| 1. |
- Chang, Xiangrong, et al.
(författare)
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A first principles investigation of the electronic, mechanical and optical properties of XPbN2 (X=Mg, Zn)
- 2024
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Ingår i: Physica. B, Condensed matter. - : Elsevier. - 0921-4526 .- 1873-2135. ; 689
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Tidskriftsartikel (refereegranskat)abstract
- The electronic, mechanical, optical and conductive properties of MgPbN2 and ZnPbN2 are investigated using hybrid-functional first-principles calculations. Our calculations show that these two compounds preferentially stabilize in the tetragonal chalcopyrite structure. The calculated phonon dispersions show that both compounds are thermodynamically stable. From the calculated elastic constants we infer that the compound are also mechanically stable. As for the electronic properties, MgPbN2 is computed to be a semiconductor with a direct band gap of 1.071 eV at the Γ point, while ZnPbN2 is semi-metallic with the valence band and conduction band crossing at the Γ point. Both MgPbN2 and ZnPbN2 show an enhanced optical absorption and electric conductivity compared to those of the related semiconductors MgSnN2 and ZnSnN2. ZnPbN2 gives a much stronger electric conductivity and a slightly larger light absorption than MgPbN2. In addition the influence of alloying is investigated for Mg1-xZnxPbN2 at different doping amount. When Mg was doped at 25% level, a band gap of 0.22 eV opens up and turns the system to semiconducting. When Sn was doped at Pb site, a similar phenomena was observed. On the other hand, the electric conductivity will increase with a small amount of Zn doping in MgPbN2. With large amount of doping, a conductivity higher than that of pristine ZnPbN2 is achieved. Our investigation widens the knowledge of II-IV-N2 family of ternary nitrides, and may hence help to boost their applications.
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| 2. |
- Dai, Jingjing, et al.
(författare)
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Behavior of intrinsic defects in BaF2 under uniaxial compressions : An ab initio investigation
- 2021
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Ingår i: Materials Today Communications. - : Elsevier. - 2352-4928. ; 28
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Tidskriftsartikel (refereegranskat)abstract
- We revisit the defects and related optical properties of one of the prototypical scintillator materials, BaF2, by means of density functional theory calculations. The interstitial F atom is found to be the most favorable defect to be formed, while it is rather difficult to create a F vacancy, Ba vacancy and Ba interstitial intrinsically, unless external forces are applied to the lattice, such as shock compression. The migration barriers for the above defects are remarkably path dependent. For a Ba vacancy, the migration barrier in the (001) (loading) direction is significantly reduced compared to those in the perpendicular (100) and (010) directions, while the migration along the (101) direction has the least value, which is surprisingly smaller than that at ambient pressure. An interstitial Ba atom prefers to move along the (100) and (010) directions in a manner similar to collective diffusion. The F vacancy has the lowest energy barrier along the (001) direction, while uniaxial strain greatly hinders the diffusion of F interstitials. We also study the role of defects on the optical absorption and find that a F vacancy, and interstitial F and Ba atoms are able to introduce pronounced changes to the spectra, whereas the Ba vacancy only shows marginal effects. The favorable migration paths for the different types of defects under strain or pressure can contribute to the design of specific ionic conductor properties under extreme conditions.
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| 3. |
- Dubrovinsky, L., et al.
(författare)
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The most incompressible metal osmium at static pressures above 750 gigapascals
- 2015
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Ingår i: Nature. - : NATURE PUBLISHING GROUP. - 0028-0836 .- 1476-4687. ; 525:7568, s. 226-
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Tidskriftsartikel (refereegranskat)abstract
- Metallic osmium (Os) is one of the most exceptional elemental materials, having, at ambient pressure, the highest known density and one of the highest cohesive energies and melting temperatures(1). It is also very incompressible(2-4), but its high-pressure behaviour is not well understood because it has been studied(2-6) so far only at pressures below 75 gigapascals. Here we report powder X-ray diffraction measurements on Os at multi-megabar pressures using both conventional and double-stage diamond anvil cells(7), with accurate pressure determination ensured by first obtaining self-consistent equations of state of gold, platinum, and tungsten in static experiments up to 500 gigapascals. These measurements allow us to show that Os retains its hexagonal close-packed structure upon compression to over 770 gigapascals. But although its molar volume monotonically decreases with pressure, the unit cell parameter ratio of Os exhibits anomalies at approximately 150 gigapascals and 440 gigapascals. Dynamical mean-field theory calculations suggest that the former anomaly is a signature of the topological change of the Fermi surface for valence electrons. However, the anomaly at 440 gigapascals might be related to an electronic transition associated with pressure-induced interactions between core electrons. The ability to affect the core electrons under static high-pressure experimental conditions, even for incompressible metals such as Os, opens up opportunities to search for new states of matter under extreme compression.
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| 4. |
- Feng, Qingguo, et al.
(författare)
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An advanced multi-orbital impurity solver for dynamical mean field theory based on the equation of motion approach
- 2012
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 24:5, s. 055603-
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Tidskriftsartikel (refereegranskat)abstract
- We propose an improved fast multi-orbital impurity solver for the dynamical mean field theory based on equations of motion (EOM) for Green's functions and a decoupling scheme. In this scheme the inter-orbital Coulomb interactions are treated fully self-consistently, and involve the inter-orbital fluctuations. As an example of the use of the derived multi-orbital impurity solver, the two-orbital Hubbard model is studied for various cases. Comparisons are made between numerical results obtained with our EOM scheme and those obtained with quantum Monte Carlo and numerical renormalization group methods. The comparison shows a good agreement, but also reveals a dissimilarity of the behaviors of the densities of states which is caused by inter-site inter-orbital hopping effects and on-site inter-orbital fluctuation effects, thus corroborating the assertion of the value of the EOM method for the study of multi-orbital strongly correlated systems.
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| 5. |
- Feng, Qingguo, et al.
(författare)
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Fast multi-orbital equation of motion impurity solver for dynamical mean field theory
- 2011
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 23:42, s. 425601-
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Tidskriftsartikel (refereegranskat)abstract
- We propose a fast multi-orbital impurity solver for dynamical mean field theory (DMFT). Our DMFT solver is based on the equations of motion (EOMs) for local Green's functions and is constructed by generalizing from the single-orbital case to the multi-orbital case with the inclusion of the inter-orbital hybridizations and applying a mean field approximation to the inter-orbital Coulomb interactions. The two-orbital Hubbard model is studied using this impurity solver within a large range of parameters. The Mott metal-insulator transition and the quasiparticle peak are well described. A comparison of the EOM method with the quantum Monte Carlo method is made for the two-orbital Hubbard model and good agreement is obtained. The developed method hence holds promise as a fast DMFT impurity solver in studies of strongly correlated systems.
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| 6. |
- Feng, Qingguo, et al.
(författare)
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Investigation of on-site interorbital single-electron hoppings in general multiorbital systems
- 2012
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 86:3, s. 035107-
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Tidskriftsartikel (refereegranskat)abstract
- A generalized multiorbital Hubbard model, which includes on-site interorbital electron hoppings, is introduced and studied on an arbitrary basis. In a general system the on-site interorbital single-electron hopping may exist realistically between two orbitals, e. g., the excitation of electrons or flow of electrons between two hybridized states, and is hence one of the most basic interactions like the intersite hopping and the on-site Coulomb interactions. It is shown in this work how the higher order interactions can be well incorporated (or generated) in the equations of motion from the basic interactions in the Hamiltonian, which has made it possible and also given a self-consistent way to investigate these higher order correlation effects. The two-orbital Hubbard model is studied numerically in the paramagnetic case to demonstrate the influence of the single-electron hopping effect, leading to a change of the shape of the bands and a shrinking of the difference between the two bands. Inclusion of the on-site interorbital hopping suppresses the so-called orbital-selective Mott transition.
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| 7. |
- Feng, Qingguo, et al.
(författare)
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Topological transitions of the Fermi surface of osmium under pressure: an LDA plus DMFT study
- 2017
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Ingår i: New Journal of Physics. - : IOP PUBLISHING LTD. - 1367-2630. ; 19
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Tidskriftsartikel (refereegranskat)abstract
- The influence of pressure on the electronic structure of Os has attracted substantial attention recently due to reports on isostructural electronic transitions in this metal. Here, we theoretically investigate the Fermi surface of Os from ambient to high pressure, using density functional theory combined with dynamical mean field theory. Weprovide a detailed discussion of the calculated Fermi surface and its dependence on the level of theory used for the treatment of the electron-electron interactions. Although we confirm that Os can be classified as weakly correlated metal, the inclusion of local quantum fluctuations between 5d electrons beyond the local density approximation explains the most recent experimental reports regarding the occurrence of electronic topological transitions in Os.
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| 8. |
- Oppeneer, Peter M., et al.
(författare)
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Spin and orbital hybridization at specifically nested Fermi surfaces in URu(2)Si(2)
- 2011
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 84:24, s. 241102-
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Tidskriftsartikel (refereegranskat)abstract
- The Fermi-surface (FS) nesting properties of URu(2)Si(2) are analyzed with particular focus on their implication for the mysterious hidden order phase. We show that there exist two Fermi surfaces that exhibit a strong nesting at the antiferromagnetic wave vector Q(0) = (0,0,1). The corresponding energy dispersions fulfill the relation epsilon(1)(k) = -epsilon(2)(k +/- Q(0)) at eight FS hot-spot lines. The spin-orbital characters of the involved 5f states are distinct (j(z) = +/-5/2 vs +/-3/2) and hence the degenerate Dirac crossings are symmetry protected in the nonmagnetic normal state. Dynamical symmetry breaking through an Ising-like spin and orbital excitation mode with Delta j(z) = +/-1 induces a hybridization of the two states, causing substantial FS gapping. Concomitant spin and orbital currents in the uranium planes give rise to a rotational symmetry breaking.
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| 9. |
- Sun, Weiwei, et al.
(författare)
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Anisotropic distortion and Lifshitz transition in alpha-Hf under pressure
- 2017
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Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 95:11
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Tidskriftsartikel (refereegranskat)abstract
- In this work we report a theoretical investigation on behavior of the elastic constant C-44 and the transverse optical phonon mode E(2)g of a-Hf under pressure within the density functional theory. In contrast to many other reported transition metals, the above two quantities do not show a synchronous relation as pressure increases. Below 13 GPa, an opposite shifting tendency has been observed. However, once the pressure is raised above 13 GPa, the trend is pulled back to be consistent. This anomalous behavior is figured out to be caused by the large lattice anisotropy of the c/a ratio along with the elastic anisotropy. The synchronous behavior is found to be in accordance with the behavior of c/a ratio with increased pressure. In our band-structure investigations the electronic topological transition has been discovered at 10 GPa, which relates to the change of c/a ratio suggested by recent literature. The presence of the Van Hove singularity shown in the densities of states has been identified and regarded as the origin of the variation of C-44 and E(2)g.
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