Scientific Computing and Modelling NV

Amsterdam, Netherlands

Scientific Computing and Modelling NV

Amsterdam, Netherlands
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Li W.,Scientific Computing and Modelling NV | Mingo N.,CEA Grenoble
Physical Review B - Condensed Matter and Materials Physics | Year: 2015

We study the lattice thermal conductivity (κ) of fully filled skutterudites YbFe4Sb12 and BaFe4Sb12 from first principles. The calculated κ of YbFe4Sb12 is ten times lower than that calculated for BaFe4Sb12, and much lower than any values ever reported for other skutterudites. The ultralow κ of YbFe4Sb12 is closely related to the flat Yb-dominated modes appearing in the frequency range from 5 to 7.2 rad/ps. The flat modes significantly increase the three-phonon scattering channels, which is reflected by the unique characteristics of weighted phase space and leads to much stronger anharmonic scattering of intermediate-frequency optical phonons. Although those flat modes are dominated by the Yb atoms, the filler-related anharmonic interaction does not play a role in the increased phonon scattering. This underlines the importance of the hybridization of the filler and the host matrix in those flat modes, which can be guaranteed by the avoided crossing with acoustic phonons. The depressed phonon spectrum mechanism, common to other skutterudites, also plays a role in the ultralow κ. Our work presents a reduction mechanism of κ by the filler in cage-like structures such as skutterudites and clathrates, and demonstrates the nonuniqueness of the reduction mechanism in these materials. © 2015 American Physical Society.


Li W.,Scientific Computing and Modelling NV | Mingo N.,CEA Grenoble
Physical Review B - Condensed Matter and Materials Physics | Year: 2014

Materials with heavier atomic masses usually possess lower lattice thermal conductivity (κ). The reported κ of IrSb3 skutterudite is about 35% higher than that of CoSb3, despite Ir being much heavier than Co. We study the lattice dynamics and κ of CoSb3 and IrSb3 from first principles. We unveil the physical reasons for the difference in κ by comparing all the influential factors: phonon velocities, anharmonicity characterized by the third-order interatomic force constants, the weighted phase space W, and the atomic mass. We find the increased mass from Co to Ir is ultimately the dominant factor resulting in the increase of κ in IrSb3, and the other factors tend to reduce κ. Larger mass leads to smaller thermal displacements causing weaker anharmonic scattering. Our work provides deeper insight to understand the correlation of κ of systems sharing the same crystal structure. We also find that the decreases in acoustic phonon frequencies and Debye temperature in IrSb3 are almost entirely due to the mass increase from Co to Ir. © 2014 American Physical Society.


Katcho N.A.,CIC EnergiGUNE | Carrete J.,CEA Grenoble | Li W.,Scientific Computing and Modelling NV | Mingo N.,CEA Grenoble
Physical Review B - Condensed Matter and Materials Physics | Year: 2014

We show that impurities and vacancies affect the thermal conductivity much more strongly than what is predicted by widely accepted models. When local distortions around point defects are strong, standard perturbative approaches fail, and phonon scattering can only be accounted for by an exact Green's function calculation. We apply the theory to the study, from first-principles, of nitrogen and vacancy defects in diamond. The thermal conductivity is computed by solving the linearized Boltzmann transport equation. The Born approximation underestimates the phonon scattering cross sections of nitrogen and vacancies by factors of 3 and 10, respectively. Thermal conductivity calculations are in good agreement with experiment. © 2014 American Physical Society.


PubMed | Wilhelm Ostwald Institute For Physikalische Und Theoretische Chemie, University Claude Bernard Lyon 1, VU University Amsterdam and Scientific Computing and Modelling NV
Type: Journal Article | Journal: The Journal of chemical physics | Year: 2016

We report a time-dependent density functional based tight-binding (TD-DFTB) scheme for the calculation of UV/Vis spectra, explicitly taking into account the excitation of nuclear vibrations via the adiabatic Hessian Franck-Condon method with a harmonic approximation for the nuclear wavefunction. The theory of vibrationally resolved UV/Vis spectroscopy is first summarized from the viewpoint of TD-DFTB. The method is benchmarked against time-dependent density functional theory (TD-DFT) calculations for strongly dipole allowed excitations in various aromatic and polar molecules. Using the recent 3ob:freq parameter set of Elstners group, very good agreement with TD-DFT calculations using local functionals was achieved.


Ruger R.,Scientific Computing and Modelling NV | Ruger R.,VU University Amsterdam | Van Lenthe E.,Scientific Computing and Modelling NV | Lu Y.,Scientific Computing and Modelling NV | And 5 more authors.
Journal of Chemical Theory and Computation | Year: 2015

During the last two decades density functional based linear response approaches have become the de facto standard for the calculation of optical properties of small- and medium-sized molecules. At the heart of these methods is the solution of an eigenvalue equation in the space of single-orbital transitions, whose quickly increasing number makes such calculations costly if not infeasible for larger molecules. This is especially true for time-dependent density functional tight binding (TD-DFTB), where the evaluation of the matrix elements is inexpensive. For the relatively large systems that can be studied the solution of the eigenvalue equation therefore determines the cost of the calculation. We propose to do an oscillator strength based truncation of the single-orbital transition space to reduce the computational effort of TD-DFTB based absorption spectra calculations. We show that even a sizable truncation does not destroy the principal features of the absorption spectrum, while naturally avoiding the unnecessary calculation of excitations with small oscillator strengths. We argue that the reduced computational cost of intensity-selected TD-DFTB together with its ease of use compared to other methods lowers the barrier of performing optical property calculations of large molecules and can serve to make such calculations possible in a wider array of applications. (Graph Presented). © 2014 American Chemical Society.


Li W.,Scientific Computing and Modelling NV
Physical Review B - Condensed Matter and Materials Physics | Year: 2015

We demonstrate the ab initio electrical transport calculation limited by electron-phonon coupling by using the full solution of the Boltzmann transport equation (BTE), which applies equally to metals and semiconductors. Numerical issues are emphasized in this work. We show that the simple linear interpolation of the electron-phonon coupling matrix elements from a relatively coarse grid to an extremely fine grid can ease the calculational burden, which makes the calculation feasible in practice. For the Brillouin zone (BZ) integration of the transition probabilities involving one δ function, the Gaussian smearing method with a physical choice of locally adaptive broadening parameters is employed. We validate the calculation in the cases of n-type Si and Al. The calculated conductivity and mobility are in good agreement with experiments. In the metal case we also demonstrate that the Gaussian smearing method with locally adaptive broadening parameters works excellently for the BZ integration with double δ functions involved in the Eliashberg spectral function and its transport variant. The simpler implementation is the advantage of the Gaussian smearing method over the tetrahedron method. The accuracy of the relaxation time approximation and the approximation made by Allen [Phys. Rev. B 17, 3725 (1978)PRBMDO0163-182910.1103/PhysRevB.17.3725] has been examined by comparing with the exact solution of BTE. We also apply our method to n-type monolayer MoS2, for which a mobility of 150 cm2 v-1 s-1 is obtained at room temperature. Moreover, the mean free paths are less than 9 nm, indicating that in the presence of grain boundaries the mobilities should not be effectively affected if the grain boundary size is tens of nanometers or larger. The ab initio approach demonstrated in this paper can be directly applied to other materials without the need for any a priori knowledge about the electron-phonon scattering processes, and can be straightforwardly extended to study cases with electron-impurity scattering. © 2015 American Physical Society.


Ma J.,Huazhong University of Science and Technology | Li W.,Scientific Computing and Modelling NV | Luo X.,Huazhong University of Science and Technology
Physical Review B - Condensed Matter and Materials Physics | Year: 2014

The Callaway model [J. Callaway, Phys. Rev. 113, 1046 (1959)PHRVAO0031- 899X10.1103/PhysRev.113.1046], regarded as an improvement over the relaxation time approximation (RTA) for the phonon Boltzmann transport equation (BTE), is widely used in studying lattice thermal conductivity (κ). However, its accuracy needs to be systematically examined. By solving BTE accurately using an iterative method along with the first principles calculation of phonon scatterings, we conduct such an examination of the Callaway model as well as a modified version proposed by Allen [Phys. Rev. B 88, 144302 (2013)PRBMDO1098- 012110.1103/PhysRevB.88.144302] for Si, diamond, and wurtzite AlN. At room temperature, the RTA underestimates κ by 5%, 32%, 11%, and 12% for Si, diamond, and in-plane and cross-plane AlN, respectively. The deviation of the original Callaway model from the accurate κ is -1%, 25%, 1%, and -12%, respectively, while the deviation of Allen's modified model is 7%, 44%, 13%, and -8%, respectively. The room temperature anisotropy of AlN is 5%, and the anisotropy predicted by RTA, the Callaway model, and Allen's modified version is 7%, 19%, and 29%, respectively. We conclude that neither the original Callaway model nor Allen's modified version can generally guarantee an improvement over RTA. In these three systems, we also find that the relaxation times for umklapp processes scale as 1/ω3 at low frequencies for both transverse acoustic (TA) and longitudinal acoustic (LA) modes, and those for normal processes scale as 1/ω and 1/ω2 for TA and LA modes, respectively. © 2014 American Physical Society.


Ma J.,Huazhong University of Science and Technology | Li W.,Scientific Computing and Modelling NV | Luo X.,Huazhong University of Science and Technology
Applied Physics Letters | Year: 2014

Despite wurtzite InN being a widely used semiconductor, its intrinsic thermal conductivity (κ) is still little known. In this work, the κ of wurtzite InN is studied from first principles. The calculated room temperature κ is 130 Wm-1K-1 and 145 Wm -1K-1 for the in-plane and out-of-plane direction, respectively, showing an anisotropy of about 11%. The anisotropy increases with decreasing temperature, and it reaches 20% at 100 K. The evident anisotropy is contrast to the conventionally used isotropic assumption, and is explained by performing comprehensive velocity analysis. We also calculate the cumulative κ as a function of mean free path, which can help understand the size dependence of κ in the non-bulk forms. The obtained cumulative κ is in good agreement with the experimental κ of InN films with thicknesses between 0.5 and 2.1 μm, and shows the size effect can persist up to 10 μm thickness at room temperature. © 2014 AIP Publishing LLC.


Zibouche N.,Jacobs University Bremen | Zibouche N.,Scientific Computing and Modelling NV | Philipsen P.,Scientific Computing and Modelling NV | Kuc A.,Jacobs University Bremen | Heine T.,Jacobs University Bremen
Physical Review B - Condensed Matter and Materials Physics | Year: 2014

We report that an external electric field applied normal to bilayers of transition-metal dichalcogenides TX2(T = Mo, W, X = S, Se) creates significant spin-orbit splittings and reduces the electronic band gap linearly with the field strength. Contrary to the TX2monolayers, spin-orbit splittings and valley polarization are absent in bilayers due to the presence of inversion symmetry. This symmetry can be broken by an electric field, and the spin-orbit splittings in the valence band quickly reach values similar to those in the monolayers (145 meV for MoS2,..., 418 meV for WSe2) at saturation fields less than 500 mV Å-1. The band gap closure results in a semiconductor-metal transition at field strength between 1.25 (WX2) and 1.50 (MoX2) V Å-1. Thus, by using a gate voltage, the spin polarization can be switched on and off in TX2bilayers, thus activating them for spintronic and valleytronic applications. © 2014 American Physical Society.


Zibouche N.,Jacobs University Bremen | Philipsen P.,Scientific Computing and Modelling NV | Heine T.,Jacobs University Bremen | Kuc A.,Jacobs University Bremen
Physical Chemistry Chemical Physics | Year: 2014

The influence of an external electric field on single-layer transition-metal dichalcogenides TX2 with T = Mo, W and X = S, Se (MoWSeS) has been investigated by means of density-functional theory within two-dimensional periodic boundary conditions under consideration of relativistic effects including the spin-orbit interactions. Our results show that the external field modifies the band structure of the monolayers, in particular, the conduction band. This modification has, however, very little influence on the band gap and effective masses of holes and electrons at the K point, and also the spin-orbit splitting of these monolayers is almost unaffected. Our results indicate a remarkable stability of the electronic properties of TX2 monolayers with respect to gate voltages. A reduction of the electronic band gap is observed starting only from field strengths of 2.0 V Å-1 (3.5 V Å-1) for selenides (sulphides), and the transition to a metallic phase would occur at fields of 4.5 V Å-1 (6.5 V Å-1). © 2014 the Partner Organisations.

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