Room Temperature Organic Superconductor?

The electron--phonon coupling in fullerene C28 has been calculated from first principles. The value of the associated coupling constant lambda/N(0) is found to be a factor three larger than that associated with C60. Assuming similar values of the density of levels at the Fermi surface N(0) and of the Coulomb pseudopotential for C28-based solids as those associated with alkali-doped fullerides A3C60, one obtains Tc(C28) \approx 8 Tc(C60).Comment: 10 pages, 2 figure

Elimination of unoccupied state summations in it ab initio self-energy calculations for large supercells

We present a new method for the computation of self-energy corrections in large supercells. It eliminates the explicit summation over unoccupied states, and uses an iterative scheme based on an expansion of the Green's function around a set of reference energies. This improves the scaling of the computational time from the fourth to the third power of the number of atoms for both the inverse dielectric matrix and the self-energy, yielding improved efficiency for 8 or more silicon atoms per unit cell

Industrial policies in Europe: an introduction

As guest editors of this Special Issue of PE/JEP we have selected a small number of rather detailed assessment of contemporary history of domestic industrial policies in the international context. The four papers included in this Special Issue can be seen as three case studies of “sectoral” innovation policies (broad band, wind energy, biotechnology) with a strong emphasis on country institutional features and policy instruments, together with one “horizontal” case of industrial policy in a specific country context (innovative startups in Italy). The heterogeneous theoretical background (industrial organization, evolutionary theory of the firm, economics of innovation, development) provides a somewhat unifying hidden thread of these case studies, without becoming a subject of analysis per se. This approach has been our intentional editorial choice and we are fully aware of its limitations. After very short non-technical summaries of the four papers (Section 1) we try to present a rather synthetic assessment of our personal views (largely shared among us even with partial minor disagreements) about the increasingly hot debate on the nature, limitations and desirable perspectives of industrial policy today. We argue for a non-ideological forward-looking role of governments as active players in helping domestic entrepreneurial resources not only to fully exploit inherited comparative advantages but also to face structural uncertainties and discover own potential competitive advantages in a rapidly changing international context (Section 2)

Ab-initio self-energy corrections in systems with metallic screening

The calculation of self-energy corrections to the electron bands of a metal requires the evaluation of the intraband contribution to the polarizability in the small-q limit. When neglected, as in standard GW codes for semiconductors and insulators, a spurious gap opens at the Fermi energy. Systematic methods to include intraband contributions to the polarizability exist, but require a computationally intensive Fermi-surface integration. We propose a numerically cheap and stable method, based on a fit of the power expansion of the polarizability in the small-q region. We test it on the homogeneous electron gas and on real metals such as sodium and aluminum.Comment: revtex, 14 pages including 5 eps figures v2: few fixe

Elementary structural building blocks encountered in silicon surface reconstructions

Driven by the reduction of dangling bonds and the minimization of surface stress, reconstruction of silicon surfaces leads to a striking diversity of outcomes. Despite this variety even very elaborate structures are generally comprised of a small number of structural building blocks. We here identify important elementary building blocks and discuss their integration into the structural models as well as their impact on the electronic structure of the surface

Electron-Phonon Coupling in Charged Buckminsterfullerene

A simple, yet accurate solution of the electron-phonon coupling problem in C_{60} is presented. The basic idea behind it is to be found in the parametrization of the ground state electronic density of the system calculated making use of ab-initio methods, in term of sp$^{2+x}$ hybridized orbitals. This parametrization allows for an economic determination of the deformation potential associated with the fullerene's normal modes. The resulting electron-phonon coupling constants are used to calculate Jahn-Teller effects in C_{60}^-, and multiple satellite peaks in the corresponding photoemission reaction. Theory provides an accurate account of the experimental findings.Comment: 11 pages, 3 figures. Accepted for publication in Chem. Phys. Let

Quasiparticle Electronic structure of Copper in the GW approximation

We show that the results of photoemission and inverse photoemission experiments on bulk copper can be quantitatively described within band-structure theory, with no evidence of effects beyond the single-quasiparticle approximation. The well known discrepancies between the experimental bandstructure and the Kohn-Sham eigenvalues of Density Functional Theory are almost completely corrected by self-energy effects. Exchange-correlation contributions to the self-energy arising from 3s and 3p core levels are shown to be crucial.Comment: 4 pages, 2 figures embedded in the text. 3 footnotes modified and 1 reference added. Small modifications also in the text. Accepted for publication in PR

Accuracy of dielectric-dependent hybrid functionals in the prediction of optoelectronic properties of metal oxide semiconductors: a comprehensive comparison with many-body GW and experiments

Understanding the electronic structure of metal oxide semiconductors is crucial to their numerous technological applications, such as photoelectrochemical water splitting and solar cells. The needed experimental and theoretical knowledge goes beyond that of pristine bulk crystals, and must include the effects of surfaces and interfaces, as well as those due to the presence of intrinsic defects (e.g. oxygen vacancies), or dopants for band engineering. In this review, we present an account of the recent efforts in predicting and understanding the optoelectronic properties of oxides using ab initio theoretical methods. In particular, we discuss the performance of recently developed dielectric-dependent hybrid functionals, providing a comparison against the results of many-body GW calculations, including G 0 W 0 as well as more refined approaches, such as quasiparticle self-consistent GW. We summarize results in the recent literature for the band gap, the band level alignment at surfaces, and optical transition energies in defective oxides, including wide gap oxide semiconductors and transition metal oxides. Correlated transition metal oxides are also discussed. For each method, we describe successes and drawbacks, emphasizing the challenges faced by the development of improved theoretical approaches. The theoretical section is preceded by a critical overview of the main experimental techniques needed to characterize the optoelectronic properties of semiconductors, including absorption and reflection spectroscopy, photoemission, and scanning tunneling spectroscopy (STS)

Excitonic effects in solids described by time-dependent density functional theory

Starting from the many-body Bethe-Salpeter equation we derive an exchange-correlation kernel $f_{xc}$ that reproduces excitonic effects in bulk materials within time-dependent density functional theory. The resulting $f_{xc}$ accounts for both self-energy corrections and the electron-hole interaction. It is {\em static}, {\em non-local} and has a long-range Coulomb tail. Taking the example of bulk silicon, we show that the $- \alpha / q^2$ divergency is crucial and can, in the case of continuum excitons, even be sufficient for reproducing the excitonic effects and yielding excellent agreement between the calculated and the experimental absorption spectrum.Comment: 6 pages, 1 figur