First-principles Pseudopotential Study Research Articles

Surface effects on the Frenkel pair defects stability in the vicinity of the Si(001) surface

A systematic first-principles pseudo-potential study of the stability of tetrahedral Frenkel pairs generated in the vicinity of the silicon Si(001)-(2×1) surface is reported. The defect formation energies and associated structures are discussed. We demonstrate that vacancies that are generated close to the surface are subject to annihilation processes either through recombination of the Frenkel pairs or through surface amorphization. On the other hand, interstitials that are positioned on the subsurface exhibit specific behaviors compared to those located deeper inside the bulk Si. Calculations indicate that the tetrahedral interstitial is unstable when located close to the surface and it relaxes to form a split-110 (dumbbell) configuration. In deeper layers, the tetrahedral interstitial remains rarely stable; it relaxes to hexagonal or to fourfold-coordinated configurations. The defect formation energies are given as a function of surface–vacancy and interstitial–vacancy distances. Results indicate that the formation energy is considerably reduced as the interstitial approaches the surface.

Whether FeTe is superconductor: Insights from first-principles calculations

We present a first-principles pseudopotential study on the electronic structure, phonon structure and the electron–phonon interaction of stoichiometric FeTe in both the nonmagnetic and double stripe antiferromagnetic phases. Our electronic structure calculations show that the nesting effect of Fermi surface is not present in stoichiometric FeTe after considering the magnetic interaction. Comparing the phonon behavior in the double stripe antiferromagnetic phase with that in the nonmagnetic phase, we find that the spin–lattice interaction can lead to the phonon softening and increase electron–phonon coupling constant λ by about 33%, which is similar to other iron-based superconductors in the single stripe antiferromagnetic phase. We suggest that the phonon softening may have no clear contact with the specific magnetic order in the ground state. Finally, we make some discussion about whether FeTe can be superconductor combining our first-principles calculations.

Spin-phonon coupling in the superconductor FeTe0.5Se0.5 from first-principles calculations

We present a first-principles pseudopotential study on the phonon structure and the electron–phonon (EP) interaction of superconductor FeTe0.5Se0.5 in both the nonmagnetic (NM) and striped antiferromagnetic (SAF) phases with the ordered and disordered structural models. The calculations give similar results for the two different structural models, implying that the superconductivity in FeTe0.5Se0.5 may be insensitive to the local atomic structure. Comparing the phonon behavior in the SAF phase with that in the NM phase, we find that the spin-lattice interaction can lead to the phonon softening and increase EP coupling constant λ by about 50%. This enhancement EP coupling may play an important role in the superconducting pairing, but this mechanism alone cannot explain the high Tc in superconductor FeTe0.5Se0.5.

First-principles investigation of hyperfine interactions for nuclear spin entanglement in photoexcited fullerenes

The study of hyperfine interactions in optically excited fullerenes has recently acquired importance within the context of nuclear spin entanglement for quantum information technology. We here report a first-principles pseudopotential study of the hyperfine coupling parameters of optically excited fullerene derivatives as well as small organic radicals. The calculations are performed within the gauge-invariant projector-augmented wave method [C. Pickard and F. Mauri, Phys. Rev. B. 63, 245101 (2001)]. In order to establish the accuracy of this methodology we compare our results with all-electron calculations and with experiment. In the case of fullerene derivatives we study the hyperfine coupling in the spin-triplet exciton state and compare our calculations with recent electron paramagnetic resonance measurements [M. Schaffry et al., Phys. Rev. Lett. 104, 200501 (2010)]. We discuss our results in light of a recent proposal for entangling remote nuclear spins in photo-excited chromophores.

Mode softening and phononic-crystal-like behavior of FeSe

We present a first-principles pseudopotential study on lattice dynamics of binary iron-based superconductor FeSe in both tetragonal nonmagnetic (NM) phase and orthorhombic striped antiferromagnetic (SAF) phase. It is found that significant softening of phonon modes induced by spin-phonon coupling in FeSe may increase the electron-phonon coupling strength. Moreover, we discover and report that an evident phononic-crystal-like behavior exists in FeSe, exhibiting absolute frequency gap in the range of 187 cm−1 < ω < 210 cm−1 in SAF phase, may manipulate and control phonon propagation, and have good application foreground.

Stability of Frenkel pairs in Si(1 0 0) surface in the presence of germanium and oxygen atoms

A first-principles pseudo-potential study of Frenkel pair generation close to the Si(1 0 0) surface in the presence of germanium and oxygen atoms is reported. The energies and structures of the defect structures (i.e. vacancy and relaxed tetrahedral Si interstitial) are calculated using supercell with up to 88 atoms. We present results obtained using the generalized gradient approximation (GGA) for the exchange–correlation energy. We examine the effect of the presence of germanium and oxygen atoms on the stability of Frenkel pairs generated near the Si(1 0 0) surface by comparing a number of individual cases, starting from vacancy interstitial pairs situated at various positions. The general tendency of the created interstitials is to climb towards the surface, but they generally remain in subsurface layers, ready to migrate into the layer. This tendency is enhanced by the presence of the Ge and/or O atoms. We show that the formation energy is lower and Si interstitials can be created with energies as low as 1.5 eV.

Half-metallic ferromagnetism of Cr-doped rutile TiO 2: A first-principles pseudopotential study

Using the first-principles plane-wave pseudopotential method, the electronic structure and the ferromagnetism of Cr-doped rutile TiO 2 have been investigated. We find that the Cr-doped rutile TiO 2 shows half-metallic behavior in both generalized gradient approximation and local density approximation for the exchange-correlation potential. The half-metallicity originates from the hybridization of Cr 3d states and nearest-neighboring O 2p states. It may be useful in semiconductor spintronics and other applications.

Stability and electronic properties ofSn3P4

We present a first-principles pseudopotential study on the stability and electronic properties of ${\mathrm{Sn}}_{3}{\mathrm{P}}_{4}$. Similar to other group-IV phosphides with stoichiometry 3:4, the pseudocubic-${\mathrm{Sn}}_{3}{\mathrm{P}}_{4}$ was found energetically stable among the fourteen structures considered. It was also found that the high-pressure phases and the defect-NaCl phases of ${\mathrm{Sn}}_{3}{\mathrm{P}}_{4}$ have metallic characteristics, while other structures show semiconducting properties within GGA. Therefore, if bulk crystalline ${\mathrm{Sn}}_{3}{\mathrm{P}}_{4}$ can be synthesized under ambient pressure, our calculations predict that it is a semiconductor with a sizable indirect energy gap. However, phase transition from the pseudocubic phase to the $\mathrm{Ca}{\mathrm{Mn}}_{2}{\mathrm{O}}_{4}$ and $\mathrm{Ca}{\mathrm{Ti}}_{2}{\mathrm{O}}_{4}$-type structures are possible at pressure of about 11.7 and $12.8\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, respectively. High-pressure phases which may also be grown on appropriate substrate are likely to be metallic.

First-principles pseudopotential study of an aluminium grain boundary containing sulphur atoms

The electronic structure of an aluminium grain boundary with segregated sulphur impurity atoms has been calculated by a first-principles pseudo-potential method. It is found that a sulphur atom bonds to only one of the neighbouring aluminium atoms. This bond is a mixed-character metallic-covalent bond which is stronger than the metallic Al-Al bonds. Electrons that participate in forming this bond are 3p electrons of sulphur but not its 3s electrons. Other Al--S bonds in the boundary contain no covalent character. From the nature of Al--S bonds in the boundary it cannot be decided whether the embrittlement promotion mechanism by sulphur segregation should be classified as a 'bond mobility model' or a 'decohesion model'.

First-Principles Pseudo-Potential Study of the Pd(110)-c(2×2)-Ethylene Adsorption System

The interaction of molecular ethylene with the Pd(110) surface has been investigated by means of periodic first-principles density functional theory slab calculations. At 0.5 ML coverage, DFT-GGA-PW91 calculations favor the formation of a short bridge (di-σ type) adsorption mode over an atop (π-bonded) ethylene state by 196 meV. On the other hand, HREELS, NEXAFS, and STM measurements unambiguously reveal the presence of π-bonded ethylene on Pd(110). The possible causes of this wrong site preference in DFT-GGA calculations are discussed.

First-principles calculations of self-interstitial defect structures and diffusion paths in silicon

A first-principles pseudopotential study of neutral self-interstitial defects in silicon is reported, together with calculations for Pandey's concerted exchange mechanism for self-diffusion. The energies and structures of the fully relaxed hexagonal, tetrahedral, split-110, `caged' (Clark S J and Ackland G J 1997 Phys. Rev. B 56 47), split-100, and bond-centred interstitials are calculated using supercells with up to 128 + 1 atoms. We present results obtained using the local density approximation (LDA) and the PW91 generalized gradient approximation (GGA) for the exchange-correlation energy. Both the LDA and PW91-GGA functionals give the hexagonal and split-110 defects as the lowest-energy self-interstitials. The hexagonal and split-110 defects are essentially degenerate in energy with formation energies of about 3.3 eV (LDA) and 3.80 eV (PW91-GGA). Energy barriers are studied by calculating saddle-point structures using a simple `ridge-walking' method. The energy barrier for a diffusive jump between the hexagonal and split-110 interstitial sites is calculated to be 0.15 eV (LDA) and 0.20 eV (PW91-GGA) and the barrier between neighbouring hexagonal sites is 0.03 eV (LDA) and 0.18 eV (PW91-GGA), but we have not found a low-energy path between split-110 interstitial sites. The results suggest that self-interstitial diffusion in silicon occurs via diffusive jumps between the hexagonal sites and between hexagonal and split-110 defects.

First-principles pseudopotential study of GaN and BN (110) surfaces

Using the first-principles pseudopotential LDA approach, we have performed calculations of the atomic and electronic structure of the (110) surface of the cubic phase of GaN and BN. It is found that these surfaces are characterised by a much smaller rotation angle ω of the top atomic layer than many other III–V(110) surfaces. We have found a linear correlation between the relative vertical displacement of the cation and the anion on the surface layer ( Δ 1⊥) and the surface bond length of the III–V, II–VI and SiC compounds. In addition, we have studied the band structure addressing the influence of the degree of ionicity on the character of the surface states.

A first-principles pseudopotential study of the compressibility of ϵ-FeSi

The experimental investigations have given conflicting results for the bulk modulus of ϵ-FeSi. We present the results of first-principles pseudopotential plane-wave calculations of the structural properties of ϵ-FeSi. In these calculations the local density and generalized gradient approximations (LDA and GGA) for the exchange-correlation potential have been used. The calculated bulk modulus, using both LDA and GGA, is found to be in good agreement with and supports the largest experimental value of 209GPa. The other structural parameters are also found to be in good agreement with the experiment.

Transverse interplanar forces and phonon spectra of strained Si, Ge, and Si/Ge superlattices.

We present a first-principles pseudopotential study of the effects of the (001) biaxial strain on the transverse phonon spectra of Si and Ge along the $\ensuremath{\Delta}$ direction, and of the transverse phonon modes of strained Si/Ge (001) superlattices (SL's). The supercell method is used to calculate the [001] transverse interplanar force constants (IPFC's) of Si and Ge at three strain configurations corresponding to coherent growth on ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$ (001) (with $x=0, 0.5, \mathrm{and} 1$) substrates. IPFC's up to the eighth-nearest-neighboring planes were included. It has been found that (i) the strain-induced frequency shift of the transverse optical (TO) branch is in excellent agreement with the available experimental data and has a weak wave vector dependence. (ii) Both the Si- and Ge-like TO confined modes in strained Si/Ge (001) SL's are well described relative to the bulk dispersions by an effective confinement length ${d}_{\mathrm{eff}}=(n+3)\frac{{a}_{\ensuremath{\perp}}}{4}$, where $n$ is the number of monolayers of Si or Ge, and $\frac{{a}_{\ensuremath{\perp}}}{4}$ is the corresponding interplanar distance. (iii) The frequencies of the interface modes are about 400 ${\mathrm{cm}}^{\ensuremath{-}1}$ and have a strong dependence on the interfaces coupling in short period SL's.

First-principles pseudopotential study of the phase stability of the III-V semiconductors GaAs and AlAs.

A first-principles pseudopotential study of the phase stability of the III-V semiconductors GaAs and AlAs is reported. For both compounds we study the zinc-blende, wurtzite, NaCl, CsCl, \ensuremath{\beta}-tin, NiAs, and sc16 structures, the latter being the binary analog of the metastable bc8 structure observed in Si and Ge. For GaAs we have also considered a structure of orthorhombic symmetry with four atoms per unit cell. This structure is the same as that recently discovered for ZnTe III [Phys. Rev. Lett. 73, 1805 (1994)], and includes as a special case the Pmm2 structure which had previously been suggested as the structure of GaAs II. Comparison is made with the experimental transition pressures, equations of state, and internal parameters of these compounds. We investigate the recent suggestion that the sc16 structure is very low in energy in III-V compounds. We find that sc16 GaAs is thermodynamically stable to all the other phases studied in a very small range of pressures, but sc16 AlAs is not thermodynamically stable at any pressure. For both GaAs and AlAs we find that the CsCl structure is thermodynamically stable at very high pressures.

First-principles pseudopotential study of the structural phases of silicon.

A first-principles pseudopotential study of 11 phases of silicon is reported: diamond (cd), body-centered cubic (bcc), face-centered cubic (fcc), body-centered tetragonal (bct), simple hexagonal (sh), hexagonal-close-packed (hcp), double-hexagonal-close-packed (dhcp), simple cubic (sc), \ensuremath{\beta}-tin, a body-centered cubic structure with eight atoms per unit cell (bc8), and a simple tetragonal structure with 12 atoms per unit cell (st12). For each structure and for each volume considered we have minimized the energy with respect to all structural degrees of freedom. We have used large basis sets and very careful integrations over the Brillouin zone to resolve the small energy differences between structures, and our pseudopotentials incorporate nonlinear core exchange-correlation corrections, leading to more accurate results than those of previous calculations. We find good agreement with experiments and with some previous calculations but a few discrepancies remain and we gain some insights into the high-pressure phase diagram of silicon.

First-principles pseudopotential study of structures of iron on copper ( 001) surface

An ab initio pseudopotential density functional study of iron on Cu(001) surfaces is presented. No surface relaxation is found on clean Cu(001) surface, while small first layer expansions are found for both the monolayer and double layer Fe coverage. The total energies of various surfaces are calculated and compared. It is concluded that a proposed bilayer growth mode is not energetically favorable. Further calculations suggest that deposited Fe atoms readily mix with Cu atoms and migrate into the substrate. This result is consistent with the observed surface patches of iron atoms. Spin-density functional calculations have been performed, and results on surface magnetization are presented.

Structural properties and electronic structure of low-compressibility materials: beta -Si3N4 and hypothetical beta -C3N4.

We present a first-principles pseudopotential study of the structural and electronic properties of \ensuremath{\beta}-${\mathrm{Si}}_{3}$${\mathrm{N}}_{4}$ and the hypothetical compound \ensuremath{\beta}-${\mathrm{C}}_{3}$${\mathrm{N}}_{4}$. \ensuremath{\beta}-${\mathrm{C}}_{3}$${\mathrm{N}}_{4}$, which is ${\mathrm{C}}_{3}$${\mathrm{N}}_{4}$ in the \ensuremath{\beta}-${\mathrm{Si}}_{3}$${\mathrm{N}}_{4}$ structure, with C substituted for Si, is used as a prototype for investigating the properties of possible covalent C-N solids. The calculated lattice constant, bulk modulus, and electronic band structure of \ensuremath{\beta}-${\mathrm{Si}}_{3}$${\mathrm{N}}_{4}$ are in good agreement with experimental results. This gives support for the predicted properties of \ensuremath{\beta}-${\mathrm{C}}_{3}$${\mathrm{N}}_{4}$. The bulk modulus of \ensuremath{\beta}-${\mathrm{C}}_{3}$${\mathrm{N}}_{4}$ is found to be comparable to diamond, and its moderately large cohesive energy suggests that the prototype structure may be metastable. Although the crystal structure and the valencies of the constituent atoms are similar in \ensuremath{\beta}-${\mathrm{Si}}_{3}$${\mathrm{N}}_{4}$ and \ensuremath{\beta}-${\mathrm{C}}_{3}$${\mathrm{N}}_{4}$, the electronic bonding properties in these two solids are found to differ. The large core size and repulsive p pseudopotential of the second-row element, Si, results in a more ionic Si-N bond compared with a covalent C-N bond.