[en] Quantum molecular dynamic simulations were used to examine the change in atomic and electronic structure in liquid rubidium along its liquid-vapor coexistence curve. Starting from the liquid at the triple point, with increasing expansion we observe a continuous increase in the electron localization leading to ion clustering near the metal-nonmetal transition at about twice the critical density, in agreement with electrical measurements, and to the presence of dimers near and below the critical density

[en] Ab initio molecular dynamics calculations are adapted to treat dense plasmas for temperatures exceeding the electronic Fermi temperature. Extended electronic states are obtained in a plane wave basis by using pseudopotentials for the ion cores in the local density approximation to density functional theory. The method reduces to conventional first principles molecular dynamics at low temperatures with the expected high level of accuracy. The occurrence of thermally excited ion cores at high temperatures is treated by means of final state pseudopotentials. The method is applied to the shock compression Hugoniot equation of state for aluminum. Good agreement with experiment is found for temperatures ranging from zero through 105K

[en] Plastic deformation in bcc metals at low temperatures and high-strain rates is controlled by the motion of a/2<111> screw dislocations, and understanding the fundamental atomistic processes of this motion is essential to develop predictive multiscale models of crystal plasticity. The multiscale modeling approach presented here for bcc Ta is based on information passing, where results of simulations at the atomic scale are used in simulations of plastic deformation at mesoscopic length scales via dislocation dynamics (DD). The relevant core properties of a/2<111> screw dislocations in Ta have been obtained using quantum-based interatomic potentials derived from model generalized pseudopotential theory and an ab-initio data base together with an accurate Green's-function simulation method that implements flexible boundary conditions. In particular, the stress-dependent activation enthalpy for the lowest-energy kink-pair mechanism has been calculated and fitted to a revealing analytic form. This is the critical quantity determining dislocation mobility in the DD simulations, and the present activation enthalpy is found to be in good agreement with the previous empirical form used to explain the temperature dependence of the yield stress

[en] High-Z metals constitute a particular challenge for large-scale ab initio calculations, as they require high resolution due to the presence of strongly localized states and require many eigenstates to be computed due to the large number of electrons and need to accurately resolve the Fermi surface. Here, we report recent findings on high-Z materials, using an efficient massively parallel planewave implementation on some of the largest computational architectures currently available. We discuss the particular architectures employed and methodological advances required to harness them effectively. We present a pair-correlation function for U, calculated using quantum molecular dynamics, and discuss relaxations of Pu atoms in the vicinity of defects in aged and alloyed Pu. We find that the self-irradiation associated with aging has a negligible effect on the compressibility of Pu relative to other factors such as alloying

[en] Ab initio molecular dynamics calculations are performed for the equation of state of aluminum, spanning condensed matter and dense plasma regimes. Electronic exchange and correlation are included with either a zero- or finite-temperature local density approximation potential. Standard methods are extended to above the Fermi temperature by using final state pseudopotentials to describe thermally excited ion cores. The predicted Hugoniot equation of state agrees well with earlier plasma theories and with experiment for temperatures from 0 to 3 x 10⁶ K

[en] Two Si-based spintronic materials, a Mn-Si digital ferromagnetic heterostructure ((delta)-layer of Mn doped in Si) with defects and dilutely doped Mn_xSi_1-x alloy are investigated using a density-functional based approach. We model the heterostructure and alloy with a supercell of 64 atoms and examine several configurations of the Mn atoms. We find that 25% substitutional defects without vacancies in the (delta) layer diminishes half metallicity of the DFH substantially. For the alloy, the magnetic moment M ranges from 1.0-9.0 μ_B/unit-cell depending on impurity configuration and concentration. Mn impurities introduce a narrow band of localized states near E_F. These alloys are not half metals though their moments are integer. We explain the substantially different magnetic moments

[en] A brief comparison of conventional electronics and spintronics is given. The key features of half metallic binary compounds with the zincblende structure are presented, using MnAs as an example. We discuss the interactions responsible for the half metallic properties. Special properties of superlattices and a digital ferromagnetic heterostructure incorporating zincblende half metals are also discussed

[en] The authors examine theoretically conduction processes near the Fermi energy of thin layers of zincblende structure half metals, using as an example a superlattice consisting of monolayers of GaAs and MnAs, a bilayer of CrAs, and a bilayer of GaAs. By artificially separating bilayers, they show that non-fourfold coordinated Cr states thwart half metallicity. However, capping the metal-As bilayers restores half metallicity and ballistic conduction of electrons around 0.3 eV above the Fermi level will give nearly 100% spin-polarized transmission in the direction of the thin superlattice. Recent developments suggest atomic layer epitaxy can be used to produce such thin layers for spintronics applications

[en] A series of Cu_x TaSe₂ (0 ≤ x ≤ 0.12) samples were prepared and characterized via x-ray diffraction, magnetization, resistivity, and heat capacity measurements. We found that the charge density wave phases in 2H-Cu_x TaSe₂ are noticeably suppressed and the superconducting critical temperatures T_c are significantly enhanced from 0.14 K for pure TaSe₂ to about 2 K in the Cu-doped samples. The maximum T _c of 2.7 K was obtained in the optimally doped sample with x = 0.07. The crystal structure of the Cu_0.07TaSe₂ was refined by the Rietveld method. Magnetic measurements revealed that Cu_0.07TaSe₂ is a bulk superconductor with upper critical field H_c2 (0) of 4.41 T. Analysis of specific heat data shows that the effective electron–phonon coupling and density of states at the Fermi level are enhanced in the Cu-doped samples. We present an electronic phase diagram for the 2H-Cu_x TaSe₂ system. (paper)

[en] Ab-initio electronic-structure calculations have been performed to investigate the adsorption structures, the energetics, and the electronic structures of a Pt monolayer (ML) deposited on a MgO(001) surface. Our calculations for a 1-ML Pt/MgO(001) structure show that the Pt atoms highly prefer to bind on the oxygen sites of the MgO(001) surfaces. Also examined were the electronic properties of 1-ML Pt/MgO(001). Interestingly, the oxide-supported ultrathin 1-ML Pt film was predicted to be ferromagnetic with a magnetic moment of 0.89 Bohr magnetons. In addition, the origin of the unusual magnetism in 1-ML Pt/MgO(001) was discussed in comparison with calculations for a freestanding Pt monolayer.