L. H. Yang

Quantum-based atomistic simulation of materials properties in transition metals

We present an overview of recent work on quantum-based atomistic simulation of materials properties in transition metals performed in the Metals and Alloys Group at Lawrence Livermore National Laboratory. Central to much of this effort has been the development, from fundamental quantum mechanics, of robust many-body interatomic potentials for bcc transition metals via model generalized pseudopotential theory (MGPT), providing close linkage between ab?initio electronic-structure calculations and large-scale static and dynamic atomistic simulations. In the case of tantalum (Ta), accurate MGPT potentials have been so obtained that are applicable to structural, thermodynamic, defect, and mechanical properties over wide ranges of pressure and temperature. Successful application areas discussed include structural phase stability, equation of state, melting, rapid resolidification, high-pressure elastic moduli, ideal shear strength, vacancy and self-interstitial formation and migration, grain-boundary atomic structure, and dislocation core structure and mobility. A number of the simulated properties allow detailed validation of the Ta potentials through comparisons with experiment and/or parallel electronic-structure calculations. Elastic and dislocation properties provide direct input into higher-length-scale multiscale simulations of plasticity and strength. Corresponding effort has also been initiated on the multiscale materials modelling of fracture and failure. Here large-scale atomistic simulations and novel real-time characterization techniques are being used to study void nucleation, growth, interaction, and coalescence in series-end fcc transition metals. We have so investigated the microscopic mechanisms of void nucleation in polycrystalline copper (Cu), and void growth in single-crystal and polycrystalline Cu, undergoing triaxial expansion at a large, constant strain rate - a process central to the initial phase of dynamic fracture. The influence of pre-existing microstructure on the void growth has been characterized both for nucleation and for growth, and these processes are found to be in agreement with the general features of void distributions observed in experiment. We have also examined some of the microscopic mechanisms of plasticity associated with void growth.

Accurate atomistic simulation of (a/2) 〈111〉 screw dislocations and other defects in bcc tantalum

Abstract The fundamental atomic-level properties of (a/2)(111) screw dislocations and other defects in bcc Ta have been simulated by means of new quantum-based multi-ion interatomic potentials derived from the model generalized pseudopotential theory (MGPT). The potentials have been validated in detail using a combination of experimental data and ab-initio electronic structure calculations on ideal shear strength, vacancy and self-interstitial formation and migration energies, grain-boundary atomic structure and generalized stacking-fault energy (γ) surfaces. Robust and accurate two- and three-dimensional Greens function (GF) techniques have been used to relax dynamically the boundary forces during the dislocation simulations. The GF techniques have been implemented in combination with a spatial domain decomposition strategy, resulting in a parallel MGPT atomistic simulation code that increases computational performance by two orders of magnitude. Our dislocation simulations predict a degenerate core structure with threefold symmetry for Ta, but one that is nearly isotropic and only weakly polarized at ambient pressure. The degenerate nature of the core structure leads to possible antiphase defects (APDs) on the dislocation line as well as multiple possible dislocation kinks and kink pairs. The APD and kink energetics are elaborated in detail in the low-stress limit. In this limit, the calculated stress-dependent activation enthalpy for the lowest-energy kink pair agrees well with that currently used in mesoscale dislocation dynamics simulations to model the temperature-dependent single-crystal yield stress. In the high-stress limit, the calculated Peierls stress displays a strong orientation dependence under pure shear and uniaxial loading conditions, with an antitwinning-twinning ratio of 2.29 for pure shear {211}-(111) loading.

Half-Metallic Digital Ferromagnetic Heterostructure Composed of a delta-Doped Layer of Mn in Si

We propose and investigate the properties of a digital ferromagnetic heterostructure consisting of a delta-doped layer of Mn in Si, using ab initio electronic-structure methods. We find that (i) ferromagnetic order of the Mn layer is energetically favorable relative to antiferromagnetic, and (ii) the heterostructure is a two-dimensional half-metallic system. The metallic behavior is contributed by three majority-spin bands originating from hybridized Mn-d and nearest-neighbor Si-p states, and the corresponding carriers are responsible for the ferromagnetic order in the Mn layer. The minority-spin channel has a calculated semiconducting gap of 0.25 eV. The band lineup is found to be favorable for retaining the half-metal character to near the Curie temperature. This kind of heterostructure may be of special interest for integration into mature Si technologies for spintronic applications.

Electronic and magnetic properties of zinc blende half-metal superlattices

Zinc blende half-metallic compounds such as CrAs, with large magnetic moments and high Curie temperatures, are promising materials for spintronic applications. We explore layered materials, consisting of alternating layers of zinc blende half-metals, by first principles calculations, and find that superlattices of (CrAs)1(MnAs)1 and (CrAs)2(MnAs)2 are half-metallic with magnetic moments of 7.0μB and 14.0μB per unit cell, respectively. We discuss the nature of the bonding and half-metallicity in these materials and, based on the understanding acquired, develop a simple expression for the magnetic moment in such materials. We explore the range of lattice constants over which half-metallicity is manifested, and suggest corresponding substrates for growth in thin film form.

Atomistic Simulations for Multiscale Modeling in bcc Metals

Quantum-based atomistic simulations are being used to study fundamental deformation and defect properties relevant to the multiscale modeling of plasticity in bcc metals at both ambient and extreme conditions. Ab initio electronic-structure calculations on the elastic and ideal-strength properties of Ta and Mo help constrain and validate many-body interatomic potentials used to study grain boundaries and dislocations. The predicted C(capital Sigma)5 (310)[100] grain boundary structure for Mo has recently been confirmed in HREM measurements. The core structure, (small gamma) surfaces, Peierls stress, and kink-pair formation energies associated with the motion of a/2(111) screw dislocations in Ta and Mo have also been calculated. Dislocation mobility and dislocation junction formation and breaking are currently under investigation.

Kink-pair mechanisms for a/2 〈111〉 screw dislocation motion in bcc tantalum

Abstract The structure, energetics, and multiplicity of kinks on an a /2 〈111〉 screw dislocation in bcc Ta have been studied in detail via atomistic computer simulation, using quantum-based multi-ion interatomic potentials derived from model generalized pseudopotential theory (MGPT) together with a robust Greens function simulation technique. The stable core structure of the rigid screw dislocation is predicted to be weakly polarized and spread out on three {110} planes in 〈112〉 directions, with two energetically equivalent configurations. This double degeneracy leads to the possibility of anti-phase defects forming on the dislocation line as well multiple kinks and kink pairs. The zero-stress formation energies of 16 possible kink-pair configurations for bcc Ta have been calculated and are in the range 0.67–1.84 eV. The lowest kink-pair energy of the perfect screw is in good agreement with the best current empirical estimate. Under an applied stress, the corresponding kink–kink interaction energy displays a λ −1 elastic attraction when the separation λ is larger than 7 b =20 A, while the stress needed to maintain the kink pair varies as λ −1.5 .

Stabilizing and increasing the magnetic moment of half-metals: The role of Li in half-Heusler LiMn Z ( Z = N , P , Si )

L. Damewood, ∗ B. Busemeyer, M. Shaughnessy, C. Y. Fong, L. H. Yang, and C. Felser Department of Physics, University of California, Davis, CA 95616 USA Sandia National Laboratories at Livermore, Livermore, CA 94551 USA Lawrence Livermore National Laboratory, Livermore, CA 94551 USA Institut für Anorganische Chemie und Analytische Chemie, Johannes Gutenberg-Universität Mainz, 55099 Mainz, Germany (Dated: December 12, 2013)

Three interacting excitons in three coupled quantum dots

Using a scaled Kohn-Sham formalism, we examine three heavy- and light-hole excitons, respectively, in three coupled quantum dots to study the effects of competition involving the electron-electron and hole-hole interactions between excitons, the electron-hole interaction within excitons and the effective masses. The particle-particle interactions play dominant roles in determining the configuration of the excitons in the coupled dots. In the absence of an external electric field, the two lowest (occupied) energy states of the heavy-hole excitons are degenerate, and the excitons have an equal probability of residing in either of the two side dots. The corresponding states of the light-hole excitons exhibit nondegenerate character and the lowest energy exciton is confined predominantly to the centre dot. Under a weak dc electric field, both the heavy- and light-hole excitons form direct excitons which are localized in the side dots. For large values of the electric field, we find that the electrons associated with the excitons become ionized as a result of strong confinement by the field and the electron-electron repulsion between excitons before the excitons can be transformed into indirect excitons. This result is in contrast to the conclusion that a single exciton in a double-quantum-well structure will be transformed into an indirect exciton in real space when it is subjected to large fields. Furthermore, we suggest that the wave function overlap between interacting excitons may diminish significantly the increase in excitonic lifetime predicted for a single exciton in two coupled quantum dot systems, implying that it may be difficult to make use of several excitons in coupled structures for nonlinear devices.

Quantum‐Based Atomistic Simulation of Transition Metals

First‐principles generalized pseudopotential theory (GPT) provides a fundamental basis for transferable multi‐ion interatomic potentials in d‐electron transition metals within density‐functional quantum mechanics. In mid‐period bcc metals, where multi‐ion angular forces are important to structural properties, simplified model GPT or MGPT potentials have been developed based on canonical d bands to allow analytic forms and large‐scale atomistic simulations. Robust, advanced‐generation MGPT potentials have now been obtained for Ta and Mo and successfully applied to a wide range of structural, thermodynamic, defect and mechanical properties at both ambient and extreme conditions of pressure and temperature. Recent algorithm improvements have also led to a more general matrix representation of MGPT beyond canonical bands allowing increased accuracy and extension to f‐electron actinide metals, an order of magnitude increase in computational speed, and the current development of temperature‐dependent potentials.

Quasi-two-dimensional quantum states of H{sub 2} in stage-2 Rb-intercalated graphite

Inelastic-incoherent-neutron scattering can be a valuable nanostructural probe of H{sub 2}-doped porous materials, provided the spectral peaks can be interpreted in terms of crystal-field-split hydrogen-molecule energy levels, which represent a signature of the local symmetry. Inelastic-neutron-scattering measurements as well as extensive theoretical analyses have been performed on stage-2 Rb-intercalated graphite (Rb-GIC), with physisorbed H{sub 2}, HD, and D{sub 2}, a layered porous system with abundant spectral peaks, to assess whether the crystal-field-state picture enables a quantitative understanding of the observed structure. Potential-energy surfaces for molecular rotational and translational motion, as well as the intermolecular interactions of hydrogen molecules in Rb-GIC, were calculated within local-density-functional theory (LDFT). Model potentials, parameterized using results of the LDFT calculations, were employed in schematic calculations of rotational and translational excited state spectra of a single physisorbed H{sub 2} molecule in Rb-GIC. Results of the analysis are basically consistent with the assignment by Stead et al. of the lowest-lying peak at 1.4 meV to a rotational-tunneling transition of an isotropic hindered-rotor oriented normal to the planes, but indicate a small azimuthal anisotropy and a lower barrier than for the isotropic case. Based on the experimental isotope shifts and the theoretically predicted states, they conclude that spectral peaks at 11 and 22 meV are most likely related to center of mass excitations.