Publications (14)25.86 Total impact

Article: The temperature dependence of electronic eigenenergies in the adiabatic harmonic approximation
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ABSTRACT: The renormalization of electronic eigenenergies due to electronphonon interactions (temperature dependence and zeropoint motion effect) is important in many materials. We address it in the adiabatic harmonic approximation, based on first principles (e.g. DensityFunctional Theory), from different points of view: directly from atomic position fluctuations or, alternatively, from Janak's theorem generalized to the case where the Helmholtz free energy, including the vibrational entropy, is used. We prove their equivalence, based on the usual form of Janak's theorem and on the dynamical equation. We then also place the AllenHeineCardona (AHC) theory of the renormalization in a firstprinciple context. The AHC theory relies on the rigidion approximation, and naturally leads to a selfenergy (Fan) contribution and a DebyeWaller contribution. Such a splitting can also be done for the complete harmonic adiabatic expression, in which the rigidion approximation is not required. A numerical study within the DensityFunctional Perturbation theory framework allows us to compare the AHC theory with frozenphonon calculations, with or without the rigidion terms. For the two different numerical approaches without rigidion terms, the agreement is better than 7 $\mu$eV in the case of diamond, which represent an agreement to 5 significant digits. The magnitude of the non rigidion terms in this case is also presented, distinguishing specific phonon modes contributions to different electronic eigenenergies.08/2014;  [Show abstract] [Hide abstract]
ABSTRACT: We demonstrate that Daubechies wavelets can be used to construct a minimal set of optimized localized contracted basis functions in which the KohnSham orbitals can be represented with an arbitrarily high, controllable precision. Ground state energies and the forces acting on the ions can be calculated in this basis with the same accuracy as if they were calculated directly in a Daubechies wavelets basis, provided that the amplitude of these contracted basis functions is sufficiently small on the surface of the localization region, which is guaranteed by the optimization procedure described in this work. This approach reduces the computational costs of DFT calculations, and can be combined with sparse matrix algebra to obtain linear scaling with respect to the number of electrons in the system. Calculations on systems of 10,000 atoms or more thus become feasible in a systematic basis set with moderate computational resources. Further computational savings can be achieved by exploiting the similarity of the contracted basis functions for closely related environments, e.g. in geometry optimizations or combined calculations of neutral and charged systems.The Journal of chemical physics. 01/2014; 140(20).  [Show abstract] [Hide abstract]
ABSTRACT: With the everincreasing sophistication of codes, the verification of the implementation of advanced theoretical formalisms becomes critical. In particular, cross comparison between different codes provides a strong hint in favor of the correctness of the implementations, and a measure of the (hopefully small) possible numerical differences. We lead a rigorous and careful study of the quantities that enter in the calculation of the zeropoint motion renormalization of the direct band gap of diamond due to electronphonon coupling, starting from the total energy, and going through the computation of phonon frequencies and electronphonon matrix elements. We rely on two independent implementations : Quantum Espresso + Yambo and ABINIT. We provide the order of magnitude of the numerical discrepancies between the codes, that are present for the different quantities: less than $10^{5}$ Hartree per atom on the total energy (5.722 Ha/at), less than 0.07 cm$^{1}$ on the $\Gamma,L,X$ phonon frequencies (555 to 1330 cm$^{1}$), less than 0.5% on the square of the electronphonon matrix elements and less than 4 meV on the zeropoint motion renormalization of each eigenenergies (44 to 264 meV). Within our approximations, the DFT converged direct band gap renormalization in diamond due to the electronphonon coupling is 0.409 eV (reduction of the band gap).Computational Materials Science. 09/2013; 83.  [Show abstract] [Hide abstract]
ABSTRACT: So far, no boron fullerenes were synthesized: more compact sp(3)bonded clusters are energetically preferred. To circumvent this, metallic clusters have been suggested by Pochet et al. [Phys. Rev. B 83, 081403(R) (2011)] as "seeds" for a possible synthesis which would topologically protect the sp(2) sector of the configuration space. In this paper, we identify a basic pentagonal unit which allows a balance between the release of strain and the selfdoping rule. We formulate a guiding principle for the stability of boron fullerenes, which takes the form of an isolated filled pentagon rule (IFPR). The role of metallic clusters is then reexamined. It is shown that the interplay of the IFPR and the seedinduced doping breaks polymorphism and its related problems: it can effectively select between different isomers and reduce the reactivity of the boron shells. The balance between self and exterior doping represents the best strategy for boron buckyball synthesis.The Journal of Chemical Physics 05/2013; 138(18):184302. · 3.12 Impact Factor  Applied Physics Letters 12/2012; 101(25). · 3.79 Impact Factor
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ABSTRACT: Magnetism in two dimensional atomic sheets has attracted considerable interest as its existence could allow the development of electronic and spintronic devices. The existence of magnetism is not sufficient for devices, however, as states must be addressable and modifiable through the application of an external drive. We show that defects in hexagonal boron nitride present a strong interplay between the NN distance in the edge and the magnetic moments of the defects. By stressinduced geometry modifications, we change the ground state magnetic moment of the defects. This control is made possible by the triangular shape of the defects as well as the strong spin localisation in the magnetic state.Applied Physics Letters 09/2012; 101(13). · 3.79 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The modifications of the electronic band structure of solids due to electronphonon interactions (temperature and zeropoint motion effects) have been explored by Manuel Cardona from both the experimental and theoretical sides. In the present contribution, we focus on the theoretical approaches to such effects. Although the situation has improved since the seventies, the wish for a fully developed theory (and associated efficient implementations) is not yet fulfilled. We review noticeable semiempirical and firstprinciple studies, with a special emphasis on the AllenHeineCardona (AHC) approach. We then focus on the nondiagonal DebyeWaller contribution, appearing beyond the rigidion approximation, in a DensityFunctional Theory (DFT) approach. A numerical study shows that they can be sizeable (10%–50%) for diatomic molecules. We also present the basic idea of a new formalism, based on DensityFunctional Perturbation Theory, that allows one to avoid the sums over a large number of empty states, and speed up the calculation by one order of magnitude, compared to the straightforward implementation of the AHC approach within DFT.Annalen der Physik 11/2010; 523(1‐2):168  178. · 1.51 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We present a firstprinciples study of Peierls distortions in transpolyacetylene, polyacene, and armchair (n,n) carbon nanotubes. Our findings suggest that the groundstate geometries of armchair (n,n) carbon nanotubes, with n up to 6, exhibit a Peierls distortion as it is found for transpolyactetylene. In contrast to previous studies in which no Peierls distortion is found with conventional local and semilocal density functionals, we use a hybrid functional whose exactexchange admixture has been specifically optimized for the problem at hand.Physical review. B, Condensed matter 07/2010; 82(3). · 3.77 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The energy bands of semiconductors exhibit significant shifts and broadening with temperature at constant volume. This is an effect of the direct renormalization of band energies due to electronphonon interactions. In search of an efficient linear response DFT approach to this effect, beyond semiempirical approximation or frozen phonon DFT, we have implemented formulas derived by Allen and Heine [J. Phys. C 9, 2305 (1976)] inside the ABINIT package. We have found that such formulas need a great number of bands, O(1000), to properly converge the thermal corrections of deep potential well atoms, i.e. elements of the first row. This leads to heavy computational costs even for simple systems like diamond. The DFPT formalism can be used to circumvent entirely the need for conduction bands by computing the firstorder wavefunctions using the selfconsistent Sternheimer equation. We will compare the results of both formalism demonstrating that the DFPT approach reproduces the correct converged results of the formulas of Allen and Heine.03/2010; 
Article: First principles electronic properties investigation of polythienoacene and its derivatives
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ABSTRACT: The electronic properties of laddertype polythiophene (polythienoacene) and its derivatives are studied using density functional theory. Upon an analysis of the variation of the band gap when comparing the nonladder and the laddertype polymers, a discrepancy is found between the thiophene and the pyrrole(nitrogensubstituted thiophene) polymer families. The polythienoacene has a larger band gap than the polythiophene whereas the opposite is found for the pyrrole polymers. Also, it is found that a simple alternation of the sulfur atom in polythienoacene structure by nitrogen or boron atoms can lead to small band gap polymers. The excitations of these polythienoacene's derivatives are investigated using timedependent density functional theory.03/2009;  [Show abstract] [Hide abstract]
ABSTRACT: ABINIT [http://www.abinit.org] allows one to study, from firstprinciples, systems made of electrons and nuclei (e.g. periodic solids, molecules, nanostructures, etc.), on the basis of DensityFunctional Theory (DFT) and ManyBody Perturbation Theory. Beyond the computation of the total energy, charge density and electronic structure of such systems, ABINIT also implements many dynamical, dielectric, thermodynamical, mechanical, or electronic properties, at different levels of approximation.The present paper provides an exhaustive account of the capabilities of ABINIT. It should be helpful to scientists that are not familiarized with ABINIT, as well as to already regular users. First, we give a broad overview of ABINIT, including the list of the capabilities and how to access them. Then, we present in more details the recent, advanced, developments of ABINIT, with adequate references to the underlying theory, as well as the relevant input variables, tests and, if available, ABINIT tutorials.Program summaryProgram title: ABINITCatalogue identifier: AEEU_v1_0Distribution format: tar.gzJournal reference: Comput. Phys. Comm.Programming language: Fortran95, PERL scripts, Python scriptsComputer: All systems with a Fortran95 compilerOperating system: All systems with a Fortran95 compilerHas the code been vectorized or parallelized?: Sequential, or parallel with proven speedup up to one thousand processors.RAM: Ranges from a few Mbytes to several hundred Gbytes, depending on the input file.Classification: 7.3, 7.8External routines: (all optional) BigDFT [1], ETSF IO [2], libxc [3], NetCDF [4], MPI [5], Wannier90 [6]Nature of problem: This package has the purpose of computing accurately material and nanostructure properties: electronic structure, bond lengths, bond angles, primitive cell size, cohesive energy, dielectric properties, vibrational properties, elastic properties, optical properties, magnetic properties, nonlinear couplings, electronic and vibrational lifetimes, etc.Solution method: Software application based on DensityFunctional Theory and ManyBody Perturbation Theory, pseudopotentials, with planewaves, ProjectorAugmented Waves (PAW) or wavelets as basis functions.Running time: From less than one second for the simplest tests, to several weeks. The vast majority of the >600 provided tests run in less than 30 seconds.References:[1] http://inac.cea.fr/LSim/BigDFT.[2] http://etsf.eu/index.php?page=standardization.[3] http://www.tddft.org/programs/octopus/wiki/index.php/Libxc.[4] http://www.unidata.ucar.edu/software/netcdf.[5] http://en.wikipedia.org/wiki/MessagePassingInterface.[6] http://www.wannier.org.Computer Physics Communications. 01/2009; 
Article: Densityfunctional study of Peierls instability in carbon nanotubes using hybrid functionals
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ABSTRACT: We present a firstprinciples study of Peierls distortions in transpolyacetylene, polyacene and armchair (n,n) carbon nanotubes. All calculations were done within density functional theory using a gaussian basis set. We show that while density only functionals (LDA, GGA) cannot reproduce the experimentally mesured dimerization in trans polyactetylene, hybrid functionals including HartreeFock exchange can give the correct geometry. These findings suggest that armchair (n,n) carbon nanotubes could have a nonsymmetric ground state; in contradiction with what is commonly accepted. Indeed, the B3LYP functional (which includes 20% of exact exchange) opens a gap of 0.26 eV and 0.12 eV for the (3,3) and (6,6) carbon nanotubes respectively. Accordingly, dimerization amplitudes of 0.005 ãnd 0.002 ãre obtained. It is found that the dimerization and the band gap are proportional to the the amount of exact exchange included in the functional.03/2007;  [Show abstract] [Hide abstract]
ABSTRACT: This article presents an \textit{ab initio} study of four polymers, polythiophene, polypyrrole, laddertype polythiophene, and laddertype polypyrrole. Upon an analysis of the variation of the band gap when comparing the unconstrained and the laddertype polymers, a discrepancy was found between the thiophene and the pyrrole polymer families. For polythiophene, the laddertype polymer has a larger gap than the unconstrained polymer whereas the opposite is found for the pyrrole polymers. The structural properties and the charge densities using the Bader charge analysis of these four compounds are investigated. The different band gap behaviors in thiophene and pyrrole polymers can be explained in terms of the competition between the bond length alternation and the effect of the charge density in the carbon backbone.Chemical Physics Letters 11/2006; · 2.15 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: A quasianalytical theory of tunnel ionization is developed that is applicable to general complex systems, such as large molecules. Our analysis reveals strong deviations from conventional tunnel ionization theories, dependent upon the system's geometry, angular momentum, and polarizability. A comparison of our theory with recent C(60) ionization experiments yields reasonable agreement.Physical Review Letters 09/2005; 95(7):073001. · 7.73 Impact Factor
Publication Stats
91  Citations  
25.86  Total Impact Points  
Top Journals
Institutions

2014

Cea Leti
Grenoble, RhôneAlpes, France


2013

Institut Néel
Grenoble, RhôneAlpes, France


2010

Université du Québec à Montréal
 Department of Chemistry
Montréal, Quebec, Canada


2009–2010

Catholic University of Louvain
 Institute of Condensed Matter and Nanosciences
LouvainlaNeuve, WAL, Belgium


2005–2006

Université de Montréal
Montréal, Quebec, Canada
