European Theoretical Spectroscopy Facility
Liege, Belgium
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Botti S.,University Claude Bernard Lyon 1 | Botti S.,European Theoretical Spectroscopy Facility | Marques M.A.L.,University Claude Bernard Lyon 1 | Marques M.A.L.,European Theoretical Spectroscopy Facility
Physical Review Letters | Year: 2013

The self-consistent GW band gaps are known to be significantly overestimated. We show that this overestimation is, to a large extent, due to the neglect of the contribution of the lattice polarization to the screening of the electron-electron interaction. To solve this problem, we derive within the GW formalism a generalized plasmon-pole model that accounts for lattice polarization. The resulting GW self-energy is used to calculate the band structures of a set of binary semiconductors and insulators. The lattice contribution always decreases the band gap. The shrinkage increases with the size of the longitudinal-transverse optical splitting and it can represent more than 15% of the band gap in highly polar compounds, reducing the band-gap percentage error by a factor of 3. © 2013 American Physical Society.

Amato M.,CNRS Fundamental Electronics Institute | Palummo M.,European Theoretical Spectroscopy Facility | Rurali R.,CSIC - Institute of Materials Science | Ossicini S.,CNR Institute of Neuroscience | Ossicini S.,University of Modena and Reggio Emilia
Chemical Reviews | Year: 2014

The different employed growth techniques for silicon-germanium (SiGe) nanowires, their morphology and structural properties were discussed. Significant progresses toward a precise control of the NWs composition were subsequently made in chemical vapor deposition using an appropriate gas inlet ratio in an optimum temperature range or tuning the total growth pressure. Long and straight, without significant tapering, SiGe NWs were obtained through the use of additional gases, other than the usual precursor. Regarding the morphology both axial and radial heterostructures have been reported so far. When laser ablation and CVD techniques were combined, axially modulated SiGe NWs of different diameters were produced. A better control of the interface sharpness, of the order of 1nm, has been then reached combining vapor-liquid-solid (VLS) and vapor-solid-solid (VSS) growth. It is clear that SiGe nanowires will play an important role in the next generation of advanced miniaturized devices.

Cannuccia E.,University of Rome Tor Vergata | Cannuccia E.,European Theoretical Spectroscopy Facility | Marini A.,CNR Institute of Structure of Matter
Physical Review Letters | Year: 2011

The quantum zero-point motion of the carbon atoms is shown to induce strong effects on the optical and electronic properties of diamond and trans-polyacetylene, a conjugated polymer. By using an ab initio approach, we interpret the subgap states experimentally observed in diamond in terms of entangled electron-phonon states. These states also appear in trans-polyacetylene causing the formation of strong structures in the band structure that even call into question the accuracy of the band theory. This imposes a critical revision of the results obtained for carbon-based nanostructures by assuming the atoms frozen in their equilibrium positions. © 2011 American Physical Society.

D'Avino G.,European Theoretical Spectroscopy Facility | Verstraete M.J.,European Theoretical Spectroscopy Facility
Physical Review Letters | Year: 2014

We present a theoretical investigation of the anomalous ferroelectricity of mixed-stack charge transfer molecular crystals, based on the Peierls-Hubbard model, and first-principles calculations for its parametrization. This approach is first validated by reproducing the temperature-induced transition and the electronic polarization of TTF-CA, and then applied to a novel series of hydrogen-bonded crystals, for which room temperature ferroelectricity has recently been claimed. Our analysis shows that the hydrogen-bonded systems present a very low degree of charge transfer and hence support a very small polarization. A critical reexamination of experimental data supports our findings, shedding doubts on the ferroelectricity of these systems. More generally, our modeling allows the rationalization of general features of the ferroelectric transition in charge transfer crystals and suggests design principles for materials optimization. © 2014 American Physical Society.

Cazzaniga M.,European Theoretical Spectroscopy Facility
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

We perform a comparative study of the performances of some standard approaches within the many-body perturbation theory. We calculate quasiparticle dispersions, lifetimes, and spectral functions of aluminum and sodium. Calculations have been carried out in the GW approximation with a plasmon pole model (PPM) or with the contour deformation technique. We also accounted for vertex corrections either only in the screening (replacing the RPA dielectric function with the TDLDA or the Hubbard one) or both in the screening and in the self-energy (using the Del Sole local vertex). Results show the failure of the PPM to describe the corrections far from the Fermi energy, as well as its inability to describe quasiparticle lifetimes and spectral functions. Calculations with a more refined screened interaction decrease the bandwidths and the lifetime of the quasiparticles compared with the GW as well as inducing tiny modifications in the spectral functions. The inclusion of the vertex also in the self-energy cancels the effects arising from the screening by pushing the results back toward the GW ones or even enlarging the differences. © 2012 American Physical Society.

Kufner S.,European Theoretical Spectroscopy Facility | Bechstedt F.,European Theoretical Spectroscopy Facility
Physical Review B - Condensed Matter and Materials Physics | Year: 2014

(HgTe)N(CdTe)M(110) and (001) superlattices are studied by means of ab initio calculations versus the thickness of the HgTe quantum wells (QWs). The used approximate quasiparticle theory including spin-orbit coupling (SOC) gives the correct band ordering, band gap, and SOC splitting for bulk HgTe and CdTe. The resulting band discontinuities indicate confinement also for occupied states. In agreement with earlier k·p calculations and experiments we find a topological transition from the topological nontrivial quantum spin Hall state into a trivial insulator with decreasing QW thickness. The spatial localization near the interfaces and the spin polarization are demonstrated for the edge states for QWs with thicknesses near the critical one. They do not depend on the QW orientation and are therefore topologically protected. Below the critical QW thickness, the trivial insulator exhibits drastic confinement effects with a significant gap opening. We show that the inclusion of inversion symmetry, the nonaxial rotation symmetry of the QWs, and the real QW barriers lead to some agreement but also significant deviations from the predictions within toy models. The deviations concern the critical thickness, the number and localization of edge states, and the possibility to find QW subbands between edge states. © 2014 American Physical Society.

Weissker H.-Ch.,CNRS Interdisciplinary Nanoscience Centre of Marseille | Weissker H.-Ch.,European Theoretical Spectroscopy Facility | Mottet C.,CNRS Interdisciplinary Nanoscience Centre of Marseille
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

Pseudopotential time-dependent density-functional theory (TDDFT) calculations are carried out to investigate the optical absorption spectra of magic-number noble-metal nanoparticles of 13, 38, 55, 140, and 147 atoms. In particular, we study the differences between isomeric structures such as Ag 13 in both cubic and icosahedral structures. Differences are well visible up to sizes of about 55 atoms, demonstrating the need for proper treatment of the structural details on the atomic level. For the largest sizes of about 150 atoms, our calculations confirm earlier results of TDDFT using a structureless jellium model. In particular, we recover the surface plasmon resonance for silver nanoclusters. The bimetallic Ag32Au6 core-shell cluster displays an intense peak corresponding to the surface-plasmon resonance in the Ag cluster, but the spectrum does not lie between the spectra of the pure Ag38 and Au38 clusters. By contrast, a copper core in a Ag38Cu6 cluster leads to a strong damping of this peak. © 2011 American Physical Society.

Ramsden J.D.,European Theoretical Spectroscopy Facility | Godby R.W.,European Theoretical Spectroscopy Facility
Physical Review Letters | Year: 2012

We calculate the exact Kohn-Sham potential that describes, within time-dependent density-functional theory, the propagation of an electron quasiparticle wave packet of nonzero crystal momentum added to a ground-state model semiconductor. The potential is observed to have a highly nonlocal functional dependence on the charge density, in both space and time, giving rise to features entirely lacking in local or adiabatic approximations. The dependence of the nonequilibrium part of the Kohn-Sham electric field on the local current and charge density is identified as a key element of the correct Kohn-Sham functional. © 2012 American Physical Society.

Rodl C.,European Theoretical Spectroscopy Facility | Bechstedt F.,European Theoretical Spectroscopy Facility
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

We calculate the frequency-dependent dielectric function for the series of antiferromagnetic transition metal oxides (TMOs) from MnO to NiO using many-body perturbation theory. Quasiparticle, excitonic, and local-field effects are taken into account by solving the Bethe-Salpeter equation in the framework of collinear spin polarization. The optical spectra are based on electronic structures which have been obtained using density-functional theory with a hybrid functional containing screened exchange (HSE03) and a subsequent quasiparticle calculation in the GW approximation to describe exchange and correlation effects adequately. These sophisticated quasiparticle band structures are mapped to electronic structures resulting from the computationally less expensive GGA+U+Δ scheme that includes an on-site interaction U and a scissors shift Δ and allows us to calculate the large number of electronic states that is necessary to construct the Bethe-Salpeter Hamiltonian. For an accurate description of the optical spectra, an appropriate treatment of the strong electron-hole attraction is mandatory to obtain agreement with the experimentally observed absorption-peak positions. The itinerant s and p states as well as the localized transition metal 3d states have to be considered on an equal footing. We find that a purely atomic picture is not suitable to understand the optical absorption spectra of the TMOs. Reflectivity spectra, absorption coefficients, and loss functions at vanishing momentum transfer are computed in a wide spectral range and discussed in light of the available experimental data. © 2012 American Physical Society.

Belabbes A.,European Theoretical Spectroscopy Facility | Furthmuller J.,European Theoretical Spectroscopy Facility | Bechstedt F.,European Theoretical Spectroscopy Facility
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

Using density-functional-based total-energy calculations and the LDA-1/2 method to compute approximately quasiparticle band structures, we have studied clean relaxed InN surfaces with varying polarity. More in detail, the c-plane, a-plane, and m-plane surfaces have been investigated. In contrast to the polar faces, which allow Fermi level pinning, the projected fundamental bulk gap of about 0.71 eV is free of surface states in the nonpolar cases. Consequently, freshly cleaved InN surfaces cannot lead to a surface accumulation layer. The different electronic structures modify the surface dipole and hence the ionization energy and electron affinity significantly when varying the surface normal from [0001] via [11̄00] or [112̄0] to [0001̄]. © 2011 American Physical Society.

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