| 1. |
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| 2. |
- Morales, J. C., et al.
(författare)
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A giant exoplanet orbiting a very-low-mass star challenges planet formation models
- 2019
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 365:6460, s. 1441-1445
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Tidskriftsartikel (refereegranskat)abstract
- Surveys have shown that super-Earth and Neptune-mass exoplanets are more frequent than gas giants around low-mass stars, as predicted by the core accretion theory of planet formation. We report the discovery of a giant planet around the very-low-mass star GJ 3512, as determined by optical and near-infrared radial-velocity observations. The planet has a minimum mass of 0.46 Jupiter masses, very high for such a small host star, and an eccentric 204-day orbit. Dynamical models show that the high eccentricity is most likely due to planet-planet interactions. We use simulations to demonstrate that the GJ 3512 planetary system challenges generally accepted formation theories, and that it puts constraints on the planet accretion and migration rates. Disk instabilities may be more efficient in forming planets than previously thought.
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| 3. |
- de Oliveira, A. C., et al.
(författare)
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Spectroscopy studies of 4H-SiC
- 2002
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Ingår i: Materials Research. ; 6, s. 43-
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Tidskriftsartikel (refereegranskat)
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| 4. |
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| 5. |
- Ahuja, Rajeev, et al.
(författare)
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Electronic and optical properties of lead iodide
- 2002
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 92:12, s. 7219-7224
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Tidskriftsartikel (refereegranskat)abstract
- The electronic properties and the optical absorption of lead iodide (PbI2) have been investigated experimentally by means of optical absorption and spectroscopic ellipsometry, and theoretically by a full-potential linear muffin-tin-orbital method. PbI2 has been recognized as a very promising detector material with a large technological applicability. Its band-gap energy as a function of temperature has also been measured by optical absorption. The temperature dependence has been fitted by two different relations, and a discussion of these fittings is given.
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| 6. |
- Marqués, M., et al.
(författare)
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Pressure effects on the structure and vibrations of β- and γ-C3N4
- 2004
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 70:10, s. 104114-
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Tidskriftsartikel (refereegranskat)abstract
- We present the results of an investigation on the pressure behavior of structural, bonding and vibrational properties of β- and γ-C3N4 phases. Emphasis is focused on the trends of the calculated properties along the A3N4 (A: C, Si, Ge) family. Geometry optimizations and electronic structure calculations are carried out in the framework of the local density functional theory using a planewave-pseudopotential scheme. The equilibrium cell geometry, the isothermal bulk modulus and its pressure derivatives have been evaluated for the two phases in a pressure range up to 400 GPa. The Bader’s Atoms in Molecules formalism is applied to characterize the chemical bonding and the atomic contributions to the bulk compressibility in β- and γ-A3N4 crystals. The calculated stability diagram reveals the occurrence of a hypothetical β-C3N4→γ-C3N4 phase transition around 370 GPa. Γ-point harmonic vibrational modes are computed at selected pressures within the density functional perturbation theory approach. In agreement with recent theoretical calculations, the comparison of the computed Raman vibrational frequencies with experimental estimations for β-C3N4 raises doubts on a previously reported synthesis of this structure. Pressure effects on the vibrational frequencies inform of a reduction of the P63∕m symmetry of the β phase at pressures around 60 GPa and contribute to look into the mechanical stability of the γ phase.
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| 7. |
- Owens, Frank J., et al.
(författare)
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Unusual room temperature ferromagnetism in bulk sintered GaP doped with copper
- 2007
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Ingår i: IEEE transactions on magnetics. - 0018-9464 .- 1941-0069. ; 43:6, s. 3043-3045
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Tidskriftsartikel (refereegranskat)abstract
- Robust room temperature ferromagnetism is obtained in single phase Gallium Phosphide doped with Cu2+ prepared by simple solid state reaction route. The saturation magnetization at 300 K is 1.5 x 10(-2) emu/g and the coercivity was found to be 125 Oe. A strong ferromagnetic resonance signal confirms the long range magnetic order which persists to temperatures as high as 739 K. X-ray absorption spectroscopy (XAS) indicate that Cu is in a +2 state. Ab initio calculations also show that the ferromagnetic ordering is energetically favorable in Cu doped GaP. When the spin-orbit coupling is included we get an enhanced total magnetic moment of 0.31 mu(B) with a local moment on Cu 0.082 and on P 0.204 mu(B). per atom.
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| 8. |
- Ahuja, Rajeev, et al.
(författare)
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Electronic and optical properties of gamma-Al2O3 from ab initio theory
- 2004
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 16:16, s. 2891-2900
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Tidskriftsartikel (refereegranskat)abstract
- We report on a density functional theory calculation of the electronic structure and optical properties of gamma-Al2O3. We have made a comparison between the optical and electronic properties of the alpha and gamma phases of alumina. The calculated bulk modulus of the gamma phase is slightly lower than that of the a phase. The calculated static dielectric function and the optical constant of the gamma phase are very close to those of the alpha phase.
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| 9. |
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| 10. |
- Kollar, J., et al.
(författare)
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Calculation of surface stress for fcc transition metals
- 2003
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 68:24, s. 245417-
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Tidskriftsartikel (refereegranskat)abstract
- Using the density functional theory, formulated within the framework of the exact muffin-tin orbitals method, we have calculated the surface stress for the (111) free surfaces of the fcc 4d and 5d transition metals. Good agreement is obtained with the available ab initio data for Pd, Ir, and Au, while for Pt we predict a surface stress, which is about 33% lower compared to former theoretical results. The present surface stress values for the 4d and 5d fcc metals show the typical trend characteristic for the cohesive or surface energies of d series.
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| 11. |
- Luo, Wei, et al.
(författare)
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Electronic structure of a thermoelectric material : CsBi4Te6
- 2008
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Ingår i: Journal of Physics and Chemistry of Solids. - : Elsevier BV. - 0022-3697 .- 1879-2553. ; 69:9, s. 2274-2276
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Tidskriftsartikel (refereegranskat)abstract
- We have calculated the electronic structure of CsBi4Te6 by means of first-principles self-consistent total-energy calculations within the local-density approximation using the full-potential linear-muffin-tin-orbital method. From our calculated electronic structure we have calculated the frequency dependent dielectric function. Our calculations shows that CsBi4Te6 a semiconductor with a band gap of 0.3 eV. The calculated dielectric function is very anisotropic. Our calculated density of state support the recent experiment of Chung et al. [Science 287 (2000) 10241 that CsBi4Te6 is a high performance thermoelectric material for low temperature applications. (C) 2008 Elsevier Ltd. All rights reserved.
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| 12. |
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| 13. |
- Ahuja, Rajeev, et al.
(författare)
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Optical properties of 4H-SiC
- 2002
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 91:4, s. 2099-2103
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Tidskriftsartikel (refereegranskat)abstract
- The optical band gap energy and the dielectric functions of n-type 4H-SiC have been investigated experimentally by transmission spectroscopy and spectroscopic ellipsometry and theoretically by an ab initio full-potential linear muffin-tin-orbital method. We present the real and imaginary parts of the dielectric functions, resolved into the transverse and longitudinal photon moment a, and we show that the anisotropy is small in 4H-SiC. The measurements and the calculations fall closely together in a wide range of energies.
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| 14. |
- Ahuja, R., et al.
(författare)
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Optical properties of 4H-SiC
- 2002
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Ingår i: J. Appl. Phys.. ; 91:4, s. 2099-2103
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Tidskriftsartikel (refereegranskat)
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| 15. |
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| 16. |
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| 17. |
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| 18. |
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| 19. |
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| 20. |
- Espinosa-Garcia, W. F., et al.
(författare)
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Dimension-dependent band alignment and excitonic effects in graphitic carbon nitride : a many-body perturbation and time-dependent density functional theory study
- 2017
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Ingår i: RSC Advances. - : Royal Society of Chemistry. - 2046-2069. ; 7:71, s. 44997-45002
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Tidskriftsartikel (refereegranskat)abstract
- First-principles many-body theory and time-dependent density functional theory were used to study the dimension effects on the band alignment and optical properties of s-triazine and graphitic C3N4. The inclusion of quasiparticle corrections is very important to describe the quantum confinement and the enhancement of the electron-electron (e-e) interaction. The calculated quasiparticle gaps range from 4.0 eV (monolayer) to 3.21 eV (tetralayer). The position of the valence band maxima is found to be almost constant, whereas the conduction band minima show a strong quantum confinement effect with a variation of similar to 0.7 eV respective to the bulk structure. The calculated frequency-dependent imaginary part of the dielectric function using the Bethe-Salpeter equation shows prominent excitonic effects, where there is a strong redistribution of the spectral weight to lower photon energies in the ultraviolet frequencies where the major part of the absorption occurs. On the other hand, a less intense excitonic absorption in the visible region is due to light polarization perpendicular to the surface plane. In contrast, time-dependent density functional theory also shows a redistribution of the spectral weight in the ultraviolet but it fails to describe the excitonic features in the visible region.
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| 21. |
- Espinosa-Garcia, W. F., et al.
(författare)
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The electronic and optical properties of the sulvanite compounds : a many-body perturbation and time-dependent density functional theory study
- 2018
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Ingår i: Journal of Physics. - : Institute of Physics (IOP). - 0953-8984 .- 1361-648X. ; 30:3
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Tidskriftsartikel (refereegranskat)abstract
- We have studied, by means of first-principles calculations, the electronic and optical properties of the sulvanite family: Cu3MX4 (M = V, Nb, Ta and X = S, Se), which, due to its broad range of gaps and chemical stability, have emerged as promising materials for technological applications such as photovoltaics and transparent conductivity. To address the reliability of those properties we have used semi-local and hybrid functionals (PBEsol, HSE06), many-body perturbation theory (G(0)W(0) approximation and Bethe-Salpeter equation), and time-dependent density functional theory (revised bootstrap kernel) to calculate the quasi-particle dispersion relation, band gaps, the imaginary part of the macroscopic dielectric function and the absorption coefficient. The calculated valence band maximum and the conduction band minimum are located at the R and X-points, respectively. The calculated gaps using PBEsol are between 0.81 and 1.88 eV, with HSE06 into 1.73 and 2.94 eV, whereas the G(0)W(0) values fall into the 1.91-3.19 eV range. The calculated dielectric functions and absorption coefficients show that all these compounds present continuous excitonic features when the Bethe-Salpeter equation is used. Contrarily, the revised bootstrap kernel is incapable of describing the excitonic spectra. The calculated optical spectra show that Cu3VS4 and Cu3MSe4 have good absorption in the visible, whereas Cu3NbS4 and Cu3TaS4 have it on the near ultraviolet.
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| 22. |
- Kaewmaraya, Thanayut, et al.
(författare)
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Electronic structure and ionic diffusion of green battery cathode material : Mg2Mo6S8
- 2014
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Ingår i: Solid State Ionics. - : Elsevier BV. - 0167-2738 .- 1872-7689. ; 261, s. 17-20
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Tidskriftsartikel (refereegranskat)abstract
- We report ab-initio density functional theory calculations of crystal and electronic structure of Mg2Mo6S8, a candidate material to be used in rechargeable magnesium batteries, by employing hybrid exchange-correlation functionals. We find that Mg2Mo6S8 crystalizes in a triclinic geometry and it is a semiconductor with an indirect band gap. Ab-initio molecular dynamics shows that Mg ions present progressive diffusion starting at 200 K with a preferable path through the channel between Mo6S8 blocks along the [010] direction. The intercalation voltage of the system is also determined and the results show that the voltage evaluated by PBE and hybrid functionals likely implies the lower and the upper limit of the experimental value. Lastly, we confirm the dynamical stability of the crystal structure by the calculated phonon dispersion relation.
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| 23. |
- Osorio-Guillen, J. M., et al.
(författare)
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A theoretical study of olivine LiMPO4 cathodes
- 2004
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Ingår i: Solid State Ionics. - : Elsevier BV. - 0167-2738 .- 1872-7689. ; 167:3-4, s. 221-227
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Tidskriftsartikel (refereegranskat)abstract
- We report on a density functional theory (DFT) calculation of the properties of LiMPO4, where M is either Fe, Mn or Co. The mixing between Fe and Mn in these structures is also examined. We have derived three relevant battery properties, namely average voltage, energy density and specific energy, as well as the lattice constants and ionic coordinates for each case examined. Our calculated values for these properties are in good agreement with recent experimental values, when available. Further insight is gained from the electronic density of states of the phases, through which conclusions about the physical properties of the various phases are made.
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| 24. |
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| 25. |
- Osorio-Guillen, J. M., et al.
(författare)
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Assessing photocatalytic power of g-C3N4 for solar fuel production : A first-principles study involving quasi-particle theory and dispersive forces
- 2015
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Ingår i: Journal of Chemical Physics. - : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 143:9
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Tidskriftsartikel (refereegranskat)abstract
- First-principles quasi-particle theory has been employed to assess catalytic power of graphitic carbon nitride, g-C3N4, for solar fuel production. A comparative study between g-h-triazine and g-h-heptazine has been carried out taking also into account van der Waals dispersive forces. The band edge potentials have been calculated using a recently developed approach where quasi-particle effects are taken into account through the OW approximation First, it was found that the description of ground state properties such as cohesive and surface formation energies requires the proper treatment of dispersive interaction. Furthermore, through the analysis of calculated band-edge potentials, it is shown that g-h-triazine has high reductive power reaching the potential to reduce CO2 to formic acid, coplanar g-h-heptazine displays the highest thermodynamics force toward H2O/O-2 oxidation reaction, and corrugated g-h-heptazine exhibits a good capacity for both reactions. This rigorous theoretical study shows a route to further improve the catalytic performance of g-C3N4. (C) 2015 AIP Publishing LLC.
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| 26. |
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| 27. |
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| 28. |
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| 29. |
- Osorio-Guillen, J. M., et al.
(författare)
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Structural phase transitions in heavy alkali metals under pressure
- 2004
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Ingår i: ChemPhysChem. - : Wiley. - 1439-4235 .- 1439-7641. ; 5:9, s. 1411-1415
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Tidskriftsartikel (refereegranskat)abstract
- We performed a theoretical study of the crystal structures of cesium and rubidium under high compressions. Our results confirm the recent high-pressure experimental observations of new complex crystal structures for the Cs lll and Rb lll phases. The calculated transition pressures agree extremely well with the measured data. Thus, it is now certain that the famous isostructural phase transition in cesium is actually a new crystallographic phase transition. A d-orbital occupation number of about 0.52 is crucial for the occurrence of these complex structures.
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| 30. |
- Rudisch, Katharina, et al.
(författare)
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Structural and Electronic Properties of Cu2MnSnS4 from Experiment and First-Principles Calculations
- 2019
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Ingår i: Physica status solidi. B, Basic research. - : Wiley-VCH Verlagsgesellschaft. - 0370-1972 .- 1521-3951. ; 256:7
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Tidskriftsartikel (refereegranskat)abstract
- Cu2MnSnS4 shares several promising properties with the widely investigated Cu2ZnSnS4 for photovoltaic applications such as containing only earth abundant and non-toxic elements, and suitable absorption characteristics for absorber materials. Thin film Cu2MnSnS4 samples with various cation compositions are co-sputtered reactively followed by a high temperature anneal. Formation of Cu2MnSnS4 and co-existence of several secondary phases is verified by XRD and Raman. Our investigation of the crystal structure based on first-principles DFT confirms that stannite crystal structure is preferred over kesterite, although, further verification considering cation disorder is needed. The direct band gap of Cu2MnSnS4 is calculated as 1.52 eV (1.62 eV) for stannite (kesterite), which coincides with the range of the measured band gaps from spectrophotometry of 1.42-1.59 eV. After further annealing treatments below 240 degrees C, the absorption shows reversible changes: the band gap blue-shifts and the Urbach tail energy is reduced. It is concluded that, just like Cu2ZnSnS4, disorder also occurs in Cu2MnSnS4. The implications of our findings are discussed and related to the current understanding of cation disorder in Cu2ZnSnS4 and related compounds. Furthermore, for the first time first-principles DFT investigations are presented for the thiospinel Cu2MnSn3S8 which is observed experimentally as a secondary phase in Sn-rich Cu2MnSnS4 thin films.
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| 31. |
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