The nonlinear refraction index in the resonant region (around 400 nm) exhibits a significant enhancement by two orders compared with that in the off-resonant region (around 800 nm)), and exhibits an sign alternation of the resonant nonlinear absorption, which results in a negligible nonlinear absorption at a certain excitation intensity. Moreover, a low degree of nonlinear absorption was measured at the edges of the resonant region (600 and 650 nm), which is attributed to the competition of the saturated absorption and the two-photon absorption processes.

Predictions of pressure-induced structural transition, mechanical and thermodynamic properties of α- and β-Si3N4 ceramics: ab initio and quasi-harmonic Debye modeling

The plane-wave pseudo-potential method within the framework of ab initio technique is used to investigate the structural and elastic properties of α- and β-Si3N4. The ground-state parameters accord quite well with the experimental data. Our calculation reveals that α-Si3N4 can retain its stability to at least 40 GPa when compressed at 300 K. The α → β phase transformation would not occur in a pressure range of 0–40 GPa and a temperature range of 0–300 K. Actually, the α → β transition occurs at 1600 K and 7.98 GPa. For α- and β-Si3N4, the c axes are slightly more incompressible than the a axes. We conclude that β-Si3N4 is a hard material and ductile in nature. On the other hand, β-Si3N4 is also found to be an ionic material and can retain its mechanical stability in a pressure range of 0–10 GPa. Besides, the thermodynamic properties such as entropy, heat capacity, and Debye temperature of α- and β-Si=N4 are determined at various temperatures and pressures. Significant features in these properties are observed at high temperature. The calculated results are in good agreement with available experimental data and previous theoretical values. Many fundamental solid-state properties are reported at high pressure and high temperature. Therefore, our results may provide useful information for theoretical and experimental investigations of the Si3N4 polymorphs.

First-principles calculations of elasticity and thermodynamic properties of LaNi5 crystal under pressure

This paper investigates the equilibrium lattice parameters, heat capacity, thermal expansion coefficient, bulk modulus and its pressure derivative of LaNi5 crystal by using the first-principles plane-wave pseudopotential method in the GGA-PBE generalized gradient approximation as well as the quasi-harmonic Debye model. The dependences of bulk modulus on temperature and on pressure are investigated. For the first time it analyses the relationships between bulk modulus B and temperature T up to 1000 K, the relationship between bulk modulus B and pressure at different temperatures are worked out. The pressure dependences of heat capacity Cv and thermal expansion α at various temperatures are also analysed. Finally, the Debye temperatures of LaNi5 at different pressures are successfully obtained. The calculated results are in excellent agreement with the experimental data.

First-principles calculation of elastic and thermodynamic properties of Ni2MnGa Heusler alloy

The equilibrium lattice parameter, heat capacity, thermal expansion coefficient and bulk modulus of Ni2MnGa Heusler alloy are successfully obtained using the first-principles plane-wave pseudopotential (PW-PP) method as well as the quasi-harmonic Debye model. We analyse the relationship between bulk modulus B and temperature T up to 800 K and obtain the relationship between bulk modulus B and pressure at different temperatures. It is found that the bulk modulus B increases monotonically with increasing pressure and decreases with increasing temperature. The pressure dependence of heat capacity Cv and thermal expansion α at various temperatures are also analysed. Finally, the Debye temperature of Ni2MnGa is determined from the non-equilibrium Gibbs function. Our calculated results are in excellent agreement with the experimental data.

Pressure-induced phase transition in silicon nitride material

The equilibrium crystal structures, lattice parameters, elastic constants, and elastic moduli of the polymorphs α-, β-, and γ-Si3N4, have been calculated by first-principles method. β-Si3N4 is ductile in nature and has an ionic bonding. γ-Si3N4 is found to be a brittle material and has covalent chemical bonds, especially at high pressures. The phase boundary of the β → γ transition is obtained and a positive slope is found. This indicates that at higher temperatures it requires higher pressures to synthesize γ-Si3N4. On the other hand, the α → γ phase boundary can be described as P = 14.37198 + 3.27 × 10−3T−7.83911 × 10−7T2−3.13552 × 10−10T3. The phase transition from α- to γ-Si3N4 occurs at 16.1 GPa and 1700 K. Then, the dependencies of bulk modulus, heat capacity, and thermal expansion on the pressure P are obtained in the ranges of 0 GPa–30 GPa and 0 K–2000 K. Significant features in these properties are observed at high temperatures. It turns out that the thermal expansion of γ-Si3N4 is larger than that of α-Si3N4 over wide pressure and temperature ranges. The evolutions of the heat capacity with temperature for the Si3N4 polymorphs are close to each other, which are important for possible applications of Si3N4.

First-Principles Calculations of Elastic Properties of LaNi5 Compound

The equilibrium lattice constants, temperature dependence of bulk modulus, the pressure dependence of the normalized volume V/V0, elastic constants Cij, and bulk modulus of LaNi5 crystal are obtained using the first-principles plane-wave pseudopotential method in the GGA-PBE generalized gradient approximation as well as the quasi-harmonic Debye model. We analyse the relationship between bulk modulus and temperature up to 1000 K and obtain the relationship between bulk modulus B and pressure at different temperatures. It is found that the bulk modulus B increases monotonously with increasing pressure. Moreover, the pressure dependences of Debye temperatures and the pressure derivatives of lattice constants are also successfully obtained. The calculated results are in agreement with the experimental data and the other theoretical results.

First-Principles Calculations of Elastic Properties of Cubic Ni2MnGa

Dependence of bulk modulus on both pressure and temperature, the elastic constants Cij and the pressure and temperature dependence of normalized volume V/V0 of cubic Ni2MnGa alloy are successfully obtained using the first-principles plane-wave pseudopotential (PW-PP) method as well as the quasi-harmonic Debye model. We analyse the relationship between bulk modulus and temperature up to 800 K and obtain the relationships between bulk modulus B and pressures at different temperatures. It is found that the bulk modulus B increases monotonically with increasing pressure. Moreover, the temperature dependences of the Debye temperature are also analysed. The calculated results are in agreement with the available experimental data and the previous theoretical results.

Role of Nuclear Coulomb Attraction in Nonsequential Double Ionization of Argon Atom

The microscopic recollision dynamics in strong-field nonsequential double ionization of Ar atoms is investigated using three-dimensional classical ensembles. By adjusting the nuclear Coulomb potential, we can excellently reproduce the experimental results both within the laser intensity regimes well above the recollision threshold and well below the recollision threshold quantitatively. More importantly, our trajectory analysis clearly reveals the particular electronic dynamics in recollision process: the momentum of the recolliding electron encounters a sudden change both in magnitude and in direction when it approaches the nucleus closely, which show that the nuclear Coulomb attraction plays a key role in the recollision process of nonsequential double ionization of Ar atoms.

First-Principles Calculations of Elastic Properties of LaNi4.75Sn0.25 Alloys under Pressure

The equilibrium lattice constants, bulk modulus, shear modulus, elastic constants and Debye temperature of LaNi4.75Sn0.25 under pressure are calculated using the full-potential linearized augmented plane wave (FP-LAPW) method as well as the quasi-harmonic Debye model. The results at zero pressure are in excellent agreement with the experimental data. The Sn atom is found to occupy the equivalent 3g site (0.5a, 0.75b, 0.5c) in the quadruple cell. The Debye temperature of LaNi4.75Sn0.25 is lower than that of LaNi5. The dependences of bulk modulus on finite temperature and on finite pressure are also investigated. The results show that the bulk modulus B increases monotonously as pressure increases.

Theoretical Study of Compressibility and Thermoelasticity of LaNi5−x Alx, (x = 0.3, 0.5 and 1.0)

The compressibility, the temperature dependence of bulk modulus, the pressure dependence of normalized volume V/V0, thermal expansion coefficient and Debye temperature of LaNi5-x Alx compounds are successfully obtained using the first-principles plane-wave pseudopotential (PW-PP) method, the EOSFIT6.0 software and the quasiharmonic Debye model. The rapid decrease of relative lattice constant a/a0 shows that the deformation is easier in directions normal to the c-axis than that along it. The relationships between bulk modulus B and pressure at different temperatures are also analysed. It is found that the bulk modulus B increases monotonically with increasing pressure. Moreover, the pressure dependences of thermal expansion and Debye temperature are also successfully obtained. The calculated results are in agreement with the experimental data.