Linear Muffin-tin Orbital Method Research Articles

First-principles quantum transport method for disordered nanoelectronics: Disorder-averaged transmission, shot noise, and device-to-device variability

Because disorders are inevitable in realistic nanodevices, the capability to quantitatively simulate the disorder effects on electron transport is indispensable for quantum transport theory. Here, we report a unified and effective first-principles quantum transport method for analyzing effects of chemical or substitutional disorder on transport properties of nanoelectronics, including averaged transmission coefficient, shot noise, and disorder-induced device-to-device variability. All our theoretical formulations and numerical implementations are worked out within the framework of the tight-binding linear muffin tin orbital method. In this method, we carry out the electronic structure calculation with the density functional theory, treat the nonequilibrium statistics by the nonequilbrium Green's function method, and include the effects of multiple impurity scattering with the generalized nonequilibrium vertex correction (NVC) method in coherent potential approximation (CPA). The generalized NVC equations are solved from first principles to obtain various disorder-averaged two-Green's-function correlators. This method provides a unified way to obtain different disorder-averaged transport properties of disordered nanoelectronics from first principles. To test our implementation, we apply the method to investigate the shot noise in the disordered copper conductor, and find all our results for different disorder concentrations approach a universal Fano factor $1/3$. As the second test, we calculate the device-to-device variability in the spin-dependent transport through the disordered Cu/Co interface and find the conductance fluctuation is very large in the minority spin channel and negligible in the majority spin channel. Our results agree well with experimental measurements and other theories. In both applications, we show the generalized nonequilibrium vertex corrections play a determinant role in electron transport simulation. Our results demonstrate the effectiveness of the first-principles generalized CPA-NVC for atomistic analysis of disordered nanoelectronics, extending the capability of quantum transport simulation.

Li9Al4Sn5as a new ordered superstructure of the Li13Sn5type

Alloys from the ternary Li-Al-Sn system have been investigated with respect to possible applications as negative electrode materials in Li-ion batteries. This led to the discovery of a new ternary compound, a superstructure of the Li13Sn5 binary compound. The ternary stannide, Li9Al4Sn5 (nonalithium tetraaluminium pentastannide; trigonal, P-3m1, hP18), crystallizes as a new structure type, which is an ordered variant of the binary Li13Sn5 structure type. One Li and one Sn site have -3m. symmetry, and all other atoms occupy sites of 3m. symmetry. The polyhedra around all types of atoms are rhombic dodecahedra. The electronic structure was calculated by the tight-binding linear muffin-tin orbital atomic spheres approximation method. The electron concentration is higher around the Sn and Al atoms, which form an [Al4Sn5]m- polyanion.

Electronic and magnetic properties of the ferromagnetic superconductor UCoGe

The electronic structure and x-ray magnetic circular dichroism (XMCD) spectra of the ferromagnetic superconductor UCoGe at the U N4,5, Ge and Co K and Co L2,3 edges were investigated theoretically from first principles, using the fully relativistic Dirac linear muffin-tin orbital band structure method. The electronic structure is obtained with the local spin-density approximation (LSDA), as well as with a generalization of the LSDA+U method which takes into account the non-diagonal occupation matrix (in spin indexes) of localized electrons. A stable ferromagnetic ground state was found. The uranium total magnetic moment is quite small (about −0.171μB) in the LSDA approximation as a result of almost complete cancellation between the spin magnetic moment of 0.657 μB and the opposite orbital magnetic moment of −0.828 μB, resulting from strong spin-orbit coupling at the uranium site. Valency of U ion in UCoGe is close to 3+. The ratio orbital and spin magnetic moments Ml/Ms ranged from 1.163 in the GGA approa...

High Pressure Band Structure, Density of States, Structural Phase Transition and Metallization in CdS

The electronic band structure, density of states, metallization and structural phase transition of cubic zinc blende type cadmium sulphide (CdS) is investigated using the full potential linear muffin-tin orbital (FP-LMTO) method. The ground state properties and band gap values are compared with the experimental results. The equilibrium lattice constant, bulk modulus and its pressure derivative and the phase transition pressure at which the compounds undergo structural phase transition from ZnS to NaCl are predicted from the total energy calculations. The density of states at the Fermi level (N(E F )) gets enhanced under pressure, which leads to metallization in CdS. The pressure corresponding to structural phase transition from ZnS structure (B3) to the NaCl structure (B1) is 0.0168 Mbar in CdS. The metallization pressure PM is 6.9133 Mbar.

Band Structure, Density of States, Structural Phase Transition and Metallization in CdO Under High Pressure

The structural phase transition and metallization of the group II-VI compound semiconductor cadmium oxide (CdO) is investigated through its band structure obtained using the full potential linear muffin-tin orbital (FP-LMTO) method. The ground state properties and band gap values are compared with the experimental results. Under high pressure CdO undergoes a first order structural phase transition from the NaCl structure (B1) to the CsCl structure (B2). The band structure, density of states (DOS) and total energy are computed as a function of reduced volume for both NaCl and CsCl phases. The values of pressure under reduced volume are calculated using Birch-Murnaghan’s equation of state. The pressure corresponding to structural phase transition from NaCl structure (B1) to the CsCl structure (B2) is 0.9 Mbar in CdO. The metallization pressure PM is 1.2 Mbar. DOI: http://dx.doi.org/10.17807/orbital.v0i0.884

FPLMTO study of new phase changes in CuX (X = Cl, Br, I) compounds under hydrostatic pressure

We study the ground-state properties of copper halides CuX (X = Cl, Br, I) and the behavior under pressure using a recent version of the full potential linear muffin-tin orbitals method (FPLMTO). It predicts the possibility of an additional local minimum in the tetragonal PbO phase for CuBr and CuI at 0.7 GPa and 01.53 GPa with a direct energy gap at Γ of about 0.678 eV and 1.265 eV respectively. The use of GGA in this work is more appropriate than LDA and correctly predicts the cubic B3 phase as ground state structure for the three binaries. Importantly, calculated transition pressure Pt revealed also the presence of PbO phase at 5.333 GPa for CuCl which is consistent with experimental reports. Other important results are that concerned with the possibility of phase transition from B3 to the hexagonal B8 phase at lower pressures (3.43, 4.11 and 6.23) GPa and from B3 to B2 at (22.96, 22.66 and 24.83) GPa respectively for CuBr, CuI and CuCl.

Comparative study on performance and physical properties of CsI, NaI, RbI, and KBr materials

In the present work we report the results of our investigation using the full potential linear muffin tin orbital method (FP-LMTO) to calculate the structural and electronic properties of NaI, KBr, RbI, and CsI materials, in particular, the lattice constant and bulk modulus. The electronic band structures are also studied and discussed. Our aim is to find the relationship between the physical properties of these materials and their efficiency for detection. For this, Geant4 simulation has been performed for studying the absolute and photopeak detection efficiencies for the different materials as scintillation detectors at several photon energies. The range of gamma ray energy was between 662 and 1332.5 keV. The measurement results are compared with the available theoretical and experimental data.

Ab Initio Study of Electronic Structure, Elastic and Transport Properties of Fluoroperovskite LiBeF3

The aim of this work is to investigate the electronic, mechanical, and transport properties of the fluoroperovskite compound LiBeF3 by first-principles calculations using the full-potential linear muffin-tin orbital method based on density functional theory within the local density approximation. The independent elastic constants and related mechanical properties including the bulk modulus (B), shear modulus (G), Young’s modulus (E), and Poisson’s ratio (ν) have been studied, yielding the elastic moduli, shear wave velocities, and Debye temperature. According to the electronic properties, this compound is an indirect-bandgap material, in good agreement with available theoretical data. The electron effective mass, hole effective mass, and energy bandgaps with their volume and pressure dependence are investigated for the first time.

Crystal, magnetic and electronic structures of 3d–5d ordered double perovskite Ba 2 CoReO 6

A comprehensive study on crystal, magnetic and electronic structures of ordered double perovskite Ba2CoReO6 was carried out using X-ray powder diffraction (XRD) and superconducting quantum interference device (SQUID). Also, the density functional theory (DFT) calculations were performed by full potential linear muffin-tin orbital (FP-LMTO) method within the localized spin density approximation (LSDA+U) and generalized gradient approximation (GGA+U). At room temperature, the crystal structure of Ba2CoReO6 is face-centered cubic, space group Fm3¯m, containing an almost completely ordered arrangement of CoO6–ReO6 octahedra. Magnetic structure showed an antiferromagnetic (AF) behavior below TN=41 K. The magnetic and electronic structures are consistent with the electronic configurations Co2+(3d7)–Re6+(5d1) having a total spin magnetic moment of about 2.0 µB/f.u. DFT electronic structures predicted half-metallic yields from 3d-t2g↓ and 5d-t2g↓ through O2−.

Ab initio study of structural and electronic properties of (GaN)n/(AlN)n superlattices

Structural and electronic properties of binary GaN and AlN compounds and their superlattices (SLs) (GaN)n/(AlN)n are investigated using the first-principles full potential linear muffin-tin orbitals method (FP-LMTO). The exchange-correlation potential is treated with the local density approximation of Perdew and Wang (LDA-PW). The ground-state properties are determined for the bulk materials GaN, AlN, and their superlattices (GaN)n/(AlN)n) in cubic phase. The calculated structural properties of GaN and AlN compounds are in good agreement with available experimental and theoretical data. It is found that AlN exhibit an indirect fundamental band gap while that GaN and the superlattices (SLs) exhibit a direct fundamental band gap, which might make the superlattices (GaN)n/(AlN)n materials promising and useful for optoelectronic applications. The fundamental band gap decreases with increasing the number of monolayer.

Elastic and electronic properties calculations of the filled skutterudite CeOs4P12

The full-potential linear muffin-tin orbital method (FP-LMTO) within the local density approximation (LDA) using the Perdew-Wang parameterization is used to calculate the structural, electronic and elastic properties of the filled skutterudite CeOs4P12. The results of the electronic properties show that this compound is an indirect band gap material (T -N). A special interest has been made to the determination of the elastic constants since there have been no available experimental and theoretical data. The energy band gaps and their volume and pressure dependence are also investigated.

Density functional study of the optical response of FeN and CoN nitrides with zinc-blend and rock-salt structures

We investigated the optical properties of iron and cobalt mono-nitrides in rock-salt and zinc-blende crystal structures. Density functional theoretic calculations were performed using the linear muffin tin orbital method in the generalized gradient approximation for exchange and correlation. The optical response was characterized by means of the dielectric function, calculated for each structural phase in the different magnetic arrangements found experimentally and theoretically. The origin of the main absorption peaks was traced back to particular interband transitions after a careful analysis of the band structures and orbital- and atom- projected electronic densities of states. The optical response of both nitrides with this structure in the nomagnetic state is very similar at photon frequencies below 2eV, but differ at high frequencies. In zinc-blende structure, both compounds are nomagnetic and present an optical gap of about 0.5eV for interband transitions in their optical response.

Electronic band structure ofMg−IV−N2compounds in the quasiparticle-self-consistentGWapproximation

We present calculations of the lattice constants, structural parameters, bulk moduli, energies of formation, and band structures of $\mathrm{Mg}\ensuremath{-}\mathrm{IV}\ensuremath{-}{\mathrm{N}}_{2}$ compounds with IV=Si, Ge, Sn by using the full-potential linearized muffin-tin orbital method and the quasiparticle-self-consistent $GW$ approach for the wurtzite-based $Pna{2}_{1}$ crystal structure. The lattice parameters calculated with the generalized gradient approximation (GGA) are found to be in good agreement (within 1%) with experiment for the cases of ${\mathrm{MgSiN}}_{2}$ and ${\mathrm{MgGeN}}_{2}$, where data are available. Similar to the Zn-IV-${\mathrm{N}}_{2}$ compounds, ${\mathrm{MgSiN}}_{2}$ is found to have an indirect gap slightly lower than the lowest direct gap, while the other materials have direct gaps. The direct gaps, calculated at the GGA lattice constant, range from 3.43 eV for ${\mathrm{MgSnN}}_{2}$ to 5.14 eV for ${\mathrm{MgGeN}}_{2}$ and 6.28 eV for ${\mathrm{MgSiN}}_{2}$ in the $0.8\mathrm{\ensuremath{\Sigma}}$ approximation, i.e., reducing the $\mathrm{QS}GW\phantom{\rule{4pt}{0ex}}\mathrm{\ensuremath{\Sigma}}$ by a factor 0.8 and including an estimated zero-point-motion correction. The symmetry character of the valence-band maximum states and their splittings and effective masses are determined. The conduction-band minima are found to have slightly higher Mg $s$- than Si $s$-like character in ${\mathrm{MgSiN}}_{2}$ but in ${\mathrm{MgGeN}}_{2}$ and ${\mathrm{MgSnN}}_{2}$, the group-IV-$s$ character becomes increasingly dominant.

First-principles evaluation of intrinsic, side-jump, and skew-scattering parts of anomalous Hall conductivities in disordered alloys

We develop a first-principles procedure for the individual evaluation of the intrinsic, side-jump, and skew-scattering contributions to the anomalous Hall conductivity $\sigma_{xy}$. This method is based on the different microscopic conductive processes of each origin of $\sigma_{xy}$ in the Kubo--Bastin formula. We also present an approach for implementing this scheme in the tight-binding linear muffin-tin orbital (TB-LMTO) method with the coherent potential approximation (CPA). The validity of this calculation method is demonstrated for disordered FePt and FePd alloys. We find that the estimated value of each origin of $\sigma_{xy}$ exhibits reasonable dependencies on the electron scattering in these disordered alloys.

Exchange and spin-orbit induced phenomena in diluted (Ga,Mn)As from first principles

Physical properties induced by exchange interactions (Curie temperature and spin stiffness) and spin-orbit coupling (anomalous Hall effect, anisotropic magnetoresistance, and Gilbert damping) in the diluted (Ga,Mn)As ferromagnetic semiconductor are studied from first principles. Recently developed Kubo-Bastin transport theory and nonlocal torque operator formulation of the Gilbert damping as formulated in the tight-binding linear muffin-tin orbital method are used. The first-principles Liechtenstein mapping is employed to construct an effective Heisenberg Hamiltonian and to estimate Curie temperature and spin stiffness in the real-space random-phase approximation. Good agreement of calculated physical quantities with experiments on well-annealed samples containing only a small amount of compensating defects is obtained.

Comment on “Structural, electronic, elastic, optical and thermodynamic properties of copper halides CuCl, CuBr and their ternary alloys CuCl1−xBrx (0.0 ≤ x ≤ 1.0) using full-potential linear muffin-tin orbital (FP-LMTO) method”[Optik 127 (2016) 4559–4573

Kamel et al. (2016) [1] have recently published a paper in Optik 127 (2016) 4559–4573. They have studied several interesting properties, such as: structural, elastic, electronic, optical and thermodynamic properties of copper halides (CuCl, CuBr and their ternary alloys CuCl1−xBrx (0.0≤x≤1.0)) using full-potential linear muffin-tin orbital (FP-LMTO) method. After carefully reading the paper of Kamel et al. (2016), it is observed that the algebraic values of the longitudinal (vl), transverse (vt) and average (vm) wave velocities, and also that of the Debye temperature θD, of CuX (X=Cl, Br) binary compounds obtained in the paper Kamel et al. (2016) [1] are not correctly calculated. Although this mistake has no influence on the conclusion of the paper Kamel et al. (2016) [1], I think, that is better to correct the numerical values of the previous quantities, which can be reused in the future by other researchers, perhaps for comparison or for other object.In the present work, I recalculate the correct numerical values of the longitudinal (vl), transverse (vt) and average (vm) wave velocities, and the Debye temperature θD, of CuX (X=Cl, Br) compounds, using the lattice parameters and the elastic moduli obtained in the original work of Kamel et al. (2016) [1].

Ab Initio Theory of the Gilbert Damping in Random Ferromagnetic Alloys

We present an ab initio theory of the Gilbert damping in ferromagnetic alloys with substitutional disorder. The theory is based on nonlocal torques that are represented by nonrandom, site-off-diagonal, and spin-independent matrices, which simplifies the configuration averaging. The formalism is developed for the relativistic tight-binding linear muffin-tin orbital (TB-LMTO) method and the coherent potential approximation (CPA). The CPA-vertex corrections play a crucial role for the internal consistency of the theory and for its exact equivalence to other first-principles approaches based on random local torques. The theory is illustrated by calculations for various random transition metal alloys: FeNi, FeCo, Heusler alloys, permalloy with impurities, Fe with vacancies, and stoichiometric FePt and CoPt alloys with a varying degree of L1 0 atomic long-range order. Results are in a reasonable agreement with other calculations and accessible experimental data.

Modelling of free positron states in TiHx

Electron energy structure, positron spectrum and positron characteristics of α-Ti and α-TiH0.125 were calculated. Self-consistent calculations of the band structure were performed by the linear muffin-tin orbital method in the atomic sphere approximation. Modelling has been made on low content of hydrogen into α-Ti with expanded close-packed hexagonal cell inclusive 8 titanium atoms. Variation of sphere radiuses permitted to consider anisotropy and spherical symmetry of potential. Positron potential and positron wave function were calculated on a base of self-consistent electron density. Then positron probability of existence into TiHx lattice and lifetime were founded. Theoretical calculation indicated a satisfactory agreement of positron characteristics absolute values with the experimental data is achieved but the tendency of values with hydrogen defects increasing is not. The reason of divergence is discussed. On the basis of experimental data and theoretical calculations it was shown that different hydrogen atom states demonstrate the different influence in the lifetime spectra.

Electronic structure and X-ray spectroscopic properties of YbNi2P2

X-ray absorption spectrum at the Yb L3 edge and X-ray emission spectra of Ni and P at the K and L2,3 edges have been studied experimentally and theoretically in the mixed valent compound YbNi2P2 with ThCr2Si2 type crystal structure. The electronic structure of YbNi2P2 is investigated using the fully relativistic Dirac linear muffin-tin orbital (LMTO) band-structure method. The effect of the spin–orbit (SO) interaction and Coulomb repulsion U on the electronic structure of YbNi2P2 is examined in the frame of the LSDA+SO+U method. The core-hole effect in the final states as well as the effect of the electric quadrupole E2 transitions have been investigated. A good agreement between the theory and the experiment was found. Both the trivalent and the divalent Yb ions in YbNi2P2 are reflected in the experimentally measured Yb L3 X-ray absorption spectrum simultaneously. We found that the best agreement between the experimental spectrum and sum of the theoretically calculated Yb2+ and Yb3+ spectra is achieved with 73% ytterbium ions in 2+ state and 27% ions in 3+ state.

Electronic structure and x-ray magnetic circular dichroism inA2FeReO6(A=Ca,Sr,andBa)oxides

A systematic electronic structure study of ${A}_{2}{\mathrm{FeReO}}_{6}(A=\mathrm{Ba},\mathrm{Sr},\text{and}\mathrm{Ca})$ has been performed by employing the local-spin-density approximation (LSDA) and $\mathrm{LSDA}+U$ methods using the fully relativistic spin-polarized Dirac linear muffin-tin orbital band-structure method. We investigated the effects of the subtle interplay between spin-orbit coupling, electron correlations, and lattice distortion on the electronic structure of double perovskites. ${\mathrm{Ca}}_{2}{\mathrm{FeReO}}_{6}$ has a large distortion in the Fe-O-Re bond, and the electronic structure is mainly determined by electron correlations and lattice distortion. In the $\mathrm{Ba}\ensuremath{-}\mathrm{Sr}\ensuremath{-}\mathrm{Ca}$ row, the correlation effects at the Fe site are increased. The correlations at the Re site are small in the Ba- and Sr-based compounds but significant in ${\mathrm{Ca}}_{2}{\mathrm{FeReO}}_{6}$. ${\mathrm{Ca}}_{2}{\mathrm{FeReO}}_{6}$ behaves like an insulator only if considered with a relatively large value of Coulomb repulsion ${U}_{\text{eff}}=2.3$ eV at the Re site in addition to ${U}_{\text{eff}}=3.1$ eV at the Fe site. ${\mathrm{Ca}}_{2}{\mathrm{FeReO}}_{6}$ possesses a phase transition at 140 K where the metal-insulator transition (MIT) occurs between metallic high-temperature and insulating low-temperature phases. The spin and orbital magnetic moments are linear functions of temperature before and after the MIT but change abruptly at the point of the phase transition. From theoretically calculated magnetocrystalline anisotropy energy (MAE), we found that the easy axis of magnetization for the low-temperature phase is along the $b$ direction, in agreement with experimental data. We found that the major contribution to the MAE is due to the orbital magnetic anisotropy at the Re site. X-ray-absorption spectra and x-ray magnetic circular dichroism at the Re, Fe, and Ba ${L}_{2,3}$ and Fe, Ca, and O $K$ edges were investigated theoretically in the frame of the $\mathrm{LSDA}+U$ method. A qualitative explanation of the x-ray magnetic circular dichroism spectra shape is provided by an analysis of the corresponding selection rules, orbital character, and occupation numbers of individual orbitals. The calculated results are compared with available experimental data.