Ab Initio Pseudopotential Calculations Research Articles

Ab initio predictions of pressure-dependent structural, elastic, and thermodynamic properties of GaMF3 (M = Ca, and Sr) halide perovskites

The structural, elastic, and thermodynamic characteristics of the GaCaF3 and GaSrF3 halide perovskites have been predicted for the first time, using ab initio pseudopotential plane wave calculations. The equilibrium lattice parameters are in good agreement with previously documented results. Both compounds exhibit substantial thermodynamic and mechanical stability. The single-crystal and polycrystalline elastic properties and related properties have been examined. GaCaF3 exhibits higher hardness than GaSrF3. The temperature dependence of the lattice parameter, bulk modulus, volume thermal expansion coefficient, heat capacity, and Debye temperature, have been computed using ab initio calculations combined with the quasi-harmonic Debye model.

The effect of the spin–orbit coupling on the physical properties of ScGa[formula omitted] and LuGa[formula omitted] superconductors

In this work, we have aimed to reveal the impacts of spin–orbit coupling (SOC) on the elastic, mechanical, electronic, phonon, and electron–phonon interaction properties of ScGa3 and LuGa3 by executing both scalar and full-relativistic ab initio pseudopotential calculations based on the density functional theory with its local density approximation. The effect of this coupling on the physical properties of LuGa3 is slightly more pronounced than those of ScGa3. This finding has a physical explanation because spin–orbit interaction in an atom is due to the interaction of electronic spin moment with the magnetic field formed by the traveling electron in the electric field of the nucleus. Since the nuclear charge enhances with the rise in atomic number of the elements, heavier atoms are characterized by larger SOC strength and the mass of Lu atom is considerably heavier than that of Sc atom. In particular, the inclusion of SOC changes the value of the electron–phonon coupling parameter for LuGa3 from 0.593 to 0.583 by less 2%, while the corresponding parameter of ScGa3 remains almost unchanged (from 0.536 to 0.537). The superconducting transition temperature is estimated to be 2.09 K for ScGa3, and 2.15 K for LuGa3 are almost equal to their experimental values of 2.1 and 2.2 K, respectively. Finally, we have estimated three superconducting parameters: the Ginzburg–Landau parameter (κ(0)), the London penetration depth (λL(0)) and the coherence length (ξ(0)). Their calculated values confirm the existence of type-I superconductivity in both superconductors.

The effect of spin–orbit coupling on the physical and superconducting properties of the Ir-rich cubic Laves superconductors AIr[formula omitted] (A = Y, Lu, and Th)

We have examined the impact of spin–orbit coupling (SOC) in cubic Laves superconductors AIr2 (A = Y, Lu, and Th) by realizing ab initio pseudopotential calculations of their physical and electron–phonon interaction properties. The inclusion of SOC removes the degeneracy of the electronic states and moderately influences the elastic and mechanical properties of all the compounds investigated. A critical examination of the elastic and mechanical properties reveals that all the compounds studied are soft (ductile) because of significant metallic bonding. The SOC effect on the phonon and electron–phonon interaction properties depends on the type of A element. This coupling has little influence on the phonon properties of YIr2. However, differently from YIr2, three optical phonon branches of LuIr2 and ThIr2 originating from the triply degenerate zone-center T2u mode are significantly affected by the inclusion of SOC. Although the effect of SOC on the electron–phonon interaction properties changes with the type of A atom, the presence of this coupling improves the agreement with experiment for the superconducting transition temperatures of all the cubic Laves superconductors investigated.

The Electronic Structure and Chemical Bonding in MgPbP2

Ab initio pseudopotential calculations have been performed for the recently predicted tetragonal MgPbP2 crystal with chalcopyrite structure. The results include band structure, total and projected densities of states, electron density maps as well as maximally localized Wannier functions, electron localization function, and effective Born charges. The MgPbP2 is found to be a direct‐gap semiconductor with a bandgap being of order 0.90 eV and mixed type of chemical bonding which is mostly ionic for MgP and covalent for PbP bonds.

Experimental and Theoretical Investigation of the Surface Electronic Structure of ZnGa2O4(100) Single‐Crystals

Herein, a detailed experimental and theoretical investigation on the surface electronic structure of ZnGa2O4(100) bulk single‐crystals, with a special emphasis on the surface preparation, is presented. The surface crystallizes in the bulk‐derived structure, even at low annealing temperatures. Thermal treatments in ultra‐high vacuum have detrimental effects, as they cannot remove the carbon contamination and induce substantial zinc losses, further exacerbated by sputtering. A short sputtering duration and annealing in oxygen atmosphere dramatically reduce the zinc and oxygen losses in the crystal surface, leading to a contamination‐free, crystalline surface of nearly stoichiometric composition. The investigation of the valence states along the high symmetry directions of the Brillouin zone compares favorably with ab initio pseudopotential calculations, indicating a good surface quality and overall agreement with theory. An in‐depth analysis of the measured and simulated valence band peak intensities reveals difficulties associated with the precise description of the metal‐oxygen hybridization. This study provides a first fundamental understanding of the electronic structure of ZnGa2O4, while also indicating that the surface thermal instability is a challenging task that should be taken into account for the fabrication of heterostructures based on ZnGa2O4.

First-principles calculations of physical properties and superconductivity of orthorhombic Mo2BC and Nb2BN

Ab initio pseudopotential calculations have made for the structural, electronic, elastic, mechanical, and electron–phonon interaction properties of molybdenum borocarbide (Mo2BC) and niobium boronitride (Nb2BN) superconductors. Analysis of the structural and electronic properties reveals that the nature of bonding in both these compounds is a combination of covalent, ionic, and metallic. The near-Fermi electronic states in both compounds are occupied by the d states of transition metal atoms. The electronic density of states at the Fermi level in Mo2BC is significantly higher than that in Nb2BN. Lattice dynamical calculations verify their dynamical stability in the base-centered orthorhombic Mo2BC-type crystal structure. We find that the total electron–phonon coupling constant is equal to 0.745 for Mo2BC and 0.539 for Nb2BN. The calculated superconducting transition temperature of 7.41 K for Mo2BC and 3.50 K for Nb2BN is comparable with their experimental values of 7.2 and 4.4 K, respectively.

Impact of spin-orbit coupling on the physical properties and superconductivity of Ir-rich superconductor Mg2Ir3Si: A first-principles investigation

In this study, we elucidate the impact of spin-orbit coupling (SOC) on the physical properties and superconductivity of ternary Laves compound Mg2Ir3Si by performing full relativistic and scalar relativistic ab initio pseudopotential calculations. The consideration of SOC shifts the scalar relativistic bands in energy and changes degeneracy of the bands, but the elastic and mechanical properties are marginally afftected. SOC decreases the electron-phonon coupling strength λ by 11% (from 1.05 to 0.93) A critical assessment of phonon spectrum and Eliashberg spectral function reveals that the origin of the SOC-induced diminishment of λ lies both in slight hardening of some low-frequency phonon modes and in a decrease in the electron-phonon coupling matrix elements. As a consequence, the inclusion of SOC reduces the value of superconducting transition temperature from 7.64 K to 7.05 K, resulting in improved agreement with its recent experimental value of 7 K.

First-principles calculations to investigate the structural, electronic, elastic, vibrational and thermodynamic properties of the full-Heusler alloys X2ScGa (X = Ir and Rh)

This study has investigated ab initio pseudopotential calculations on the structural, electronic, elastic, vibrational and thermodynamic properties of the full-Heusler X2ScGa (X = Ir and Rh) alloys. The calculations have taken place under consideration of the generalized gradient approximation (GGA) of the density functional theory (DFT) with using the plane-wave ab initio pseudopotential method. According to the calculations, the major contribution to electronic states at the Fermi energy has been achieved by d orbitals, revealing a more active role for transition metals Ir (Rh) and Sc atoms. The reckonings point out that the Ir2ScGa and Rh2ScGa have metallic behavior at the equilibrium lattice constant with the density of states (DOS) at the Fermi level (N (EF)) of 1.412 states/eV and 1.821 states/eV, respectively. The results of the elastic constants showed that these compounds met the criteria for Born mechanical stability. It was also observed that they have a ductile structure and exhibit anisotropic behavior according to Pugh criteria. Besides, the full phonon spectra and their projected partial density of states of the alloys have been analyzed with the first-principle linear-response approach of the density-functional perturbation theory. All the alloys behaved dynamically stable in the L21 phase. Furthermore, internal free energy, entropy, specific heat capacity at constant volume and vibrational free energy changes of Ir2ScGa and Rh2ScGa alloys were analyzed and discussed between the temperature range of 0–800 K using the quasi harmonic approximation. According to the results, these alloys are potential candidate for industrial use.

First-principles calculations of physical properties and superconductivity of orthorhombic ScRuSi and ZrRhSi

We have performed ab initio pseudopotential calculations of the structural, electronic, elastic, mechanical, vibrational, electron-phonon interaction, and superconducting properties of ScRuSi and ZrRhSi. An assessment of their elastic and mechanical properties reveals that ScRuSi is a ductile compound, while ZrRhSi is a brittle compound. For both compounds electronic energy bands in the vicinity of the Fermi level originate from the $d$ states of transition-metal atoms. The Eliashberg spectral function reveals that low-frequency phonon modes due to coupled motions of transition-metal atoms are more involved in the process of scattering of electrons than the high-frequency modes due to the vibrations of Si atoms. The average electron-phonon coupling parameter is calculated to be 0.54 for ScRuSi and 0.84 for ZrRhSi from the integration of the Eliashberg spectral function. It is found that the relatively weaker electron-phonon interaction in ScRuSi makes its superconducting transition temperature value $({T}_{c}=3.23 \mathrm{K})$ considerably lower than that of ZrRhSi $({T}_{c}=10.45 \mathrm{K})$.

Probing physical properties and superconductivity of noncentrosymmetric superconductors LaPtGe and LaPtGe3: A first-principles study

In this study, both scalar and full-relativistic ab initio pseudopotential calculations have been actualized on the physical and superconducting properties of noncentrosymmetric superconductors LaPtGe and LaPtGe3 in order to discern the effect of spin-orbit coupling. The spin-orbit coupling makes slightly more effect on the physical properties of LaPtGe than those of LaPtGe3 since the near-Fermi band structure of the former is dominated by d shell electrons rather than p shell electrons as observed in the latter. For both compounds, the splitting into sub-bands arising from the antisymmetric spin-orbit effect remains small around the Fermi level. The phonon spectrum and Eliashberg spectral function of LaPtGe are weakly affected by the inclusion of spin-orbit coupling, while this coupling makes almost no effect on them for LaPtGe3. Therefore, we can conclude that superconductivity in both noncentrosymmetric superconductors has s-wave pairing symmetry and its mechanism arises from the electron-phonon interaction in the medium coupling limit for LaPtGe and the weak-coupling limit for LaPtGe3.

Probing the physical and superconducting properties of hexagonal ZrRuAs: A first-principles calculation

In the presented work, ab initio pseudopotential calculations have been realized on structural, electronic, elastic, mechanical, phonon and electron-phonon interaction properties for ZrRuAs with the hexagonal ZrNiAl-type layer crystal structure. In the vicinity of the Fermi level, Ru 4d and Zr 4d states dominate and mainly contribute to the conduction properties of ZrRuAs. A critical assessment of elastic and mechanical properties for ZrRuAs reveals that, this superconductor is soft (ductile) due to remarkable metallic bonding. Phonon anomalies have been observed in the phonon spectrum of ZrRuAs for the lowest transverse acoustic branch and low-frequency transverse optical branches along the Γ-K and Γ-M-K symmetry directions. Our electron-phonon interaction calculations indicated that ZrRuAs is a conventional phonon-mediated superconductor with strong electron-phonon coupling parameter of 1.32. The strong electron-phonon coupling parameter of ZrRuAs produces its superconducting transition temperature to be 11.12 K which accords nicely with its experimental values of 12 K.

The effect of spin–orbit interaction on superconductivity in the filled skutterudites MPt4Ge12(M=Ba, Sr and Th)

ABSTRACT Ab initio pseudopotential calculations have been conducted for the superconducting filled skutterudite compounds BaPtGe, SrPtGe, and ThPtGe in order to explore the effect of spin–orbit interaction(SOI) by using the planewave pseudopotential method and the density functional theory. The electronic structure calculations suggest that the density of states at the Fermi level is dominated by Ge atom's p states, with the guest atoms (Ba, Sr, or Th) making very little or no contribution. However, when the SOI is included, the phonon frequencies of BaPtGe and SrPtGe are hardened, which in turn decreases the electron–phonon interaction. By integrating the Eliashberg spectral function F(ω) with SOI included, the average electron–phonon coupling parameters are found to be 0.74 for BaPtGe, 0.79 for SrPtGe, and 0.69 for ThPtGe. Using a reasonable value of = 0.10 for the effective Coulomb repulsion parameter, the superconducting critical temperature is found to be 5.36 K for BaPtGe, 5.43 K for SrPtGe, and 4.45 K for ThPtGe with SOI. These values are in excellent accordance with their reported experimental values of 5.35 K, 5.4 K and 4.62 K.

Physical properties and superconductivity of Heusler compound LiGa[formula omitted]Rh: A first-principles calculation

We have conducted ab initio pseudopotential calculations with and without spin orbit coupling on the physical properties of LiGa2Rh in order to explain the formation of superconductivity. Although the inclusion of spin orbit coupling lifts the degeneracies of electronic bands, its influence remains small near the Fermi level. The elastic calculations reveal that this Heusler superconductor is ductile. Phonon and electron–phonon interaction properties of LiGa2Rh are weakly influenced by the inclusion of spin orbit coupling. An analysis of electron–phonon interaction properties reveals that acoustic phonon branches are more involved in the process of scattering of electrons than optical phonon branches. From the Allen–Dynes modified McMillan formula, the value of superconducting transition temperature is obtained to be 2.68 K which is compatible with the recently reported experimental value of 2.4 K.

Investigating the normal state and superconducting state properties of orthorhombic and hexagonal ZrRuP: A first-principles study

We have executed ab initio pseudopotential calculations on the structural, electronic, elastic, mechanical, vibrational, and electron-phonon interaction properties of hexagonal ZrRuP (h-ZrRuP) and orthorhombic ZrRuP (o-ZrRuP). The electronic states of both phases near the Fermi energy are found to be dominated by the $d$ electrons of transition metal atoms, suggesting that they play a more active role in the generation of superconducting state for both phases of ZrRuP. A critical assessment of their elastic and mechanical properties reveals that the lattice of h-ZrRuP is softer than that of o-ZrRuP. A comparison of phonon dispersion curves for both phases indicates that the lower transverse acoustic branch of h-ZrRuP is much softer than that of o-ZrRuP. The soft character of this phonon branch gives rise to strong electron-phonon interaction in h-ZrRuP. Therefore the electron-phonon coupling parameter for h-ZrRuP equals to 1.25 which is considerably larger than the corresponding value of 0.57 for o-ZrRuP. As a consequence, phonon and electron-phonon interaction properties are crucial in making superconducting transition temperature much higher for h-ZrRuP than o-ZrRuP. At the end, the value of this temperature is found to be 12.49 K for h-ZrRuP and 3.89 K for o-ZrRuP which coincide with their experimental values of 12.93 and 3.82 K.

First-principles investigation of monatomic gold wires under tension

Ab initio pseudopotential total-energy calculations on infinite monatomic chains of Au are performed within density functional theory. We use the density functional perturbation theory to study the phonon spectra of these gold wires as a function of strain. Our results show that there does not seem to be a range of strain for which the linear chain is stable, contrary to what was stated by Ribeiro and Cohen [Phys. Rev. B 68 (2003) 035423]. For low strain, the zigzag chain is the stable geometry; while for higher strain, the chains with two or more aligned gold atoms are found to be more stable. At the limit between these two regimes, we predict a transition structure (an asymmetric zigzag chain) to be the most stable.

Location-dependent Stone-Wales defect in C/BN heteronanotube: A density functional theory approach

Ab initio pseudopotential density functional calculations of location-dependent Stone-Wales (SW) defect are performed to investigate the electronic and optical characteristics of (10,0) zigzag-edged hybrid Carbone/Boron-Nitride (C/BN) heteronanotubes which recently have been proposed to exhibit promising properties. These heteronanotubes are circumferentially formed from two curled carbon and boron nitride nanoribbons. Here, we report a systematic study on the effective role of SW defect located at different radial positions along the heteronanotube. The calculated band gap of the structures and also SW formation energies reveal that the defects preferentially localize at the very interface of the BN and graphene segments. Moreover, the critical importance of the boundary-oriented SW defect (either BN or graphene edges) is comprehensively studied using optical absorption spectra, electron charge density 2D maps, and bonding configurations of occupied/unoccupied π orbitals corresponding to the B–C/C–N bonds. Our results provide new insights into the band gap engineering of C/BN heterostructures which are the potential candidate for the future optoelectronic applications.

Pressure-Induced Phase Transitions in Germanium Telluride: Raman Signatures of Anharmonicity and Oxidation.

Pressure-induced phase transitions in GeTe, a prototype phase change material, have been studied to date with diffraction which is not sensitive to anharmonicity-induced dynamical effects. GeTe is also prone to surface oxidation which may compromise surface sensitive measurements. These factors could be responsible for the lack of clarity about the phases and transitions intervening in the phase diagram of GeTe. We have used high-pressure Raman scattering and abinitio pseudopotential density functional calculations to unambiguously establish the high-pressure phase diagram and identify three phases up to 57GPa, a low-pressure rhombohedral phase, an intermediate pressure cubic phase, and a high-pressure orthorhombic phase. We detect substantial broadening and softening of Raman modes at low pressure and identify the transition regions and possible intermediate phases.

Theoretical Investigations for Surface Reconstructions of Submonolayer InAs Grown on GaAs(001)

Surface reconstructions of submonolayer InAs grown on GaAs(001) are investigated using ab initio pseudopotential calculations. Decreasing As chemical potential, calculated formation energies as a function of As chemical potential reveals that the surface reconstructions change from (4 × 3) with one In-As and two As dimers at InAs coverage θ = 0.71 monolayer (ML) to (2 × 4) at θ = 1.38 ML via c(4 × 4) with two In-As and one As dimers at θ = 0.88 ML. It should be noted that the (4 × 3) with two In-As and one As dimers is less stable than the c(4 × 4) with the same dimer constituents. The c(4 × 4) with two In-As and one As dimers is still stable even due to the lattice constraint of GaAs that tend to destabilize c(4 × 4) and (2 × 4) but stabilize many (4 × 3) phases. Consequently, the newly found c(4 × 4) is an intermediate phase to form the (2 × 4) domain on the (4 × 3) surface in InAs/GaAs(001).

<i>Ab initio</i> calculation of the structural, mechanical and thermodynamic properties of Beryllium Sulphide (BeS)

An ab initio plane-wave Pseudopotential calculations using the density functional theory (DFT) implementing the generalised gradient approximation (GGA) to study the structural, elastic constants, phonon dispersion curves, density of state and thermal properties of BeS. Also we calculated the shear modulus, Young’s modulus, Poisson’s ratio, and the Zener’s anisotropic factors. The calculated properties are agreement with previous experimental and theoretical results. The quasi-harmonic approximation is applied to determine the thermal properties, and these properties are in good agreement with available literatures. The major results of the properties determined were discussed.Keywords: Poisson’s ratio, Plane-Wave Pseudopotential, Quasi-Harmonic Approximation, Lattice Parameter

Electron-phonon superconductivity in the ternary phosphidesBaM2P2(M=Ni,Rh,and Ir)

Ab initio plane-wave pseudopotential calculations of electronic and vibrational properties have been carried out for the ternary phosphides $\mathrm{Ba}{M}_{2}{\mathrm{P}}_{2}$ $(M=\text{Ni},\text{Rh}\text{ and Ir})$ with a ${\mathrm{ThCr}}_{2}{\mathrm{Si}}_{2}$-type structure. The calculated electronic results show the metallic character of $\mathrm{Ba}{M}_{2}{\mathrm{P}}_{2}$, and the plots of total and partial density of states of $\mathrm{Ba}{M}_{2}{\mathrm{P}}_{2}$ exhibit strong hybridization between the $d$ states of the $M$ atom and the $p$ states of the P atom below the Fermi energy. Differences in the phonon spectrum and density of states both in the acoustical and optical ranges for these compounds are presented and discussed. The Eliashberg spectral function for these compounds has been calculated by using a linear response approach based on the density functional theory. By integrating the Eliashberg spectral function, the average electron-phonon coupling parameter $(\ensuremath{\lambda})$ is determined to be 0.61 for ${\mathrm{BaNi}}_{2}{\mathrm{P}}_{2}$, 0.55 for ${\mathrm{BaIr}}_{2}{\mathrm{P}}_{2}$, and 0.43 for ${\mathrm{BaRh}}_{2}{\mathrm{P}}_{2}$. Using the calculated values of $\ensuremath{\lambda}$ and the logarithmically averaged phonon frequency ${\ensuremath{\omega}}_{ln}$ the superconducting critical temperature $({T}_{c})$ values for ${\mathrm{BaNi}}_{2}{\mathrm{P}}_{2},{\mathrm{BaIr}}_{2}{\mathrm{P}}_{2}$, and ${\mathrm{BaRh}}_{2}{\mathrm{P}}_{2}$ are obtained to be 2.80, 1.97, and 0.70 K, respectively, which compare very well with their experimental values of 3.0, 2.1, and 1.0 K.