Calculations of the potential energy surface for tracer Ga and In adatoms above three GaAs (111)A surface reconstructions are presented in order to understand the growth conditions required to form axial heterostructures in GaAs/InGaAs nano-pillars. In all calculations the Ga adatom has a stronger bond energy to the surface than the In adatom. The diffusion barriers for Ga adatoms are 140meV larger than for In adatoms on the Ga vacancy surface, but they are comparable on the As trimer surface. Also the binding energy for an In adatom is closer to that of a Ga adatom on the As trimer surface. We conclude that the As trimer surface is preferable for adsorption of In and thus for selective formation of hetero-interfaces on (111) facets. This work helps explain the recent successful formation of axial GaAs/InGaAs hetero-interfaces in catalyst free nano-pillars.
Employing first principles total energy calculations we have studied the behavior of Ga and Al adatoms on the GaAs(001)-beta2 surface. The adsorption site and two relevant diffusion channels are identified. The channels are characterized by different adatom-surface dimer interaction. Both affect in a novel way the adatom migration: in one channel the diffusing adatom jumps across the surface dimers and leaves the dimer bonds intact, in the other one the surface dimer bonds are broken. The two channels are taken into account to derive effective adatom diffusion barriers. From the diffusion barr…
The diffusivity of Ga and Al adatoms on the (2x4) reconstructed GaAs(001) surface are evaluated using detailed ab initio total energy calculations of the potential energy surface together with transition state theory. A strong diffusion anisotropy is found, with the direction of fastest diffusion being parallel to the surface As-dimer orientation. In contrast to previous calculations we identify a short--bridge position between the two As atoms of a surface dimer as the adsorption site for Al and Ga adatoms.
The state of the art ab initio calculations reveal the effect of a scanning tunnelling microscopy tip on magnetic properties and conductance of a benzene-adatom sandwich on Cu(001). We concentrate on a benzene-Co system interacting with a Cr tip. Our studies give a clear evidence that magnetism and conductance in molecule-adatom junctions can be tailored by the STM tip. Varying the tip-substrate distance the magnetic moment of the Co adatom can be switched on/off. The interplay between spin-polarized electron transport through the junction and its magnetic properties is demonstrated. A spin-fi…
Density functional calculations with a large unit cell have been conducted to investigate adsorption, segregation and magnetization of Mn monomer on GaAs(110). The Mn adatom is rather mobile along the trench on GaAs(110), with an energy barrier of 0.56 eV. The energy barrier for segregation across the trenches is nevertheless very high, 1.67 eV. The plots of density of states display a wide gap in the majority spin channel, but show plenty of metal-induced gap states in the minority spin channel. The Mn atoms might be invisibl in scanning tunneling microscope (STM) images taken with small bias…
We carry out density-functional ab initio molecular dynamics (AIMD) simulations of Ti adatom (Tiad) migration on, and descent from, TiN <100>-faceted epitaxial islands on TiN(001) at temperatures T ranging from 1200 to 2400 K. Adatom-descent energy-barriers determined via ab initio nudged-elastic-band calculations at 0 Kelvin suggest that Ti interlayer transport on TiN(001) occurs essentially exclusively via direct hopping onto a lower layer. However, AIMD simulations reveal comparable rates for Tiad descent via direct-hopping vs. push-out/exchange with a Ti island edge atom for T >= 1500 K. W…
The addition of boron to GaAs nanowires grown by self-catalyzed molecular beam epitaxy was found to have a strong effect on the nanowire morphology, with axial growth greatly reduced as the nominal boron concentration was increased. Transmission electron microscopy measurements show that the Ga catalyst droplet was unintentionally consumed during growth. Concurrent radial growth, a rough surface morphology and tapering of nanowires grown under boron flux suggest that this droplet consumption is due to reduced Ga adatom diffusion on the nanowire sidewalls in the presence of boron. Modelling of …
Heteroepitaxy on nanopatterned substrates is a means of defect reduction at semiconductor heterointerfaces by exploiting substrate compliance and enhanced elastic lattice relaxation resulting from reduced dimensions. We explore this possibility in the InAs/GaAs(111)A system using a combination of nanosphere lithography and reactive ion etching of the GaAs(111)A substrate for nano-patterning of the substrate, yielding pillars with honeycomb and hexagonal arrangements and varied nearest neighbor distances. Substrate patterning is followed by MBE growth of InAs at temperatures of 150 - 350 C and …
The state of the art ab initio calculations of quantum mirages,the spin-polarization of surface-state electrons and the exchange interaction between magnetic adatoms in Cu and Co corrals on Cu(111) are presented. We find that the spin-polarization of the surface-state electrons caused by magnetic adatoms can be projected to a remote location and can be strongly enhanced in corrals compared to an open surface.Our studies give a clear evidence that quantum corrals could permit to tailor the exchange interaction between magnetic adatoms at large separations. The spin-polarization of surface-state…
We have investigated ab initio the existence of localized states and resonances in abrupt GaAs/Si/GaAs (110)- and (100)-oriented heterostructures incorporating 1 or 2 monolayers (MLs) of Si, as well as in the fully developed Si/GaAs (110) heterojunction. In (100)-oriented structures, we find both valence- and conduction-band related near-band edge states localized at the Si/GaAs interface. In the (110) systems, instead, interface states occur deeper in the valence band; the highest valence-related resonances being about 1 eV below the GaAs valence-band maximum. Using their characteristic bondi…
We present ab initio quasiparticle self-energy calculations in crystalline GaAs for cases of intense electronic excitation (~ 10% of valence electrons excited into conduction band), relevant for high-intensity ultra-short pulsed laser experiments. Calculations are performed using an out-of-equilibrium generalization of the GW approximation based on the Keldysh Green's function approach. Our results indicate that while the band gap is a sensitive function of the amount of excitation, it is not possible to induce complete band gap closure in this system by purely electronic means.
We use ab initio static relaxation methods and semi-empirical molecular-dynamics simulations to investigate the energetics and dynamics of the diffusion of adatoms, dimers, and vacancies on Cu(100). It is found that the dynamical energy barriers for diffusion are well approximated by the static, 0 K barriers and that prefactors do not depend sensitively on the species undergoing diffusion. The ab initio barriers are observed to be significantly lower when calculated within the generalized-gradient approximation (GGA) rather than in the local-density approximation (LDA). Our calculations predic…
In polar semiconductors and oxides, the long-range nature of the electron-phonon (\textit{e}-ph) interaction is a bottleneck to compute charge transport from first principles. Here, we develop an efficient ab initio scheme to compute and converge the \textit{e}-ph relaxation times (RTs) and electron mobility in polar materials. We apply our approach to GaAs, where using the Boltzmann equation with state-dependent RTs, we compute mobilities in excellent agreement with experiment at 250$-$500~K. The $e$-ph RTs and the phonon contributions to intravalley and intervalley $e$-ph scattering are also…
In this paper we present an ab initio real-time analysis of free polarization decay and photon echo in extended systems. As a prototype material, we study bulk GaAs driven by ultra-short laser pulses of 10 fs (energy spread of 0.4 eV), with frequency tuned in the continuum of the optical spectrum. We compute the electronic polarization P(t), and define a computational procedure to extract the echo signal in the dipole approximation. Results are obtained in both the low and high field regime, and compared with a two-levels system (TLS) model, with parameters extracted from the ab initio simulat…
We report the fabrication of self-assembled, strain-free GaAs/Al$_{0.27}$Ga$_{0.73}$As quantum dot pairs which are laterally aligned in the growth plane, utilizing the droplet epitaxy technique and the anisotropic surface potentials of the GaAs (100) surface for the migration of Ga adatoms. Photoluminescence spectra from a single quantum dot pair, consisting of a doublet, have been observed. Finite element energy level calculations of a model quantum dot pair are also presented.
We have investigated the initial growth of Fe on GaAs(110) by means of density functional theory. In contrast to the conventionally used (001)-surface the (110)-surface does not reconstruct. Therefore, a flat interface and small diffusion can be expected, which makes Fe/GaAs(110) a possible candidate for spintronic applications. Since experimentally, the actual quality of the interface seems to depend on the growth conditions, e.g., on the flux rate, we simulate the effect of different flux rates by different Fe coverages of the semiconductor surface. Systems with low coverages are highly diff…
Ab initio calculations of surface-state mediated interactions between Cu adatoms on transition metal surfaces are presented. We concentrate on Co/Cu(111) and Co(0001) substrates and compare results with our calculations for Cu(111). Our studies show that surface states of Co/Cu(111) and Co(0001) are spin-polarized. We reveal that long-range interactions between adatoms are mainly determined by sp-majority states. In contrast to Cu(111) and Co/Cu(111), the interaction between adatoms on Co(0001) is strongly suppressed at large adsorbate separations.
We have performed an ab initio investigation of the energetic stability, and the electronic properties of transition metals (TMs = Mn, Fe, Co, and Ru) adsorbed on graphene upon the presence of grain boundaries (GBs). Our results reveal an energetic preference for the TMs lying along the GB sites (TM/GB). Such an energetic preference has been strengthened by increasing the concentration of the TM adatoms; giving rise to TM nanolines on graphene ruled by GBs. Further diffusion barrier calculations for Fe adatoms support the formation of those TM nanolines. We find that the energy barriers parall…
In this work we perform a first-principles study of the adsorption properties of an In adatom deposited on 1.75 monolayers (ML) InAs, forming a wetting layer on GaAs$(001)$ with the $α_2 (2\times4)$ or $β_2 (2\times4)$ reconstruction. The structural properties of these reconstructions have been studied: we determine the equilibrium geometry of the surfaces and their stability for various growth conditions. We have then carried out a detailed study of the potential energy surface (PES) for an In adsorbate, finding the minima and the saddle points. The main characteristics of the PES and the bon…
Many recent calculations have been performed to study a Co atom adsorbed on graphene, with significantly varying results on the nature of the bonding. We use auxiliary-field quantum Monte Carlo (AFQMC) and a size-correction embedding scheme to accurately calculate the binding energy of Co on graphene. We find that as a function of the distance h between the Co atom and the six-fold hollow site, there are three distinct ground states corresponding to three electronic configurations of the Co atom. Two of these states provide binding and exhibit a double-well feature with nearly equal binding en…
We present an ab initio study of spin dependent transport in armchair carbon nanotubes with transition metal adsorbates, iron or vanadium. We neglect the effect of tube curvature and model the nanotube by graphene with periodic boundary conditions. A density functional theory based nonequilibrium Green's function method is used to compute the electronic structure and zero-bias conductance. The presence of the adsorbate causes a strong scattering of electrons of one spin type only. The scattering is shown to be due to coupling of the two armchair band states to the metal 3d orbitals with matchi…
Based on ab initio calculations, we examine the incorporation of Li atoms in the MoS2/graphene interface. We find that the intercalated Li atoms are energetically more stable than Li atoms adsorbed on the MoS2 surface. The intercalated atoms interact with both graphene sheet and MoS2 layer, increasing the Li binding energies. However, the equilibrium geometries are ruled by the MoS2 layer, where the intercalated Li atoms lie on the top (Li_T) and hollow (Li_H) sites of the MoS2 layer. We calculate the Li diffusion barriers, along the Li_T -> Li_H diffusion path, where we find similar energy ba…
Motivated by the radiation damage of solar panels in space, firstly, the results of Monte Carlo particle transport simulations are presented for proton impact on triple-junction Ga$_{0.5}$In$_{0.5}$P/GaAs/Ge solar cells, showing the proton projectile penetration in the cells as a function of energy. It is followed by a systematic {\it ab initio} investigation of the electronic stopping power for protons in different layers of the cell at the relevant velocities via real-time time-dependent density functional theory (RT-TDDFT) calculations. The electronic stopping power is found to depend signi…
We report here a reexamination of the static properties of vacancies in GaAs by means of first-principles density-functional calculations using localized basis sets. Our calculated formation energies yields results that are in good agreement with recent experimental and {\it ab-initio} calculation and provide a complete description of the relaxation geometry and energetic for various charge state of vacancies from both sublattices. Gallium vacancies are stable in the 0, -, -2, -3 charge state, but V_Ga^-3 remains the dominant charge state for intrinsic and n-type GaAs, confirming results from …
Even though it has been known for decades that the magnetocrystalline anisotropy is linked to the spin-orbit coupling (SOC), the mechanism how it arises for specific systems is still subject of debate. We focused on finding markers of SOC in the density of states (DOS) and on employing them for understanding the source of magnetocrystalline anisotropy for the case of adatoms and monolayers. Fully relativistic ab-initio KKR-Green function calculations were performed for Fe, Co, and Ni adatoms and monolayers on Au(111) to investigate changes in the orbital-resolved DOS due to a rotation of magne…
Ultra-thin FeAs is of interest both as the active component in the newly identified pnictide superconductors, and in spintronic applications at the interface between ferromagnetic Fe and semiconducting GaAs. Here we use first-principles density functional theory to investigate the properties of FeAs/GaAs heterostructures. We find that the Fermi surface is modified from that characteristic of the pnictide superconductors by interactions between the FeAs layer and the As atoms in the GaAs layers. Regardless of the number of FeAs layers, the Fe to As ratio, or the strain state, the lowest energy …
The atomic structure of well-ordered single steps on the Si(111)-\sqrt{3}\times\sqrt{3}-Sn surface and the dynamics of Sn adatoms in the vicinity of these steps was studied. The work was performed using low-temperature scanning tunneling microscopy (STM) and ab initio calculations based on the density-functional theory. The STM tip was used to record the tunneling current versus time on top of oscillating adatoms, keeping the feedback loop turned off. The dynamics of adatoms, detected as the telegraph noise present in the tunneling current, was registered near the steps at 80 K. The atomic str…
Empirical tight binding(ETB) methods are widely used in atomistic device simulations. Traditional ways of generating the ETB parameters rely on direct fitting to bulk experiments or theoretical electronic bands. However, ETB calculations based on existing parameters lead to unphysical results in ultra small structures like the As terminated GaAs ultra thin bodies(UTBs). In this work, it is shown that more reliable parameterizations can be obtained by a process of mapping ab-initio bands and wave functions to tight binding models. This process enables the calibration of not only the ETB energy …
We present ab initio local-density-functional electronic structure calculations for the (111) and (-1-1-1) surfaces of cubic BN. The energetically stable reconstructions, namely the N adatom, N3 triangle models on the (111), the (2x1), boron and nitrogen triangle patterns on the (-1-1-1) surface are investigated. Band structure and properties of the surface states are discussed in detail.
Employing density functional theory calculations we explore initial stage of competitive alloying of co-deposited silver and indium atoms into a silicon surface. Particularly, we identify respective adsorption positions and activation barriers governing their diffusion on the dimer-reconstructed silicon surface. Further, we develop a growth model that properly describes diffusion mechanisms and silicon morphology with the account of silicon dimerization and the presence of C-type defects. Based on the surface kinetic Monte Carlo simulations we examine dynamics of bimetallic adsorption and elab…
We present a novel approach to spin manipulation in atomic-scale nanostructures. Our ab initio calculations clearly demonstrate that it is possible to tune magnetic properties of sub-nanometer structures by adjusting the geometry of the system. By the example of two surface-based systems we demonstrate that (i) the magnetic moment of a single adatom coupled to a buried magnetic Co layer can be stabilized in either a ferromagnetic or an antiferromagnetic configuration depending on the spacer thickness. It is found that a buried Co layer has a profound effect on the exchange interaction between …
Results are presented of a fully ab-initio calculation of impact ionization rates in GaAs within the density functional theory framework, using a screened-exchange formalism and the highly precise all-electron full-potential linearized augmented plane wave (FLAPW) method. The calculated impact ionization rates show a marked orientation dependence in {\bf k} space, indicating the strong restrictions imposed by the conservation of energy and momentum. This anisotropy diminishes as the impacting electron energy increases. A Keldysh type fit performed on the energy-dependent rate shows a rather so…
Using the plane wave pseudopotential method we performed density functional theory calculations on the stability of steps and self-diffusion processes on Ag(100). Our calculated step formation energies show that the {111}-faceted step is more stable than the {110}-faceted step. In accordance with experimental observations we find that the equilibrium island shape should be octagonal very close to a square with predominately {111}-faceted steps. For the (100) surface of fcc metals atomic migration proceeds by a hopping or an exchange process. For Ag(100) we find that adatoms diffuse across flat…
GaAs nanopillars with 150 nm - 200 nm long axial InGaAs inserts are grown by MOCVD via catalyst-free selective-area-epitaxy (SAE). The alloy composition of the InGaAs region, as determined by room-temperature photoluminescence (PL), depends critically on the pitch and diameter of the selective-area pattern geometry. The PL emission varies based on pattern geometry from 1.0 \{mu}m to 1.25 \{mu}m corresponding to a In to Ga ratio from 0.15 to > 0.3. This In enrichment is explained by a pattern dependent change in the incorporation rate for In and Ga. Capture coefficients for Ga and In adatoms ar…
We report on the quenching of single Ni adatom moments on Te-terminated Bi2Te2Se and Bi2Te3 topological insulator surfaces. The effect becomes manifested as a missing X-ray magnetic circular dichroism for resonant L3,2 transitions into partially filled Ni 3d states of occupancy n_d = 9.2. On the basis of a comparative study of Ni and Fe using scanning tunneling microscopy and ab initio calculations we are able to relate the element specific moment formation to a local Stoner criterion. While Fe adatoms form large spin moments of m_s = 2.54 mu_B with out-of-plane anisotropy due to a sufficientl…
We investigate to what extent the magnetic properties of deposited nanostructures can be influenced by selecting as a support different surfaces of the same substrate material. Fully relativistic ab initio calculations were performed for Co monolayers and adatoms on Pd(100), Pd(111), and Pd(110) surfaces. Changing the crystallographic orientation of the surface has a moderate effect on the spin magnetic moment and on the number of holes in the d band, a larger effect on the orbital magnetic moment but sometimes a dramatic effect on the magnetocrystalline anisotropy energy (MAE) and on the magn…
We examine whether essence and quantitative aspects of electronic excitation spectra are correctly captured by an effective low-energy model constructed from an {\em ab initio} downfolding scheme. A global electronic structure is first calculated by {\em ab initio} density-functional calculations with the generalized gradient approximation. With the help of constrained density functional theory, the low-energy effective Hamiltonian for bands near the Fermi level is constructed by the downfolding procedure in the basis of maximally localized Wannier functions. The excited states of this low-ene…
The atomic and electronic structures of a graphene layer on top of the $(2\times2)$ reconstruction of the SiC (000$\bar{1}$) surface are studied from ab initio calculations. At variance with the (0001) face, no C bufferlayer is found here. Si adatoms passivate the substrate surface so that the very first C layer presents a linear dispersion characteristic of graphene. A small graphene-substrate interaction remains in agreement with scanning tunneling experiments (F.Hiebel et al. {\it Phys. Rev. B} {\bf 78} 153412 (2008)). The stacking geometry has little influence on the interaction which expl…
Using density-functional theory we investigate several properties of Al(111), Al(100), Al(110), and stepped Al(111) surfaces. We report results of formation energies of surfaces, steps, adatoms, and vacancies. For the adsorption and diffusion of Al on flat regions of Al(111) surfaces we find the hcp site energetically slightly preferred over the fcc site. The energy barrier for self-diffusion on Al(111) is very low (0.04eV). Coming close to one of the two sorts of close packed, monoatomic steps on Al(111), labeled according to their {111} and {100} micro-facets, Al adatoms experience an attrac…
We report on a theoretical study of the electronic, optical and magneto-optical properties of digital ferromagnetic hetero-structures based on $Mn$ $δ$--doped $GaAs$. We consider different structures corresponding to $Mn$ contents within the range 12-50\% and we study how the system changes as a function of the doping concentration. Our first--principles approach includes the spin-orbit interaction in a fully relativistic pseudopotential scheme and the local fields effect in the description of the optical absorption. We show that $Mn$ $δ$-doped $GaAs$ shares many properties with the uniformly …
We study the growth of the Fe films on GaAs(100) at a low temperature, 140 K, by $in$-$situ$ UHV x-ray reflectivity using synchrotron radiation. We find rough surface with the growth exponent, $β_S$ = 0.51$\pm$0.04. This indicates that the growth of the Fe film proceeds via the restrictive relaxation due to insufficient thermal diffusion of the adatoms. The XRR curves are nicely fit by a model with a uniform Fe film, implying that the surface segregation and interface alloying of both Ga and As are negligible. When the Fe film is annealed to 300 K, however, the corresponding XRR can be fit onl…
\textit{Ab initio} studies of influence of periodic-direction electric fields on spin lifetime and spin diffusion length and the validation of an \textit{ab initio} matrix-drift-diffusion model
Adsorption of carbon, nitrogen, and oxygen on the (111) surface of delta-Plutonium has been studied within the framework of density functional theory using the full-potential linear augmented plane wave plus local basis (FP-LAPW+lo) method. All adatoms prefer to bind at the higher coordinated hollow sites, with the chemisorption energies for C, N, and O being 6.539 eV, 6.714 eV, and 8.2 eV respectively. The work function and the surface Pu magnetic moments respectively increased and decreased in all cases upon chemisorption. The partial charges inside the muffin tins spheres, difference charge…
An extended elastic eigenvector approach had earlier been developed to interpret ab-initio calculations of adatom interactions. It shows oscillating interactions as well as trio- and quarto (multisite) terms within clusters. It is now applied to the interaction of Fe adatoms on Cu(111). The extended approach differs from previous calculations by using a sharp cutoff - generating oscillating interactions - and by taking into account interacting dimers - generating strong anisotropies and multisite terms. Additional weak anisotropy stems from the substrate and from the surface Brillouin zone sha…
We compute from first principles the infrared dispersion of the nonlinear susceptibility $χ^{(2)}$ in zincblende semiconductors. At terahertz frequencies the nonlinear susceptibility depends not only on the purely electronic response $χ^{(2)}_{\infty}$, but also on three other parameters $C_1$, $C_2$ and $C_3$ describing the contributions from ionic motion. They relate to the TO Raman polarizability, the second-order displacement-induced dielectric polarization, and the third-order lattice potential. Contrary to previous theory, we find that mechanical anharmonicity ($C_3$) dominates over elec…
Ga interstitials in GaAs ($I_{Ga}$) are studied using the local-orbital {ab-initio} code SIESTA in a supercell of {216+1} atoms. Starting from eight different initial configurations, we find five metastable structures: the two tetrahedral sites in addition to the 110-split$\mathrm{_{[Ga-As]}}$, 111-split$\mathrm{_{[Ga-As]}}$, and 100-split$\mathrm{_{[Ga-Ga]}}$. Studying the competition between various configuration and charges of $I_{Ga}$, we find that predominant gallium interstitials in GaAs are charged +1, neutral or at most -1 depending on doping conditions and prefer to occupy the tetrahe…
We use a fully relativistic layer Green's functions approach to investigate spin-dependent tunneling through a symmetric indirect band gap barrier like GaAs/AlAs/GaAs heterostructure along [100] direction. The method is based on Linear Muffin Tin Orbitals and it is within the Density Functional Theory (DFT) in the Local Density Approximation (LDA). We find that the results of our {\it ab-initio} calculations are in good agreement with the predictions of our previous empirical tight binding model [Phys. Rev. {\bf B}, 075313 (2006)]. In addition we show the $k_{||}$-dependence of the spin polari…
We demonstrate how first-principles calculations using density-functional theory (DFT) can be applied to gain insight into the molecular processes that rule the physics of materials processing. Specifically, we study the molecular beam epitaxy (MBE) of arsenic compound semiconductors. For homoepitaxy of GaAs on GaAs(001), a growth model is presented that builds on results of DFT calculations for molecular processes on the beta2-reconstructed GaAs(001) surface, including adsorption, desorption, surface diffusion and nucleation. Kinetic Monte Carlo simulations on the basis of the calculated ener…
A modified core-to-valence band maximum approach is applied to calculate band offsets of strained III/V semiconductor hetero junctions. The method is used for the analysis of (In,Ga)As/GaAs/Ga(As,Sb) multi-quantum well structures. The obtained offsets and the resulting bandstructure are used as input for the microscopic calculation of photoluminescence spectra yielding very good agreement with recent experimental results.
We present a first-principles analysis of the strain renormalization of the cation diffusivity on the GaAs(001) surface. For the example of In/GaAs(001)-c(4x4) it is shown that the binding of In is increased when the substrate lattice is expanded. The diffusion barrier ΔE(e) has a non-monotonic strain dependence with a maximum at compressive strain values (e < 0), while being a decreasing function for any tensile strain (e > 0) studied. We discuss the consequences of spatial variations of both the binding energy and the diffusion barrier of an adatom caused by the strain field around a heteroe…
The DFT calculations were performed of densities of states of semiconductor GaAs and magnetic semiconductor Ga0.9375Mn0.0625As. It is obtained that a part of Mn3d- states is hybridized with valence band at Fermi level. The exchange integrals of Anderson impurity model were calculated making use of atomic Hartree-Fock package and angular momentum coupling technique. Theoretical Tc of Ga0.9375Mn0.0625As obtained in the multiscale ab initio method is in reasonable agreement with experiment. The application of Hubbard parameters in DFT calculations using ultrasoft pseudopotentials is discussed.
We propose a (three-dimension) -> (one-dimension) shift of paradigm for basic understanding of Raman spectra of random Si-Ge. Fair contour modeling of Raman spectra is achieved along the linear chain approximation via 1D-cluster version of the phenomenological Percolation scheme, originally developed for zincblende alloys, after ab initio calibration of the intrinsic Si-Si, Ge-Ge and Si-Ge Raman efficiencies. The 1D-cluster scheme introduces a seven-oscillator [1x(Ge-Ge), 4x(Si-Ge), 2x(Si-Si)] Raman behavior for SiGe, which considerably deviates from the currently admitted six-oscillator [1x(G…
MnAs/GaAs superlattices, made by $δ$-doping GaAs with Mn, are known as digital ferromagnetic heterostructures. Here we present a theoretical density functional study of the electronic, magnetic and transport properties of such heterostructures. In the absence of intrinsic donors these systems show an half metallic density of states, with an exchange interaction much stronger than that of a random alloy with the same Mn concentration. {\it Ab initio} ballistic transport calculations show that the carriers with energies close to the Fermi energy are strongly confined within a few monolayers arou…
We describe a new approach to compute the electron-phonon self-energy and carrier mobilities in semiconductors. Our implementation does not require a localized basis set to interpolate the electron-phonon matrix elements, with the advantage that computations can be easily automated. Scattering potentials are interpolated on dense $\mathbf{q}$ meshes using Fourier transforms and ab initio models to describe the long-range potentials generated by dipoles and quadrupoles. To reduce significantly the computational cost, we take advantage of crystal symmetries and employ the linear tetrahedron meth…
We study the effect of semicore states on the self-energy corrections and electronic energy gaps of silicon, germanium and GaAs. Self-energy effects are computed within the GW approach, and electronic states are expanded in a plane-wave basis. For these materials, we generate {\it ab initio} pseudopotentials treating as valence states the outermost two shells of atomic orbitals, rather than only the outermost valence shell as in traditional pseudopotential calculations. The resulting direct and indirect energy gaps are compared with experimental measurements and with previous calculations base…
We present theoretical studies for the third-order elastic constants $C_{ijk}$ in zinc-blende nitrides AlN, GaN, and InN. Our predictions for these compounds are based on detailed ab initio calculations of strain-energy and strain-stress relations in the framework of the density functional theory. To judge the computational accuracy, we compare the ab initio calculated results for $C_{ijk}$ with experimental data available for Si and GaAs. We also underline the relation of the third-order elastic constants to other quantities characterizing anharmonic behaviour of materials, such as pressure d…
