[Show abstract][Hide abstract] ABSTRACT: The effect of nanocrystal orientation on the energy loss spectra of
monoclinic hafnia (m-HfO$_2$) is measured by high resolution transmission
electron microscopy (HRTEM) and valence energy loss spectroscopy (VEELS) on
high quality samples. For the same momentum-transfer directions, the dielectric
properties are also calculated ab initio by time-dependent density-functional
theory (TDDFT). Experiments and simulations evidence anisotropy in the
dielectric properties of m-HfO$_2$, most notably with the direction-dependent
oscillator strength of the main bulk plasmon. The anisotropic nature of
m-HfO$_2$ may contribute to the differences among VEELS spectra reported in
literature. The good agreement between the complex dielectric permittivity
extracted from VEELS with nanometer spatial resolution, TDDFT modeling, and
past literature demonstrates that the present HRTEM-VEELS device-oriented
methodology is a possible solution to the difficult nanocharacterization
challenges given in the International Technology Roadmap for Semiconductors.
[Show abstract][Hide abstract] ABSTRACT: Within the framework of ab initio time-dependent density-functional theory
(TD-DFT), we propose a static approximation to the exchange-correlation kernel
based on the jellium-with-gap model. This kernel accounts for electron-hole
interactions and it is able to address both strongly bound excitons and weak
excitonic effects. TD-DFT absorption spectra of several bulk materials (both
semiconductor and insulators) are reproduced in very good agreement with the
experiments and with a low computational cost.
[Show abstract][Hide abstract] ABSTRACT: We present two possible approaches to calculate the momentum distribution
n(p) and the Compton profile within the framework of the ab initio GW
approximation on the self-energy. The approaches are based on integration of
the Green's function along either the real or the imaginary axes. Examples will
be presented on the jellium model and on real bulk sodium. Advantages and
drawbacks of both methods are discussed in comparison with accurate quantum
Monte Carlo (QMC) calculations and x-ray Compton scattering experiments. We
illustrate the effect of many-body correlations and disentangle them from
band-structure and anisotropy effects by a comparison with density functional
theory in the local density approximation. Our results suggest the use of G0W0
momentum distributions as reference for future experiments and theory
[Show abstract][Hide abstract] ABSTRACT: We present both theoretical ab-initio results within the Hedin's GW approximation and experimental angle-resolved photoemission and scanning tunneling spectroscopymeasurements on TiSe2. With respect to the density-functional Kohn-Sham metallic picture, the many-body GW self-energy leads to a approximate to 0.2- eV band-gap insulator consistent with our STS spectra at 5 K. The highest valence and the lowest conduction bands are strongly renormalized, with a loss of k(2) parabolic dispersion toward a k(4) shape. In particular, GW moves the top of valence moved toward a circle of points away from Gamma, arising in a Mexican hat shape commonly associated with an excitonic insulator. Our calculations are in good agreement with experiment.
Physical Review B 05/2012; 85(19). · 3.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The band structure of gold is calculated using ab initio many-body perturbation theory. Different approximations within the GW approach are considered. Standard single-shot G0W0 corrections modify the sp-like bands while leaving unchanged the 5d occupied bands. Beyond G0W0, quasiparticle self-consistency on the wave functions lowers the 5d bands. Globally, many-body effects achieve an opening of the 5d-6sp interband gap of ∼0.4 to ∼0.8 eV, reducing the discrepancy with the experiment. Finally, the quasiparticle band structure is compared to the one obtained by the widely used HSE (Heyd, Scuseria, and Ernzerhof) hybrid functional.
[Show abstract][Hide abstract] ABSTRACT: We calculate the off-diagonal density matrix of the homogeneous electron gas at zero temperature using unbiased reptation Monte Carlo calculations for various densities and extrapolate the momentum distribution and the kinetic and potential energies to the thermodynamic limit. Our results on the renormalization factor allow us to validate approximate G0W0 calculations concerning quasiparticle properties over a broad density region (1≤r(s)≲10) and show that, near the Fermi surface, vertex corrections and self-consistency aspects almost cancel each other out.
[Show abstract][Hide abstract] ABSTRACT: On the basis of first-principles GW calculations, we study the quasiparticle properties of the guanine, adenine, cytosine, thymine, and uracil DNA and RNA nucleobases. Beyond standard G0W0 calculations, starting from Kohn-Sham eigenstates obtained with (semi)local functionals, a simple self-consistency on the eigenvalues allows us to obtain vertical ionization energies and electron affinities within an average 0.11 and 0.18 eV error, respectively, as compared to state-of-the-art coupled-cluster and multiconfigurational perturbative quantum chemistry approaches. Further, GW calculations predict the correct π-character of the highest occupied state, due to several level crossings between density functional and GW calculations. Our study is based on a recent Gaussian-basis implementation of GW calculations with explicit treatment of dynamical screening through contour deformation techniques.
[Show abstract][Hide abstract] ABSTRACT: We present both theoretical ab initio GW and experimental angle-resolved
photoemission (ARPES) and scanning tunneling (STS) spectroscopy results on
TiSe2. With respect to the density-functional Kohn-Sham metallic picture, the
many-body GW self-energy leads to a ~ 0.2 eV band gap insulator consistent with
our STS spectra at 5 K. The band shape is strongly renormalized, with the
top-of-valence moved towards a circle of points away from \Gamma, arising in a
mexican hat feature typical of an excitonic insulator. Our calculations are in
good agreement with experiment.
[Show abstract][Hide abstract] ABSTRACT: The conductance of single molecule junctions is calculated using a Landauer
approach combined to many-body perturbation theory MBPT) to account for
electron correlation. The mere correction of the density-functional theory
eigenvalues, which is the standard procedure for quasiparticle calculations
within MBPT, is found not to affect noticeably the zero-bias conductance. To
reduce it and so improve the agreement with the experiments, the wavefunctions
also need to be updated by including the non-diagonal elements of the
[Show abstract][Hide abstract] ABSTRACT: We evaluate the performances of ab initio GW calculations for the ionization energies and highest occupied molecular orbital-lowest unoccupied molecular orbital gaps of 13 gas phase molecules of interest for organic electronic and photovoltaic applications, including the C60 fullerene, pentacene, free-base porphyrins and phtalocyanine, PTCDA, and standard monomers such as thiophene, fluorene, benzothiazole, or thiadiazole. Standard G0W0 calculations, that is, starting from eigenstates obtained with local or semilocal functionals, significantly improve the ionization energy and band gap as compared to density functional theory Kohn-Sham results, but the calculated quasiparticle values remain too small as a result of overscreening. Starting from Hartree-Fock-like eigenvalues provides much better results and is equivalent to performing self-consistency on the eigenvalues, with a resulting accuracy of 2%–4% as compared to experiment. Our calculations are based on an efficient Gaussian-basis implementation of GW with explicit treatment of the dynamical screening through contour deformation techniques.
[Show abstract][Hide abstract] ABSTRACT: We investigate some aspects of the self-consistency in the Dyson-Schwinger approach to both the QED and the self-interacting scalar field theories. We prove that the set of the Dyson-Schwinger equations, together with the Green-Ward-Takahashi identity, is equivalent to the analogous set of integral equations studied in condensed matter, namely, many-body perturbation theory, where it is solved self-consistently and iteratively. In this framework, we compute the nonperturbative solution of the gap equation for the self-interacting scalar field theory.
Physical review D: Particles and fields 11/2010; 82(9).
[Show abstract][Hide abstract] ABSTRACT: We present experimental and theoretical results on the momentum distribution and the quasiparticle renormalization factor in sodium. From an x-ray Compton-profile measurement of the valence-electron-momentum density, we derive its discontinuity at the Fermi wave vector. This yields an accurate measure of the renormalization factor that we compare with quantum Monte Carlo and G0W0 calculations performed both on crystalline sodium and on the homogeneous electron gas.
[Show abstract][Hide abstract] ABSTRACT: We present experimental inelastic x-ray scattering (IXS) and ab initio time-dependent density-functional-theory (TDDFT) studies of YBa2Cu3O7−δ. The response of the low-lying Ba 5p and Y 4p core electrons is shown to interact strongly with the Cu 3d and O 2p excitations, with important consequences on screening. The agreement between IXS and TDDFT results is excellent, apart from a new type of excitations, mainly related to loosely bound Ba electrons and significantly affected by correlations. This points to correlation mechanisms not fully described by TDDFT that might have a role in giving rise to antiscreening.
[Show abstract][Hide abstract] ABSTRACT: We introduce a quantum transport formalism based on a map of a real three-dimensional lead-conductor-lead system into an effective one-dimensional (1D) system. The resulting effective 1D theory is an in principle exact formalism to calculate the conductance. Besides being more efficient than the principal layers approach, it naturally leads to a five-partitioned workbench (instead of three) where each part of the device (the true central device, the ballistic and the nonballistic leads) is explicitely treated, allowing better physical insight into the contact resistance mechanisms. Independently, we derive a generalized Fisher-Lee formula and a generalized Meir-Wingreen formula for the correlated and uncorrelated conductance and current of the system where the initial restrictions to ballistic leads are generalized to the case of resistive contacts. We present an application to graphene nanoribbons.
[Show abstract][Hide abstract] ABSTRACT: We present ab initio many-body calculations of the optical absorption in bulk graphite, graphene and bilayer of graphene. Electron-hole interaction is included solving the Bethe-Salpeter equation on top of a GW quasiparticle electronic structure. For all three systems, we observe strong excitonic effects at high energy, well beyond the continuum of pi-->pi* transitions. In graphite, these affect the onset of sigma-->sigma* transitions. In graphene, we predict an excitonic resonance at 8.3 eV arising from a background continuum of dipole forbidden transitions. In the graphene bilayer, the resonance is shifted to 9.6 eV. Our results for graphite are in good agreement with experiments.
[Show abstract][Hide abstract] ABSTRACT: We present a relativistic covariant form of many-body theory. The many-body covariant Lagrangian is derived from QED by integrating out the internal non-quantized electromagnetic field. The ordinary many-body Hamiltonian is recovered as an approximation to the exact covariant theory that contains many-body terms beyond the solely electrostatic interaction, e.g. the Lorentz force among electrons, spin-spin etc. Spin and relativistic terms, e.g. spin-orbit, are also automatically accounted. Moreover, the theory is compact, gauge-invariant and respects causality. Comment: Secondary subj-class: hep-ph
[Show abstract][Hide abstract] ABSTRACT: We present a detailed investigation of the dynamic structure factor S(Q,ω) as well as of the dielectric function εM(Q,ω) of the prototypical semiconductor silicon for finite momentum transfer, combining inelastic x-ray scattering measurements and ab initio calculations. We show that, in contrast to optical spectra, for finite momentum transfer, time-dependent density-functional theory in adiabatic local-density approximation (TDLDA) together with the inclusion of lifetime effects in a modified independent-particle polarizability χ0,LT describes the physics of valence excitations with high precision. This applies to the dynamic structure factor as well as to the dielectric function, which demonstrates that TDLDA contains the short-range many-body effects that are crucial for a correct description of εM(Q,ω) in silicon at finite momentum transfer. The form of a nonlocal and energy-dependent exchange-correlation kernel is presented which provides the inclusion of the lifetime effects using the true independent-particle polarizability χ0. The description of the silicon L2,3 absorption edge has been possible by including the outer core electrons 2s and 2p in the valence electrons of the pseudopotential. The energy of the edge is underestimated but a scissor shift of the respective states by the self-energy correction for these states yields good agreement with experiment. Short-range crystal local-field effects and exchange-correlation effects become important with increasing momentum transfer. The inclusion of crystal local-field effects in the random-phase approximation is able to describe the anisotropy of the response well. Our results demonstrate the quantitative predictive power of the first-principles description.