Dynamic stability of fcc crystals under isotropic loading from first principles

Lattice dynamics and stability of four fcc crystals (Al, Ir, Pt and Au) under isotropic (hydrostatic) tensile loading are studied from first principles using the linear response method and the harmonic approximation. The results reveal that, contrary to former expectations, strengths of all the studied crystals are limited by instabilities related to soft phonons with finite or vanishing wavevectors. The critical strains associated with such instabilities are remarkably lower than those related to the volumetric instability. On the other hand, the corresponding reduction of the tensile strength is by 20% at the most. An analysis of elastic stability conditions is also performed and the results obtained by means of both approaches are compared.     

Predicting the acidity constant of a goethite hydroxyl group from first principles

Accurate predictions of the acid-base behavior of hydroxyl groups at mineral surfaces are critical for understanding the trapping of toxic and radioactive ions in soil samples. In this work, we apply ab initio molecular dynamics (AIMD) simulations and potential-of-mean-force techniques to calculate the pK(a) of a doubly protonated oxygen atom bonded to a single Fe atom (Fe(I)OH(2)) on the goethite (101) surface. Using formic acid as a reference system, pK(a) = 7.0 is predicted, suggesting that isolated, positively charged groups of this type are marginally stable at neutral pH. Similarities and differences between AIMD and the more empirical multi-site complexation methodology are highlighted, particularly with respect to the treatment of hydrogen bonding with water and proton sharing among surface hydroxyl groups. We also highlight the importance of an electronic structure method that can accurately predict transition metal ion properties for goethite pK(a) calculations.     

Derivation from first principles of the infrared structure of quantum electrodynamics

It is shown how a contribution to the action from a surface at spatial infinity determines the infrared structure of quantum electrodynamics.     

Electronic structure and magnetic properties of rare-earth perovskite gallates from first principles

The density functional calculation is performed for centrosymmetric(La–Pm) GaO_3 rare earth gallates, using a full potential linear augmented plane wave method with the LSDA and LSDA+U exchange correlation to treat highly correlated electrons due to the very localized 4f orbitals of rare earth elements, and explore the influence of U = 0.478 Ry on the magnetic phase stability and the densities of states. LSDA+U calculation shows that the ferromagnetic(FM) state of RGaO_3 is energetically more favorable than the anti-ferromagnetic(AFM) one, except for LaGaO_3 where the NM state is the lowest in energy. The energy band gaps of RGaO_3 are found to be in the range of 3.8–4.0 eV, indicating the semiconductor character with a large gap.     

Electronic structure and optical properties of TaC from the first principles calculation

The electronic and optical properties of tantalum carbide TaC have been calculated using the full-potential linearized augmented-plane-wave method within the local density approximation scheme for the exchange-correlation potential. We find that the optical spectra can be extremely sensitive to the Brillouin zone sampling. The influence of relativistic effects on the dielectric function is investigated. It is shown that the scalar-relativistic correction is much more important than spin-orbit coupling. Our results are found to be in good agreement with the available experimental data. The determinant role of a band structure computation with respect to the analysis of optical properties is discussed.     

Effective Hamiltonians from first principles

The equations of motion for a quantum system of the electromagnetic field interacting with matter are derived from the first principle Hamiltonian (two-level atomic systems are assumed) and from the effective Hamiltonian. The derivation is based on the memoryfunction theory of Mori and on the theory of random frequency modulation of Tokuyama and Mori. On the ground of comparison of the equations of motion from the first principle Hamiltonian with those from the effective Hamiltonian a justification of the use of effective Hamiltonians is discussed.     

Generalized entropies from first principles

A derivation of power law canonical distributions from first principle statistical mechanics, including the exponential distribution as a particular case is presented. It is shown that these distributions arise naturally, and that the heat capacity of the heat bath is the condition that determines its type. As a consequence, a physical interpretation for the parameter q of the generalized entropy is given.     

Thermoelasticity of CaO from first principles

The thermoelastic properties of CaO over a wide range of pressure and temperature are studied using density functional theory in the generalized gradient approximation. The transition pressure taken from the enthalpy calculations is 66.7GPa for CaO, which accords with the experimental result very well. The athermal elastic moduli of the two phases of CaO are calculated as a function of pressure up to 200GPa. The calculated results are in excellent agreement with existing experimental data at ambient pressure and compared favourably with other pseudopotential predictions over the pressure regime studied. It is also found that the degree of the anisotropy rapidly decreases with pressure increasing in the B1 phase, whereas it strongly increases as the pressure increases in the B2 phase. The thermodynamic properties of the B1 phase of CaO are predicted using the quasi-harmonic Debye model; the heat capacity and entropy are consistent with other previous results at zero pressure.     

Predicting spinor condensate dynamics from simple principles

We study the spin dynamics of quasi-one-dimensional F=1 condensates both at zero and finite temperatures for arbitrary initial spin configurations. The rich dynamical evolution exhibited by these nonlinear systems is explained by surprisingly simple principles: minimization of energy at zero temperature and maximization of entropy at high temperature. Our analytical results for the homogeneous case are corroborated by numerical simulations for confined condensates in a wide variety of initial conditions. These predictions compare qualitatively well with recent experimental observations and can, therefore, serve as a guidance for ongoing experiments.     

CoO under pressure from first principles

Density functional theory and the generalized gradient approximation with correction for Hubbard energy was used to study the behavior of cobaltous oxide (CoO) under pressure. CoO undergoes an insulator-metal transition which is accompanied by a magnetic collapse. The antiferromagnetic phase of CoO transforms to nonmagnetic phase with the 6-7% reduction in the fractional volume. The magnetic collapse and the energy band gap closure are driven by the lost of correlation which results from the delocalization of 3d electrons. Delocalization process is due to the band broadening with compression. The Hubbard energy influences the transitions pressure. The lower Hubbard terms result in the lower values of transition pressure. The evolution of magnetic moment, energy band gap, and the bandwidth versus increasing pressure is analyzed. The results of calculations are compared to the existing theoretical and experimental data.     

Exploring at nanoscale from first principles

Systems at the nanoscale can exhibit distinctive and unexpected properties in electrical, magnetic, mechanical, and chemical aspects. Understanding these properties not only is of importance from the fundamental scientific view but also offers great opportunities for future applications. Theoretical calculations can provide important information to interpret, modify, and predict the novel properties of objects at the nanoscale and therefore play a significant role in the process of exploring the nano world. In this review, six different areas are briefly presented, namely, prediction of new stable structures, modification of properties (especially the electronic structures), design of novel devices for applications, the structures and catalytic effects of clusters, the mechanical and transport properties of gold nanowires, and improvement of materials for hydrogen storage. Based on these examples, we show what can be done and what can be found in the investigations of nanoscale systems with participation of theoretical calculations.      

The structural, elastic and thermodynamical properties of zinc-blend structure InN from first principles

A theoretical study of the structural, elastic and thermodynamic properties of the cubic zinc-blende (ZB) structure InN are presented in this paper by performing first principles calculations within local density approximation. The values of lattice constant, bulk modulus and its pressure derivatives and elastic constants are in excellent agreement with the available experimental data and other theoretical results. It is found that the ZB structure InN should be unstable above 20 GPa mechanically. The pressure and temperature dependencies of the bulk modulus, the heat capacity and the thermal expansion coefficient and the entropy S, as well as the Grüneisen parameter are obtained by the quasi-harmonic Debye model in the ranges of 0-1500 K and 0-25 GPa.     

Electronic structure of EuPtSi3 studied from the first principles calculation

We investigate the electronic structure of EuPtSi₃ using full potential linearized augmented plane wave method within the generalized gradient approximation. We reproduce the observed easy axis of magnetization as well as magnetic moment and propose that the magnetic configuration for this compound is incommensurate or noncollinear. We find that the spin configuration has only negligible effect on the band around Fermi energy. EuPtSi₃ and its isostructural superconductor BaPtSi₃ have similar band structures. Despite the shorter Pt-Si bond, EuPtSi₃ possesses larger density of state near the Fermi energy compared with BaPtSi₃, thus the structural difference and the associated difference in band structure cannot explain the non-superconductivity of EuPtSi₃.     

分子线的电子输运特性

从第一性原理出发利用密度泛函理论计算了共扼分子2氨基5硝基1,4二乙炔基4′苯硫醇基苯(2amino5nitro1,4diethyny4′benzenethiolbenzene)的电子结构,并利用前线轨道理论和微扰理论定量地确定了该分子与金表面的相互作用能常数.然后,利用弹性散射格林函数方法研究了该分子与金表面形成的分子线的伏安特性.计算结果表明,分子通过硫氢官能团可以很强地吸附于金表面上,形成以共价键为主的混合键,从而为电子的转移提供了通道.当外加偏压很低时,分子线的电流存在禁区,禁区的宽度约08eV. 关键词： 化学吸附 分子线 分子电子学     

Anisotropic spin Hall effect from first principles

We report on first principles calculations of the anisotropy of the intrinsic spin Hall conductivity (SHC) in nonmagnetic hcp metals and in antiferromagnetic Cr. For most of the metals of this study we find large anisotropies. We derive the general relation between the SHC vector and the direction of spin polarization and discuss its consequences for hcp metals. Especially, it is predicted that for systems where the SHC changes sign due to the anisotropy the spin Hall effect may be tuned such that the spin polarization is parallel either to the electric field or to the spin current.     

Superradiance from crystals of molecular nanomagnets

We show that crystals of molecular nanomagnets can exhibit giant magnetic relaxation due to the Dicke superradiance of electromagnetic waves. Rigorous theory is presented that combines superradiance with the Landau-Zener effect.     

Electronic structure and optical properties of rutile RuO2 from first principles

The systematic trends of electrionic structure and optical properties of rutile (P42 /mnm) RuO2 have been calculated by using the plane-wave norm-conserving pseudopotential density functional theory (DFT) method within the generalised gradient approximation (GGA) for the exchange-correlation potential.The obtained equilibrium structure parameters are in excellent agreement with the experimental data.The calculated bulk modulus and elastic constants are also in good agreement with the experimental data and available theoretical calculations.Analysis based on electronic structure and pseudogap reveals that the bonding nature in RuO2 is a combination of covalent,ionic and metallic bonds.Based on a Kramers-Kronig analysis of the reflectivity,we have obtained the spectral dependence of the real and imaginary parts of the complex dielectric constant (ε1 and ε2,respectively) and the refractive index (n);and comparisons have shown that the theoretical results agree well with the experimental data as well.Meanwhile,we have also calculated the absorption coefficient,reflectivity index,electron energy loss function of RuO2 for radiation up to 30 eV.As a result,the predicted reflectivity index is in good agreement with the experimental data at low energies.