First-principles Local Density Approximation Research Articles

Electronic structures and optical properties of monoclinic ZrO2 studied by first-principles local density approximation + U approach

The electronic structures and optical properties of the monoclinic ZrO2 (m-ZrO2) are investigated by means of first-principles local density approximation (LDA) + U approach. Without on-site Coulomb interactions, the band gap of m-ZrO2 is 3.60 eV, much lower than the experimental value (5.8 eV). By introducing the Coulomb interactions of 4d orbitals on Zr atom (Ud) and of 2p orbitals on O atom (Up), we can reproduce the experimental value of the band gap. The calculated dielectric function of m-ZrO2 exhibits a small shoulder at the edge of the band gap in its imaginary part, while in the tetragonal ZrO2 and cubic ZrO2 it is absent, which is consistent with the experimental observations. The origin of the shoulder is attributed to the difference of electronic structures near the edge of the valence and conduction bands.

Optical properties of monoclinic HfO2 studied by first-principles local density approximation + U approach

The band structures and optical properties of monoclinic HfO2 are investigated by the local density approximation + U approach. With the on-site Coulomb interaction being introduced to 5d orbitals of Hf atom and 2p orbitals of O atom, the experimental band gap is reproduced. The imaginary part of the complex dielectric function shows a small shoulder at the edge of the band gap, coinciding with the experiments. This intrinsic property of crystallized monoclinic HfO2, which is absent in both the tetragonal phase and cubic phase, can be understood as a consequence of the reconstruction of the electronic states near the band edge following the adjustment of the crystal structure. The existence of a similar shoulder-like-structure in the monoclinic phase of ZrO2 is predicted.

Secondary phase Cu2SnSe3 vs. kesterite Cu2ZnSnSe4: Similarities and differences in lattice vibration modes

The crystal structure of monoclinic semiconductor Cu2SnSe3 is optimized, in a first-principles local density approximation calculation by Siesta method, to be found in good agreement with available experimental data, on which base zone-center transversal phonon modes are further calculated. The comparison with a similar calculation for kesterite-phase Cu2ZnSnSe4 helps to identify vibration modes promising to serve as fingerprints for discrimination of these two materials from their lattice-dynamical properties. Moreover, a full analysis of vibration modes is done, which emphasizes an importance of structural motives present in Cu2SnSe3 but absent in kesterite, namely continuous planar chains and stripes of like cations/anions, for the manifestation of structure-specific vibration lines.

Evidence for a spin singlet state in the intermetallic semiconductorFeGa3

${\text{FeGa}}_{3}$ is unusual for an intermetallic compound in displaying a semiconducting gap, on the order of 0.5 eV. Conventional density-functional based electronic-structure calculations in the local-density approximation (LDA) give a similar gap but it is expected that Fe, a $3d$ transition metal, is likely to display an on-site Coulomb repulsion $U$ that should be taken into account, particularly in an insulating compound with some narrow bands. We analyze first-principles LDA calculations for ${\text{FeGa}}_{3}$, and then include on-site Coulomb repulsion in a mean-field way ($\text{LDA}+U$ method) to show that, with the moderate value $U$ on the order of 2 eV, one obtains Fe moments in antialigned ${\text{Fe}}_{2}$ dimers (band theory ``singlets'') with a band gap that still coincides with the observed gap. Counterintuitively, increasing the value of $U$ gradually reduces the gap and finally produces an incorrect metallic state. We suggest that more experimental study should be done to distinguish between the ``${\text{Fe}}_{2}$ singlet'' and nonmagnetic descriptions and provide calculations of the optical properties for comparison with data.

Investigation of local symmetry effects on the electronic structure of manganites: HexagonalYMnO3versus orthorhombicLaMnO3

The local site symmetry effects on electronic structure of manganites were investigated by using theoretical analyses in terms of the cluster model calculations for the $\text{O}\text{ }K$- and $\text{Mn}\text{ }{L}_{2,3}$-edge x-ray-absorption spectroscopy (XAS) results of hexagonal ${\text{YMnO}}_{3}$ and orthorhombic ${\text{LaMnO}}_{3}$. ${\text{YMnO}}_{3}$, which has an unusual ${\text{MnO}}_{5}$ $({D}_{3h})$ site symmetry, exhibits remarkably different electronic structure and XAS line shape from those of ${\text{LaMnO}}_{3}$ with the conventional ${\text{MnO}}_{6}$ $({D}_{4h})$ site symmetry, despite the same ${\text{Mn}}^{3+}$ $(3{d}^{4})$ trivalent ionic state. We developed an algebraic formalism, which takes into account the crystal-field energies and hybridization strengths of the ${D}_{3h}$ $\text{Mn}\text{ }3d$ orbital in ${\text{YMnO}}_{3}$. The deduced values are very consistent with the first-principles local-density approximation plus Hubbard U calculation results. The cluster model calculations involving the different site symmetries well reproduce the $\text{O}\text{ }K$-edge and $\text{Mn}\text{ }{L}_{2,3}$-edge spectra of both manganites. In the analyses of the $\text{O}\text{ }K$-edge spectra, we also took into account the $\text{Y}\text{ }4d$ and $\text{La}\text{ }4f/5d$ orbital states.

Optical and piezoelectric anomalies of ordered (Sc, Ga) N and (Sc, In) N ternaries

Theoretical results are presented regarding the incorporation of Scandium into wurtzite GaN and InN binaries. The electric, optical and piezoelectric properties of the resulting ScGaN and ScInN systems are reported by using first-principles Local-density approximation (LDA) within density functional theory (DFT), Berry phase approach within modern theory of polarization and phonon calculations within the density functional perturbation theory. Our results predict the existence of breaking-symmetry structural phase transition in ordered Sc0.5Ga0.5N and Sc0.5In0.5N alloys when subjected to a compressive or tensile strain. Moreover, our results demonstrate the existence of symmetry preserving pressure-induced isostructural phase transitions in ordered ScGaN and ScInN systems for different Sc concentrations. It has been shown that the existence of isostructural phase transitions leads to dramatic changes in optical, acoustic, and piezoelectric properties of ordered ScGaN and ScInN systems under high pressure. In particular, this study demonstrates that the existence of first-order isostructural phase transitions in Sc1Ga1N2 at a critical hydrostatic pressure of 12.3 GPa leads to a huge enhancement of piezoelectricity (i.e., the e 33 piezoelectric coefficient adopts a huge value as large as 13 C/m2). In addition, It has been shown that ordered Sc0.5Ga0.5N and Sc0.5In0.5N alloys exhibit tremendous piezoelectric response, associated with a breaking-symmetry phase transition from nonpolar P63/mcc(D6h) space group to a polar P63 mc(C6v) structure, at fixed Ga, In and Sc compositions, as a function of the in-plane compressive and tensile strains. We also reveal the reason behind, and consequences of, these unusual properties associated with the strain-induced and pressure-induced structural phase transitions in the novel ScGaN and ScInN ordered structures.

Stability mechanism of cuboctahedral clusters inUO2+x: First-principles calculations

The stability mechanism of cuboctahedral clusters in nonstoichiometric uranium dioxide is investigated by first-principles local density approximation with Hubbard correction method. Calculations reveal that the structural stability is inherited from ${\mathrm{U}}_{6}{\mathrm{O}}_{12}$ molecular cluster, whereas the energy gain through occupying its center with an additional oxygen makes the cluster win out by competition with point oxygen interstitials. Local displacement of the center oxygen along the ⟨111⟩ direction also leads the cluster eightfolded degeneracy and increases relatively the concentration at finite temperatures. However, totally, elevation of temperature, i.e., the effect of entropy, favors point interstitial over cuboctahedral clusters.

First-principles local density approximation (generalized gradient approximation) +U study of catalytic CenOm clusters: U value differs from bulk

Ceria possesses strong catalytic properties for CONO(x) removal and H(2) production. Clusters often show more intriguing functionalities than their bulk counterparts. Here, the geometric and electronic structures of Ce(n)O(m) (n=1-4,m=2n-1,2n) clusters are studied for the first time using the projected augmented wave method in density functional theory with detailed assessment of the exchange-correlation functional and the Hubbard parameter U. We note that the U value strongly affects the electronic structures of the oxygen-deficient Ce(n)O(2n-1) clusters, though less so on the stoichiometric Ce(n)O(2n). Furthermore, the local density approximation (LDA)+U method is more accurate than the generalized gradient approximation+U in describing the localization of the 4f electrons of the Ce(n)O(m) clusters. The calculated vibration frequency of the CeO molecule with the LDA+U (U=4 eV) is 818.4 cm(-1), in close agreement with experimental values of 820-825 cm(-1) for the low lying states. Different optimal U values were noted for the ceria cluster (4 eV) and its bulk (6 eV), due to quantum-size and geometric effects. The largely reduced formation energy of an oxygen vacancy indicates that the catalytic effect of the Ce(n)O(m) clusters are far greater than bulk CeO(2).

First-principles local density approximation (LDA) + U and generalized gradient approximation (GGA) + U studies of plutonium oxides

The electronic structures and properties of PuO2 and Pu2O3 have been studied according to the first principles by using the all-electron projector-augmented-wave (PAW) method. The local density approximation (LDA) + U and the generalized gradient approximation (GGA) + U formalisms have been used to account for the strong on-site Coulomb repulsion among the localized Pu 5f electrons. We discuss how the properties of PuO2 and Pu2O3 are affected by choosing the values of U and exchange-correlation potential. Also, the oxidation reaction of Pu2O3, leading to the formation of PuO2, and its dependence on U and exchange-correlation potential have been studied. Our results show that by choosing an appropriate U it is possible to consistently describe structural, electronic, and thermodynamic properties of PuO2 and Pu2O3, which enable the modelling of the redox process involving Pu-based materials.

First-principles local density approximation+U and generalized gradient approximation+U study of plutonium oxides

The electronic structure and properties of PuO2 and Pu2O3 have been studied from first principles by the all-electron projector-augmented-wave method. The local density approximation+U and the generalized gradient approximation+U formalisms have been used to account for the strong on-site Coulomb repulsion among the localized Pu 5f electrons. We discuss how the properties of PuO2 and Pu2O3 are affected by the choice of U as well as the choice of exchange-correlation potential. Also, oxidation reaction of Pu2O3, leading to formation of PuO2, and its dependence on U and exchange-correlation potential have been studied. Our results show that by choosing an appropriate U, it is promising to correctly and consistently describe structural, electronic, and thermodynamic properties of PuO2 and Pu2O3, which enable the modeling of redox process involving Pu-based materials possible.

First-principles calculations of piezoelectricity and polarization rotation inPb(Zr0.5Ti0.5)O3

First-principles local-density approximation calculations of the piezoelectric response of $\mathrm{Pb}({\mathrm{Zr}}_{0.5}{\mathrm{Ti}}_{0.5}){\mathrm{O}}_{3}$ (PZT 50/50) are presented for tetragonal and monoclinic phases. We use the linearized augmented plane wave with the local orbital extension method and [001]1:1 ordered PZT 50/50 supercells, constrained with $P4\mathrm{mm}$ tetragonal and $\mathrm{Cm}$ monoclinic symmetry. Calculated internal coordinates of monoclinic PZT 50/50 at the experimental $c/a$ value are in good agreement with measurements of $\mathrm{Pb}({\mathrm{Zr}}_{0.52}{\mathrm{Ti}}_{0.48}){\mathrm{O}}_{3}.$ Bulk spontaneous polarization, Born effective charges ${Z}^{*},$ and piezoelectric coefficients are computed using the Berry's phase approach. Greatly enhanced piezoelectric coefficients are observed due to polarization rotation as a function of applied strain in the monoclinic phase with fully relaxed internal atomic coordinates. As the polarization rotates within the $(11\ifmmode\bar\else\textasciimacron\fi{}0)$ mirror plane between the [001] and $[\ensuremath{\nu}\ensuremath{\nu}1]$ (pseudocubic) directions, values as large as ${e}_{33}=12.6{\mathrm{C}/\mathrm{m}}^{2},$ ${e}_{15}=10.9$ ${\mathrm{C}/\mathrm{m}}^{2},$ ${e}_{13}=\ensuremath{-}33{\mathrm{C}/\mathrm{m}}^{2},$ and ${e}_{11}^{\ensuremath{'}}=36$ ${\mathrm{C}/\mathrm{m}}^{2}$ are observed at $\ensuremath{\nu}=1.27$, where ${e}_{11}^{\ensuremath{'}}$ is defined as ${0.5(e}_{11}{+e}_{12}).$ Such large values are consistent with the measured piezoelectric response in ceramic PZT. This supports the polarization rotation mechanism as the origin of the enhanced electromechanical response.

STUDY ON THE ELECTRONIC STRUCTURE OF HIGH-TC SUPERCONDUCTOR MgB2

We report a first-principles local density approximation band structure calculation on the electronic structure of high-TC superconductor MgB2.It is revealed that the band structure of magnesium diboride is of metallic type with a wide valence band.The density of states of electrons at Fermi energy is only about 0.72 (states/eV).We conclude that the present calculation excludes the possibility of BCS mechanism in MgB2.

Properties of Polycrystalline Diamond: Multiscale Modeling Approach

Two modeling techniques to characterize fracture behavior of polycrystalline diamond films are discussed. The first technique is a multiscale modeling method in which first-principles local density approximation calculations on selected structures are combined with an analytic mesoscale model to obtain energies and cleavage fracture energies for symmetric ⟨001⟩ tilt grain boundaries (GBs) over the entire misorientation range. The second technique is large-scale atomistic simulation of the dynamics of failure in notched polycrystalline diamond samples under an applied strain. Electronic characteristics of selected ⟨001⟩ symmetrical tilt GBs calculated with a semiempirical tight-binding Hamiltonian are also presented, and the possible role of graphitic defects on field emission from polycrystalline diamond is briefly discussed.

Optical and acoustical phonon properties of BeTe

We present an investigation of the vibrational properties, the elastic constant c 11 and the infrared dielectric constant ε ∞ of BeTe. For this purpose we applied far-infrared reflectance and Raman spectroscopy to single BeTe layers and BeTe/ZnSe superlattices. The experimental data are used to access the bond ionicity in the framework of an empirical tight binding approach. A low bond polarity of 0.61 is determined for BeTe, which corresponds to a high lattice stability. Furthermore, the elastic constant c 11 is determined as to be 8.9 ± 0.9.10 10 N/m 2 from the frequency positions of folded acoustic phonon (FAP) modes in BeTe/ZnSe superlattices. The experimental results are correlated to data obtained by first-principles local density approximation (LDA) calculations.

Energetics of steps on Pt(111).

First-principles local-density approximation optimizations for Pt(331), Pt(221), Pt(211), and Pt(533) slabs, the surfaces of which are vicinal to Pt(111), are used to extract approximate theoretical energies per unit length for the two varieties of unkinked step on Pt(111). As expected on the basis of bond counting, the calculated energies of the (111)-type and (100)-type steps are nearly equal, about 0.46--0.47 eV per step-edge atom. Near equality of the step-formation energies does not agree with Michely and Comsa`s finding that (111)-type edges are about 50% longer than (100) edges for islands on Pt(111) equilibrated at 700 K, corresponding to a formation free energy that is 13% lower.

Positron Annihilation Studies of Organic Superconductivity

The positron lifetimes of two organic superconductors, {kappa}-(ET){sub 2}Cu(NCS){sub 2} and {kappa}-(ET){sub 2}Cu[N(CN){sub 2}]Br, are measured as a function of temperature across {Tc}. A drop of positron lifetime below {Tc} is observed. Positron-electron momentum densities are measured by using 2D-ACAR to search for the Fermi surface in {kappa}-(ET){sub 2}Cu[N(CN){sub 2}]Br. Positron density distributions and positron-electron overlaps are calculated by using the orthogonalized linear combination atomic orbital (OLCAO) method to interprete the temperature dependence due to the local charge transfer which is inferred to relate to the superconducting transition. 2D-ACAR results in {kappa}-(ET){sub 2}Cu[N(CN){sub 2}]Br are compared with theoretical band calculations based on a first-principles local density approximation. Importance of performing accurate band calculations for the interpretation of positron annihilation data is emphasized.

Relation between atomic structure and surface-stress anisotropy: Calculations for the clean Si(001) surface

The surface-stress tensor is an important quantity entering the elastic theory of interaction between atomic steps on solid surfaces. For the clean Si(001) surface the difference between its diagonal elements, the surface stress anisotropy Δσ, comes out by about a factor of two smaller in experiment than compared to the value obtained from previous first-principles calculations. This discrepancy was usually assigned to the presence of surface defects; as other plausible causes one could quote unrealistic atomic surface structures used for the calculations or even a failure of the local-density approximation (LDA) on which the calculations were based. This constituted a serious deficiency in the understanding of the Si(001) surface. We demonstrate that, when first-principles LDA calculations are carried out for the perfect Si(001) surface with the correct character of reconstruction, the calculated and measured Δσ agree within the experimental and numerical uncertainties. We conclude that standard LDA calculations can properly describe elastic properties of the Si(001) surface and that the effect of surface defects on the measured surface stress anisotropy is, at least for the measurements reported in the literature, much smaller than it has been previously anticipated.

First-principles simulation of STM/STS of Si(100) reconstructed surfaces

The STM images and the STS spectra of silicon (100) reconstructed surfaces are theoretically simulated by the first-principles local density approximation (LDA) method and the tunneling Hamiltonian formalism. A new method for evaluating a tail of the surface wave function, which is important for tunneling of electrons, is also employed. Our simulation shows that the symmetric dimer structure seems to reproduce STM images of occupied states, but it cannot reproduce STM images of unoccupied states or STS spectra. On the other hand, simulations of all asymmetric dimer structures reproduce STS spectra in the seemingly symmetric dimer region as well as the zigzag dimer row patterns of the STM images. These results imply that observed symmetric images by STM may be asymmetric dimer structures.

Recent Advances in Non Self-Consistent Total Energy Calculations in Alloys

In recent years, total energy calculations based on the local density approximation (LDA) have begun to find applications in materials science [1]. In the context of the present symposium, the most relevant application is to the calculation of total and relative energies of ordered alloy phases and their mixing enthalpies. The first-principles LDA approach is now taking over from tight-binding or simple empirical schemes in providing input to phase diagram calculations, for example, using Connolly-Williams inversion [2]. We wish to make two points in our contribution to the symposium.