Ab Initio Transport Coefficients of Gaseous Hydrogen

The spherical version of the hydrogen intermolecular potential ${\phi_{\rm P}}$ recently determined in ab initio calculations by Patkowski et al. was used to calculate the viscosity and thermal conductivity of hydrogen using a full quantum-mechanical formalism. Viscosities in the temperature range 203 K to 394 K were compared with recent high-accuracy (uncertainty of 0.084 %) measurements of May et al. The measured viscosities all fall in a range between 0.02 % and 0.06 % below the calculated viscosities. This close agreement supports the accuracy of ${\phi_{\rm P}}$ . Classical calculations of the viscosity with ${\phi_{\rm P}}$ fall in a range between 0.4 % and 1.3 % below the experimental values. In the lower temperature range 20 K to 300 K, other measurements typically lie above the theoretical values by a few percent. Above 400 K, measurements fall below the theoretical values by a fraction that increases with temperature, reaching ?4% at 2000 K. For normal hydrogen, the average fractional difference between the calculated thermal conductivity in the temperature range 21 K to 384 K and measurements reported in six publications is (0.1 ± 1.1) %. For para-hydrogen in the temperature range 20 K to 275 K, the average fractional difference between calculations and measurements reported in three publications is (?0.7 ± 1.2) %. At higher temperatures (600 K to 2000 K), measurements range between 4 % and 10 % below the calculated values.     

Ferromagnetism in Mo-doped TiO2 Rutile from Ab Initio Study

First-principles calculations based on spin density functional theory (DFT) within the general gradient approximation (GGA) are performed to study the spin-resolved electronic properties of TiO₂ Rutile doped with 6.25 and 12.5% of Mo. The Mo impurity is found spin polarized and the calculated band structures suggest a 100% polarization of the conduction carriers. The local moment of Molybdenum is slightly dependent on its concentration. The Fermi level is shifted to the bottom of the conduction band with increasing concentration of Mo. This leads to the increase of states density, just above the Fermi level, and consequently the 4d orbitals are strongly hybridized with Ti 3d ones to form a d-nature conduction band, without impurity states in the in-gap region. The Mo-doped TiO₂ favors ferromagnetic ground state which can be explained in terms of p–d hybridization mechanism for 6.25% of Mo, this mechanism depends on the Mo concentration. This suggests that the Mo-doped TiO₂ is a promising dilute magnetic semiconductor and may find applications in the field of spintronics.     

Tunable Magnetic Interaction of Co-Doped SiC Monolayer Under Electric Field: Ab Initio Study

The magnetic properties of Co-doped silicon carbide monolayer under an external electric field are investigated using a first-principles method. In the absence of the electric field, magnetism is observed for both Si (Co_Si) and C (Co_C) sites. Then, the interaction between two Co atoms has been studied in the Co_Si system. Both antiferromagnetic (AFM) and ferromagnetic (FM) states have been found. The results show that the FM behavior is originated by the p–d hybridization between Co and its neighboring C atoms. When an electric field was introduced along the c-axis, the interaction between two Co dopants switched from FM to AFM, which could be dominated by the competition between p–d exchange and superexchange. Moreover, the magnetic anisotropy (MA) prefers to parallel to the a-axis and seems not to be turned into the c-axis under the electric field.     

Ab Initio Study of Curie Temperatures of Diluted Magnetic Semiconductors

The Curie temperature of diluted (Ga,Mn)As magnetic semiconductors in the presence of As antisites is studied from first principles. We map total energies associated with rotations of Mn-magnetic moments onto the effective classical Heisenberg Hamiltonian which is treated in the mean-field approximation to find the Curie temperature. The presence of donors strongly reduces the Curie temperature and gives rise to a ground state with a partial disorder of local moments. We show that the observed dependence of the Curie temperature on the Mn concentration indicates that the concentration of As antisites increases with the Mn content.     

过渡金属掺杂金红石相TiO2能带结构的第一性原理计算

本文采用第一性原理能带计算方法和超晶胞模型计算金红石相TiO2掺杂过渡金属元素的电子结构.计算结果表明,Zn掺杂对TiO2的带隙宽度影响不明显,V、Cr、Mn、Fe、Co、Ni、Cu的掺杂都有可能使TiO2吸收带出现红移现象或产生在可见光区的吸收,其中杂质原子的t2g态起了重要作用.     

Accurate Ab Initio Calculation of Molecular Constants

Molecular constants have been computed for the ground states ²∏ of ¹⁷OH and ¹Σ of ¹⁰⁷AgH⁺. The valence-bond method and advanced computational technique were used to perform all-electron ab initio calculation of molecular electronic structures. The basic idea behind the model is to introduce the molecular wave functions in terms of Hartree-Fock many-electron atomic determinants. Full configuration interaction (CI) with nonorthogonal basis leads to the accurate calculation of molecular constants such as dissociation energy, equilibrium bond distance, vibrational and rotational constants with an agreement to the experimental data within a few percent.     

Ab Initio Study of Electronic and Magnetic Properties of Germanene with Different Nonmagnetic Metal Adatoms

Electronic and magnetic properties of two-dimensional (2D) germanene (Ge) with five different adatoms have been analyzed and discussed by the DFT method. Different nonmagnetic metals adsorbed at different sites, on one hand, a magnetic moment is induced by Mg adatom in the germanene. On the other hand, the adsorptions of Al, Ga, Li, and Na show no magnetism. We further study the effect of strain on the magnetism in Mg-adsorbed germanene; we apply an isotropic tensile and compressive strain on the system. On the basis of our calculations, a tunable magnetism shows as the strain increases. The analysis of the PDOS shows that the s–p hybridization between Mg and Ge atoms results in such magnetic behavior. These magneticadsorbed germanene materials might show potential applications in the nanoscale devices.     

Ab Initio Study of Magnetism in III-V- and II-VI-Based Diluted Magnetic Semiconductors

Electronic structure and magnetic properties of Ga_1–x Mn _x As, Ga_1–x Mn _x N, Zn_1–x M _x O, and Zn_1–x M _x Te (M=V, Cr, Mn, Fe, and Co) diluted magnetic semiconductors (DMS) are calculated by the tight-binding LMTO method in the 64-atom supercell. Calculations are made at several x with varied spatial distribution of dopant atoms and codoping of DMSs. The results show that stability of the ferro- and antiferromagnetic (FM and AFM) states in DMSs strongly correlates with the occupation and energy position of 3d-dopant bands. Adequacy of the double exchange and superexchange mechanisms for explanation of the FM vs. AFM competition is discussed.     

Study of Interatomic Potentials in ZnS—Crystal-GRID Experiments Versus Ab Initio Calculations

Crystal-GRID measurements have been performed with ZnS single crystals. For the first time, an asymmetric Crystal-GRID line shape could be observed. The preliminary data evaluation indicates that the reported lifetime of the 3221 keV level in ³³S is too short. A value of about 60 fs has been found. Due to this “long” lifetime the line shape is much less structured than calculated with the reported lifetime.     

Thermal oxidation behavior of hexagonal BC2N

Non-isothermal thermogravimetry and differential scanning calorimetry (DSC) kinetic analysis methods were used to study the thermal oxidation behavior of hexagonal BC₂N. The results show that the oxidation of hexagonal BC₂N begins at about 900 °C under a dry air atmosphere. The apparent activation energy E_a for the oxidation of hexagonal BC₂N is calculated to be 223.57 and 231.48 kJ/mol from the Kissinger and Ozawa methods, respectively. These values are higher than those (153.24 and 161.10 kJ/mol) for carbon, which means that hexagonal BC₂N has higher thermal stability than carbon at high temperature in an oxygen-containing environment.     

Probing properties of boron alpha-tubes by Ab Initio calculations

We investigate the properties of nanotubes obtained from recently described boron alpha-sheet, using density functional theory. Computations confirm their high stability and identify mechanical stiffness parameters. This allows one to further analyze the basic vibrations, including the radial breathing mode Raman frequency, fRBM = 210(nm/ d) cm (-1). Careful relaxation reveals the curvature-induced buckling of certain atoms off the original plane. This distortion changes the overlap of the orbitals near the Fermi level and opens up the gap in narrow tubes, rendering them semiconducting. Wider tubes with the diameter d greater, similar 1.7 nm retain original metallic character of the alpha-sheet. This combination of properties could make boron alpha-tubes (BT) an important material for electronic, bio- and chemical sensing, and optical applications.     

Heat Capacity of Defective Semiconducting Carbon Nanotubes

Using a random tight-binding model within the coherent potential approximation, the effects of boron and nitrogen doping on the temperature dependence of the specific heat of semiconducting zigzag carbon nanotubes are studied. It is shown that the electronic specific heat capacity behaves anomalously when the temperature is lowered. The presence of this anomaly is clarified on the basis of the idea of the so-called Schottky anomaly. More importantly, in the presence of dopants, the position of this anomaly moves towards higher temperatures and its height shifts down as the dopant concentration is increased. Such behavior is attributed to the substantial modifications in the density of states.     

Proximity Effect of Magnesium Diboride on Single-Walled Carbon Nanotube: an Ab Initio Study

Magnesium diboride (MgB₂) superconductor with excellent physical properties continues to attract the attention of researchers since its discovery. It derives its versatility from the absence of weak links, large coherence length, and small anisotropy. On the other hand, reports of superconductivity in small-diameter single-walled carbon nanotubes (SWCNTs) suspended between superconducting contacts and proximity induced supercurrents in Ta/SWNTs/Au junctions have also aroused great interest in the scientific community. Proximity induced superconductivity in SWCNTs has opened up new frontiers of research which will lead to many novel discoveries. This paper reports ab initio investigations on the proximity effect of MgB₂ on the electronic structure of a SWCNT. Condensation of electronic states is observed in the electronic band structure of the pristine SWCNT when MgB₂ is held in proximity. An additional band gap is generated below the lowest energy state of the valence band of the pristine CNT which we suggest, is due to Cooper pair formation. This leads to the prediction that SWCNTs will show superconducting properties in proximity of MgB₂. We envision MgB₂-coated SWCNTs as a novel nanomaterial that has a combination of proximity induced superconductivity and inherently unique mechanical and optical properties of SWCNTs.     

Ab Initio Probing of the Magnetic and Electronic Properties of ThCr2Si2-Like Charge-Balanced KFeAgTe2

Based on density functional theory band structure calculations, we address the magnetic and electronic properties of KFeAgTe₂ system. This phase belongs to the newest family of ThCr₂Si₂-type ternary iron-chalcogenide materials A _x Fe_2−y Ch ₂ (A= alkali metals or Tl, Ch= Se or S, discovered in 2010) which have aroused recently a tremendous interest and triggered intense studies owing to intriguing coexistence of metal vacancy ordering, antiferromagnetism, and superconductivity. But, unlike A _x Fe_2−y Ch ₂, KFeAgTe₂ is a Te-containing ThCr₂Si₂-type phase, includes a large amount of the noble metal and is free of vacancies. Here, by means of first-principles FLAPW-GGA calculations, the spin ordering of the magnetic ground state, total and partial densities of states for KFeAgTe₂ were obtained for the first time and analyzed in comparison with the related A _x Fe_2−y Ch ₂ systems.     

Interaction of NbVNi with Hydrogen

The interaction of NbVNi with hydrogen is studied over wide pressure and temperature ranges. Hydrogen absorption–desorption isotherms are measured, and the compositions of the resulting hydride phases are determined. X-ray diffraction analysis demonstrates that the incorporation of hydrogen into NbVNi leads to an isotropic expansion of its crystal lattice.     

Ab Initio Calculations of the Electronic Structures of Copper Pyrites CuS2, CuSe2 and CuTe2

The electronic structures of CuS2, CuSe2 and CuTe2 with pyrite structures, within the framework the density-functional theory have been investigated. The calculated results explained the recent experimental results which show that there is no clear indication of strong electron correlations in the electronic properties of Cu pyrites, due to the dominant chalcogen p character rather than d characteristic of Cu at the Fermi level.     

Study Ab Initio of the Effect of A-Site Substitution on the Fe1.12Te System

In the present work, our aim is to verify the structural, electronic and magnetic properties of both systems Fe1.12Te and <RTX> (R = Fe, X = Te and T = Ni, Co) in the P4/nmm structure. For this task, we use the density functional theory (DFT) as a theoretical tool integrated into wien2k code (Blaha 2001). The solid Fe1 ? x Mx Te (M = Ni, Co) have been synthesised by Kazakov et al. (Chem. Met. Alloys 3, 155–160 2010). They have observed a systematic shift of the lattice parameters for both systems for M = Ni and Co till x = 0.1, then a secondary phase with the NiAs-type structure appeared when x passes 0.15. Fe1 ? x Nix Te retains its structure in a concentration between x = 0.1 and 0.15, and Fe1 ? x CoxTe retains its structure when x is between 0.05 and 0.1 (Blaha 2001).     

First-principles characterisation of new ternary heterodiamond BC2N phases

In order to discover the existence of new ultrahard materials, we have performed the substitution of some carbon atoms with boron and nitrogen in two different diamond forms: cubic and hexagonal. The number of substituted carbon atoms was fixed in order to obtain BC₂N heterodiamond phases isoelectronic with diamond, which is the hardest known material. After the carbon atom replacement, a full geometry relaxation was performed with a first-principle pseudopotential (PP) method to find the fundamental electronic ground state. These hypothetical ternary compounds are expected to be more thermally and chemically stable than diamond and harder than cubic boron nitride. This possibility makes them the most interesting class of compounds that can replace the expensive diamond in many mechanical applications. In the present work, we employ the PP method to predict the mechanical properties of the new BC₂N phases. An estimation of the hardness is given with the calculation of the bulk and shear moduli. The relative stability between the phases has also been studied by using both the full potential linearised augmented plane waves (FP-LAPW) and the PP methods. Further, with the help of the FP-LAPW approach, the electronic properties are discussed by means of the density-of-state (DOS) analysis.