C. Y. Fong

Finite-element methods in electronic-structure theory

Abstract We discuss the application of the finite-element (FE) method to ab initio solid-state electronic-structure calculations. In this method, the basis functions are strictly local, piecewise polynomials. Because the basis is composed of polynomials, the method is completely general and its convergence can be controlled systematically. Because the basis functions are strictly local in real space, the method allows for variable resolution in real space; produces sparse, structured matrices, enabling the effective use of iterative solution methods; and is well suited to parallel implementation. The method thus combines the significant advantages of both real-space-grid and basis-oriented approaches and so promises to be particularly well suited for large, accurate ab initio calculations. We discuss the construction and properties of the required FE bases and develop in detail their use in the solution of the Schrodinger and Poisson equations subject to boundary conditions appropriate for a periodic solid. We present results for the Schrodinger equation illustrating the rapid, variational convergence of the method in electronic band-structure calculations. We present results for the Poisson equation illustrating the rapid convergence of the method, both pointwise and in the L 2 norm, and its linear scaling with system size in the context of a model charge-density and Si pseudo-charge-density. Finally, we discuss the application of the method to large-scale ab initio positron distribution and lifetime calculations in solids and present results for a host of systems within the range of a conventional LMTO based approach for comparison, as well as results for systems well beyond the range of the conventional approach. The largest such calculation, involving a unit cell of 4092 atoms, was shown to be well within the range of the FE approach on existing computational platforms.

Electronic and optical properties of SbSBr, SbSI and SbSeI

Abstract Band structures of SbSBr and SbSeI have been obtained by using the empirical pseudopotential method (EPM) to fit our measured optical reflectivity data and earlier gap measurements. An SbSI band structure has been determined by fitting to earlier reflectivity and Raman spectroscopic data, and the results agree better with the data than do the results of an earlier preliminary EPM calculation. Secondary conduction band minima may in part be responsible for the observed microwave oscillation (Gunn effect) in SbSI. Similar minima in SbSBr and SbSeI are reported, suggesting these crystals might also show microwave properties. The total densities of states are presented.

Electronic structure of cubic silicon–carbide doped by 3d magnetic ions

We have studied the electronic properties of cubic silicon–carbide (3C-SiC) doped with Cr, Mn, Fe, and Co magnetic atoms using the tight-binding linear combination of muffin-tin orbitals with atomic sphere approximation method. By directly comparing the difference of the total energy between a vacancy and a dopant filling the vacant site, we found that the Mn doped at C site gains the least energy as compared to the other cases. Heavier Fe and Co atoms appear to be nonmagnetic. For lighter Cr and Mn atoms at the Si site, the dopings result in 1.6 μB (Bohr magneton) for Cr and 0.7 μB for Mn, respectively. The magnetic moment for Cr atom substituting a C atom is 0.907 μB. 3d down spin hole states exist, but the mobility associated with these states is not expected to be large. Photoluminescence measurements are suggested to probe the narrow 3d structures in the gap.

Structural variants and the modified Slater-Pauling curve for transition-metal-based half-Heusler alloys

We compare the physical differences between two atomic configurations, found in the literature, of the half-Heusler alloys—XMnY, where X is a transition-metal element and Y is a nonmetallic element. The structural differences arise from the placement of the X and Y atoms and the vacancy within the full-Heusler (L21) structure. In one configuration, Y and Mn are nearest neighbors and the vacancy is at (1/4,3/4,1/4)a (4d) while in the other configuration, X and Mn are nearest neighbors and the vacancy is located at (0,0,1/2)a (4b), where a is the lattice constant of face-centered cube. We suggest that the important difference between the two configurations is the identity of the transition-metal element nearest to the non-metal element. Physical properties, in particular the bonding features, reflect this difference. The general validity of the modified Slater-Pauling curve, which gives successful zeroth-order prediction of the magnetic moments of many half-Heusler alloys including CrMnSb in the second conf...

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.

Doping in cubic silicon–carbide

We studied the energetics and the properties of impurity states that result from doping cubic silicon–carbide (3C–SiC) with aluminum (Al), boron (B), and nitrogen (N) atoms using the tight-binding linear combination of muffin-tin orbital atomic sphere approximation method. For Al doping, it is only favorable to substitute Al for Si atoms. The corresponding hole states contribute to a partially filled weak peak near the Fermi energy. For B doping, it is possible to replace either Si or C atoms in the crystal. When a B atom is at a Si site, the hole states exhibit behavior similar to the case of Al doping. However, when a B atom is at a C site, the hole states form a partially filled strong peak above the Fermi energy. This localized feature is explained in terms of the screening effect of the neighboring atoms. For n-type doping, a N atom can enter either the Si or C site. The latter site is more energetically favorable. Furthermore, the corresponding donor states form deep impurity states within the gap. ...

Structural and electronic properties induced by hydrogen adsorption on the GaAs(110) surface

Abstract The structural and electronic properties for hydrogen adsorbed on the GaAs(110) surface have been studied by the self-consistent pseudopotential method with a slab geometry and half and one monolayer coverages. The positions of the hydrogen atoms and the surface atoms for the stable configuration are given. The bonding nature is presented and the states at the Fermi energy are also determined.

Half-metallic properties of atomic chains of carbon-transition-metal compounds

We found that magnetic ground state of one-dimensional atomic chains of carbon\char21{}transition-metal compounds exhibit half-metallic properties. They are semiconductors for one spin direction, but show metallic properties for the opposite direction. The spins are fully polarized at the Fermi level and net magnetic moment per unit cell is an integer multiple of Bohr magneton. The spin-dependent electronic structure can be engineered by changing the number of carbon atoms and type of transition metal atoms. These chains, which are stable even at high temperatures and some of which keep their spin-dependent electronic properties even under moderate axial strain, hold the promise of potential applications in nanospintronics.

Theoretical investigation of (111) stacking faults in aluminium

Abstract We have investigated (111) stacking faults in aluminium using the Kohn-Sham formulation of density-functional theory (DFT) along with plane-wave expansions for the Kohn-Sham functions and pseudopotentials to describe the interactions between the Kohn-Sham functions and the ions. We find that the energies of the intrinsic, extrinsic and twin stacking faults are 161, 151 and 74 mJ m-2 respectively. These values are in reasonable agreement both with estimates based on experimental observations and with values obtained from previous calculations also based on DFT. In addition, we have considered relaxations of the atoms within the stacking-fault region and find that this has a negligible effect on the stacking-fault energies.