Full-potential Linear Muffin-tin Orbital Method Research Articles

Optical properties of (Ge2)1−x (InP)x solid solutions

The optical properties of (Ge2)1−x(InP)x solid solutions have been studied by means of ab initio calculations of the electron band structure performed using a self-consistent relativistic scalar full-potential linear muffin-tin orbital method and the density functional theory within the framework of a local charge density approximation.

High pressure study of structural and electronic properties of magnesium telluride

The full-potential linear muffin-tin orbital method (FP-LMTO) within the local density approximation (LDA) is used to calculate the electronic band structures and the total energies of MgTe in its stable (NiAs-B8) and high pressure phases. The latter provide us with the ground state properties such us lattice parameter, bulk modulus and its pressure derivatives. The transition pressure at which this compound undergoes the structural phase transition from the NiAs to CsCl phase is calculated. The energy band gaps and their volume and pressure dependence in the stable NiAs-B8 phase are investigated. The ground state properties, the transition pressure are found to agree with the experimental and other theoretical results. The elastic constants at equilibrium in both NiAs and CsCl structure are also determined.

First-principles calculations of elastic properties of LiBC

We employ all-electron full-potential linearized muffin-tin orbital method (FP-LMTO) within the frame of density functional theory (DFT) to calculate the equilibrium lattice parameters, five independent elastic constants and Debye temperature of LiBC. The equilibrium lattice parameters obtained are in good agreement with experimental data and those by others. The Debye temperature Θ D from the calculated elastic constants of LiBC is higher than that of MgB 2 and TiB 2. The anisotropies of the three types of elastic waves in LiBC at zero pressure have been discussed. It is found that the anisotropy of compressional waves in LiBC is considerably larger than that in MgB 2 and TiB 2.

Electronic spectra and magnetic properties of RB6, RB12 and RB2C2 borides

The electronic structures of RB 6 , RB 1 2 and RB 2 C 2 borides are studied ab initio by using the full-potential linear muffin-tin orbital method. This study includes the promising materials for spin electronics with reported high temperature ferromagnetism, namely, doped divalent hexaborides CaB 6 , SrB 6 , BaB 6 , and the CaB 2 C 2 compound, as well as Kondo semiconductors, SmB 6 and YbB 1 2 . For CaB 6 and SrB 6 a semiconducting band structure has been obtained, whereas a semimetallic ground state is revealed for CaB 2 C 2 and doped hexaborides. For YB 6 , LaB 6 , CaB 2 C 2 and the semimetallic Ba 1 - x La x B 6 alloys we have performed spin-polarized band structure calculations in an external field to evaluate the induced spin and orbital magnetic moments. These calculations indicate a feasibility of the field-induced weak ferromagnetic phase in CaB 2 C 2 and the La doped hexaborides. The LSDA and GGA calculations for different spin configurations of YbB 1 2 point to a possibility of antiferromagnetic coupling between Yb 3 + ions. For SmB 6 and YbB 1 2 our LSDA, GGA, and LSDA+U calculations have not revealed the hybridization gap for configurations with trivalent Sm 3 + and Yb 3 + .

Ab initio study of ZnO- and GaN-based diluted magnetic semiconductors and its magnetic properties

We study the magnetic and electronic properties of V- and Mn-doped hexagonal ZnO and cubic GaN by using the full-potential linear muffin-tin orbital method. The calculations are made at several concentrations from about 4% to 12% of dopant atoms in the 48 and 64 atoms supercell for ZnO and GaN, respectively. For Zn 1− x M x O ( M = V and Mn) at x = 0.0 8 3 (pair impurities), the energetically favorable magnetism is the antiferromagnetic states. For V-doped ZnO with the defect, the results show that it is strongly correlated between the energy states by the defect of Zn or O site and those by V impurity in ZnO.

Adsorption, Bond Formation and Graphitization of Carbon Atoms on Ni(111) Surface

Adsorption, bond formation and graphitization of carbon atoms on Ni(111) surface are investigated using full-potential linear muffin-tin orbital method with Ni(111) surface modeled by a bilayer slab. The present calculations show that the adsorption of a single carbon atom onto the Ni surface occurs on fcc and hcp hollow sites and on the on-top site with strong C-Ni bonds. When another carbon atom approaches the adatom, they form stronger C-C bonds to form single-layer graphene. At the same time, C-Ni bonds are weakened and the graphene tends to float on the surface. The stationary properties of the floating graphene are also examined.

Electronic structure of the ferromagnetic double-perovskites Sr2CrReO6, Sr2CrWO6, and Ba2FeReO6

We have studied the electronic structure of the ferromagnetic double perovskites Sr2CrReO6, Sr2CrWO6 and Ba2FeReO6 by means of a full-potential linear muffin-tin orbital density-functional method. Our scalar-relativistic calculations predict these compounds to be half-metallic with a total magnetic moment of 1, 2, and 3 µB respectively. However, when the spin-orbit coupling is included, the 5d transition Re and W ions exhibit substantial unquenched orbital magnetic moments, resulting in a significant increase of the total magnetic moment. The half-metallic gap turns into a pseudo-gap in Sr2CrReO6 and Ba2FeReO6 when the spin-orbit coupling is included whereas Sr2CrWO6 remains half-metallic even with spin-orbit coupling. The calculated spin and orbital magnetic moments agrees well with the recent experimental XMCD measurements.

Effect of chromium on the electronic structure of the cementite Fe3C

The effect of alloying of the cementite Fe3C with chromium on the band structure, atomic interactions, electric field gradients, and asymmetry parameters for iron nuclei is investigated using the self-consistent full-potential linear muffin-tin orbital (FPLMTO) method. An increase in the cohesive energy for the Fe3C-Cr system indicates an enhancement of the atomic interactions in the lattice of the cementite alloyed with chromium. It is found that the substitution of chromium for iron in the FeII positions containing eight equivalent iron atoms is energetically most favorable.

Superconductivity in boron under pressure: A full-potential linear muffin-tin orbitals study

Using the full-potential linear muffin-tin orbitals (FP-LMTO) method we examine the pressure dependence of superconductivity in the two metallic phases of boron (B), body-centered-tetragonal (bct) and fcc. Linear response calculations are carried out to examine the phonon frequencies and electron-phonon coupling for various lattice parameters, and superconducting transition temperatures are obtained from the isotropic Eliashberg equation. The fcc phase is found to be stable only at very high pressure $(\text{volume per atom}<21.3\phantom{\rule{0.3em}{0ex}}{\text{bohrs}}^{3})$, estimated to be in excess of 360 GPa. The bct phase $(\text{volume per atom}>21.3\phantom{\rule{0.3em}{0ex}}{\text{bohrs}}^{3})$ is stable at lower pressures in the range 210--360 GPa. In both bct and fcc phases the superconducting transition temperature ${T}_{c}$ is found to decrease with increasing pressure, due to the stiffening of phonons with an accompanying decrease in electron-phonon coupling. This is in contrast to a recent report, where ${T}_{c}$ is found to increase with pressure. Even more drastic is the difference between the measured ${T}_{c}$, in the range 4--11 K, and the calculated values for both bct and fcc phases, in the range 60--100 K. The calculation reveals that the transition from the fcc to bct phase, as a result of increasing volume or decreasing pressure, is caused by the softening of the $X$-point transverse phonons. This phonon softening also causes large electron-phonon coupling for high volumes in the fcc phase, resulting in coupling constants in excess of 2.5 and ${T}_{c}$ nearing 100 K. Although it is possible that the method used somewhat overestimates the electron-phonon coupling, its success in studying several other systems, including ${\mathrm{MgB}}_{2}$, clearly suggests that the experimental work should be reinvestigated. We discuss possible causes as to why the experiment might have revealed ${T}_{c}$'s much lower than what is suggested by the present study. The main assertion of this paper is that the possibility of high ${T}_{c}$, in excess of 50 K, in high pressure pure metallic phases of B cannot be ruled out, thus pointing to (substantiating) the need for further experimental investigations of the superconducting properties of high pressure pure phases of B.

First-principle study of structural, electronic and elastic properties of beryllium chalcogenides BeS, BeSe and BeTe

First-principles calculations have been used to investigated the structural, electronic and elastic properties of beryllium chalcogenides BeS, BeSe and BeTe, using the full-potential linear muffin-tin orbital (FP-LMTO) method. The exchange correlation energy is described in the local density approximation (LDA) using the Perdew–Wang parameterisation. From the results of the electronic properties, we find that these materials have indirect bandgaps. The structural parameters and the transition pressure from zinc-blende (B3) to the NiAs (B8) phase are determined. An agreement was found between our results and those of other theoretical calculations and the experimental data. A special interest has been made to the determination of the elastic constants since there have been no available experimental data. The calculated elastic constants found for BeS, BeSe and BeTe are C 11 = 1.4032, 1.1311, 0.8556 Mbar, C 12 = 0.9592, 0.7736, 0.5994 Mbar, C 44 = 0.2389, 0.1938, 0.136 Mbar, respectively. These results compared to other theoretical work show a discrepancy which is due to the use of LDA.

Magnetic moments of W5dinCa2CrWO6andSr2CrWO6double perovskites

We have investigated the magnetic moment of the W ion in the ferrimagnetic double perovskites Sr2CrWO6 and Ca2CrWO6 by X-ray magnetic circular dichroism (XMCD) at the W L(2,3) edges. In both compounds a finite negative spin and positive orbital magnetic moment was detected. The experimental results are in good agreement with band-structure calculations for (Sr/Ca)2CrWO6 using the full-potential linear muffin-tin orbital method. It is remarkable, that the magnetic ordering temperature, TC, is correlated with the magnetic moment at the 'non-magnetic' W atom.

Electronic structure of GaAs 1− xN x under pressure

The electronic band structures of GaAs 1− x N x for x=0.009, 0.016, 0.031 and 0.062 are calculated ab initio using a supercell approach in connection with the full-potential linear muffin–tin orbital method. Corrections for the ‘LDA gap errors’ are made by adding external potentials which are adjusted to yield correct gaps in pure GaAs. Even small amounts of nitrogen modify significantly the conduction bands, which become strongly non-parabolic. The effective mass in the lowest conduction band thus exhibits strong k-vector dependence. Calculated variations of gaps and effective masses with x and externally applied pressure are presented and compared to a variety of experimental data. There are significant error bars on our results due to the use of the supercell approach. These are estimated by examining the effects of varying the geometrical arrangement of the N-atoms substituting As. However, the calculations show that the electron mass for x>0.009 is much larger than that of pure GaAs, and that it decreases with x.

Theory of the temperature dependence of the easy axis of magnetization in hcp Gd

The magnetic anisotropy energy (MAE) of hcp Gd was calculated from first principles using the full-potential linear muffin-tin orbital method. It was found that the principal contributions to the MAE are the dipole-dipole interaction between the localized $4f$ spins and the spin-orbit interaction of the valence band states. The dipole contribution has the form $\frac{1}{2}{K}_{1}^{d}(1\ensuremath{-}\mathrm{cos}\phantom{\rule{0.2em}{0ex}}2\ensuremath{\theta})$, where $\ensuremath{\theta}$ is the angle between the magnetization direction and the $c$ axis. The contribution of the spin-orbit interaction is shown to arise from the polarization of the conduction band that becomes exchange split due to exchange interaction with the localized $4f$ electrons. We argue that this leads to significant contributions from higher order anisotropy constants. An imposed reduced $4f$ moment leads to a repopulation of the electronic states at the Fermi level and a reduced exchange splitting of the valence states, which we demonstrate leads to a modification of the MAE. This modification is in qualitative agreement with the observed temperature dependence of the MAE. In addition, the dependence of the MAE on the $c∕a$ ratio has been studied.

Theoretical study of cation-related point defects inZnGeP2

First-principles calculations are presented for the ${V}_{\mathrm{Zn}}$ and ${V}_{\mathrm{Ge}}$ cation vacancies and the ${\mathrm{Zn}}_{\mathrm{Ge}}$ and ${\mathrm{Ge}}_{\mathrm{Zn}}$ antisites in ${\mathrm{ZnGeP}}_{2}$, using full-potential linearized muffin-tin orbital method supercell calculations in the local-density approximation to density-functional theory. Under Zn-poor conditions, the lowest Gibbs energy defects are found to be the ${\mathrm{Ge}}_{\mathrm{Zn}}$ and ${V}_{\mathrm{Zn}}$ defects, leading to a compensated $p$-type material in agreement with experimental evidence. The occupation energy levels of the defects are determined and compared with available experimental information. As expected, the ${\mathrm{Ge}}_{\mathrm{Zn}}$ is found to be a donor while the other three are acceptors. Good agreement is obtained with optical quenching and activation of electron paramagnetic resonance signal studies if a direct transfer of electrons from ${\mathrm{V}}_{\mathrm{Zn}}^{2\ensuremath{-}}$ to ${\mathrm{Ge}}_{\mathrm{Zn}}^{2+}$ is assumed rather than a process via the conduction band. This suggests a close association of the dominant acceptors and donors. This is further confirmed by showing that the formation of complexes consisting of two ${V}_{\mathrm{Zn}}^{\ensuremath{-}}$ with a single ${\mathrm{Ge}}_{\mathrm{Zn}}^{2+}$ antisite are favorable in energy. The ${V}_{\mathrm{Ge}}$ on the other hand is found to have high energy of formation under any chemical potential conditions and is found to be unstable toward formation of a ${V}_{\mathrm{Zn}}$ and ${\mathrm{Zn}}_{\mathrm{Ge}}$ pair. Structural relaxation of all defects is performed but no symmetry breaking distortions are found. As a result, the defect wave functions of the unpaired electron in the ${V}_{\mathrm{Zn}}^{\ensuremath{-}}$ is found to be spread equally over the four neighboring P atoms, in disagreement with electron nuclear double resonance data which indicate primary localization on a pair of P atoms. Several possible origins for this discrepancy are discussed.

First-principles study of the structural and magnetic properties of iron indium nitride

A full-potential linear muffin-tin orbital method is used to study the magnetic and electronic properties of Fe1−xInxN alloys in zino-blende-derived structures. The lattice constant is found to vary linearly with concentration and to become matched to GaN near x=0.25. The materials are found to exhibit a magnetic moment beyond a critical lattice constant.

Electronic, elastic, and optical properties of Y2O2S

We have calculated the electronic structure, elastic constants, and dielectric function of Y2O2S by means of an accurate first-principles method using the full-potential linear muffin-tin orbital method. The conductivity anisotropy is analyzed using the electronic band structure. A bulk modulus of 125 GPa has been calculated from the elastic constants and the elastic anisotropy is investigated. Our calculated dielectric functions are in good agreement with the data of Itoh and Inabe [Phys. Rev. B 68, 035107 (2003)]. The calculated band gap of 3.0 eV is consistent with the calculation by Mikami and Oshiyama. We believe that the band-gap energy 6.77 eV reported by Itoh and Inabe is overestimated and that the true band gap should be around 4.6 eV. The calculated dielectric constant of 5.3 is comparable to the experimental data of 4.67.

Electronic structure of cadmium fluoride

The electronic structure, the densities of states, the dielectric function, and the localization of optical transitions in Brillouin zone of ${\mathrm{CdF}}_{2}$ crystals are calculated using the full-potential linear muffin-tin orbital method in the local density approximation. We have also calculated the dielectric function from the two experimental reflectivity spectra using the Kramers-Kronig relations. We have decomposed these calculated ${\ensuremath{\epsilon}}_{2}(E)$ spectra into components and compared their parameters with the theoretical results.

Pressure effect on the Fermi surface and electronic structure of LuGa3 and TmGa3

The Fermi surfaces and cyclotron masses of LuGa3 and TmGa3 compounds are studied by means of the de Haas-van Alphen effect technique under pressure. Highly anisotropic pressure dependences of the de Haas-van Alphen frequencies and cyclotron masses are observed in both compounds. Concurrently, ab initio calculations of the volume-dependent band structures are been carried out for these compounds, including the ferromagnetic-configuration phase of TmGa3, by employing a relativistic version of the full-potential linear muffin-tin orbital method within the local spin-density approximation. The experimental data are analyzed on the basis of the calculated volume-dependent band structures and compared with the corresponding pressure effects in the isostructural compound ErGa3.

Pseudo-half-metallicity in the double perovskite Sr2CrReO6 from density-functional calculations

The electronic structure of the spintronic material Sr2CrReO6 is studied by means of full-potential linear muffin-tin orbital method. Scalar relativistic calculations predict Sr2CrReO6 to be half-metallic with a magnetic moment of 1μB. When spin–orbit coupling is included, the half-metallic gap closes into a pseudo-gap, and an unquenched rhenium orbital moment appears, resulting in a significant increase of the total magnetic moment to 1.28μB. This moment is significantly larger than the experimental moment of 0.9μB. A possible explanation of this discrepancy is that the anti-site disorder in Sr2CrReO6 is significantly larger than hitherto assumed.

Hcp-bcc structural phase transformation of titanium: Analytic model calculations

We show that the hcp $(\ensuremath{\alpha})\ensuremath{\rightarrow}\mathrm{bcc}\phantom{\rule{0.3em}{0ex}}(\ensuremath{\beta})$ structural phase transformation of titanium occurs via anharmonicity effect of thermal lattice vibrations. The full-potential linear muffin-tin orbital method in the local density approximation with the generalized gradient correction is used to derive the embedded atom potential for Ti, which allows us the analytic and realistic calculations of the thermodynamic quantities. We also discuss the similar phase transformation but slightly different mechanism occurring in zirconium in terms of the anharmonicity of thermal lattice vibrations.