Delta-resonance effective mass in medium

Based on the relativistic chiral effective field theory, we study the effective mass of the Delta-resonance in medium by investigating the self-energy of the Delta-resonance related to the pi N decay channel in symmetric nuclear matter. We find that the effective mass of Delta-resonance decreases evidently with increasing nuclear density rho. In our calculation, we also consider the influence of the shifts of the nucleon mass, pion mass and its decay constant due to the restoration of chiral symmetry in medium. The results are roughly consistent with the data given by Lawrence Berkeley National Laboratory.

Josephson dynamics of a Bose-Einstein condensate trapped in a double-well potential

The Josephson equations for a Bose-Einstein Condensate gas trapped in a double-well potential are derived with the two-mode approximation by the Gross-Pitaevskii equation. The dynamical characteristics of the equations are obtained by the numerical phase diagrams. The nonlinear self-trapping effect appeared in the phase diagrams are emphatically discussed, and the condition EcN > 4E(J) is presented.

DBHF approach and thermodynamic consistency for nuclear matter calculations

Within the framework of Dirac Brueckner-Hartree-Fock (DBHF) approach, we calculate the energy per nucleon, the pressure, the nucleon self-energy, and the single-nucleon energy in the nuclear matter by adopting two different covariant representations for T-matrix. We mainly investigate the influence of different covariant representations on the satisfiable extent of the Hugenholtz-Van Hove (HVH) theorem in the nuclear medium in the framework of DBHF. By adopting the two different covariant representations of T-matrix, the predicted nucleon self-energy shows a quite different momentum and density dependence. Different covariant representations affect remarkably the satisfiable extent of the HVH theorem. By adopting the complete pseudo-vector representation of the T-matrix, HVH theorem is largely violated, which is in agreement with the result in the non-relativistic Brueckner-Hartree-Fock approach and reflects the importance of ground state correlations for single nucleon properties in nuclear medium, whereas by using the pseudoscalar representation, the ground state correlation cannot be shown. It indicates that the complete pseudo-vector presentation is more feasible than the pseudo-scalar one.

On-line concentration of neutral and charged species in capillary electrochromatography with a methacrylate-based monolithic stationary phase

A method involving self-concentration, on-column enrichment and field-amplified sample stacking for on-line concentration in capillary electrochromatography with a polymer monolithic column is presented. Since monolithic columns eliminate the frit fabrication and the problems associated with frits, the experimental conditions could be more flexibly adjusted to obtain higher concentration factor in comparison with conventional particulate packed columns. With self-concentration effect, the detection sensitivity of benzene and hexylbenzene is improved by a factor of 4 and 8, respectively. With on-column enrichment and ultralong injection, improvement as high as 22 000 times in detection sensitivity of benzoin is achieved. Furthermore, a combination of the three above-mentioned methods yields up to a 24000-fold improvement in detection sensitivity for caffeine, a charged compound. Parameters affecting the efficiency of on-line concentration are investigated systematically. In addition, equations describing on-line concentration process are deduced.

yambo: An ab initio tool for excited state calculations

yambo is an ab initio code for calculating quasiparticle energies and optical properties of electronic systems within the framework of many-body perturbation theory and time-dependent density functional theory. Quasiparticle energies are calculated within the GW approximation for the self-energy. Optical properties are evaluated either by solving the Bethe-Salpeter equation or by using the adiabatic local density approximation. yambo is a plane-wave code that, although particularly suited for calculations of periodic bulk systems, has been applied to a large variety of physical systems. yambo relies on efficient numerical techniques devised to treat systems with reduced dimensionality, or with a large number of degrees of freedom. The code has a user-friendly command-line based interface, flexible 110 procedures and is interfaced to several publicly available density functional ground-state codes.

Electronic properties of interfaces and defects from many-body perturbation theory: Recent developments and applications

We review some recent developments in many body perturbation theory (MBPT) calculations that have enabled the study of interfaces and defects. Starting from the theoretical basis of MBPT, Hedin's equations are presented, leading to the CW and CWI' approximations. We introduce the perturbative approach, that is the one most commonly used for obtaining quasiparticle (QP) energies. The practical strategy presented for dealing with the frequency dependence of the self energy operator is based on either plasmon-pole models (PPM) or the contour deformation technique, with the latter being more accurate. We also discuss the extrapolar method for reducing the number of unoccupied states which need to be included explicity in the calculations. The use of the PAW method in the framework of MBPT is also described. Finally, results which have been obtained using, MBPT for band offsets a interfaces and for defects presented, with companies on the main difficulties and cancels.

Schematic representation of the QP corrections (marked with ) to the band edges (E and E-v) and a defect level (F) for a Si/SiO2 interface (Si and O atoms are represented in blue and red, respectively, in the ball and stick model) with an oxygen vacancy leading to a Si-Si bond (the Si atoms involved in this bond are colored light blue).

Electronic properties of zircon and hafnon from many-body perturbation theory

The electronic properties of zircon and hafnon, two wide-gap high-kappa materials, are investigated using many-body perturbation theory (MBPT) combined with the Wannier interpolation technique. For both materials, the calculated band structures differ from those obtained within density-functional theory and MBPT by (i) a slight displacement of the highest valence-band maximum from the Gamma point and (ii) an opening of the indirect band gap to 7.6 and 8.0 eV for zircon and hafnon, respectively. The introduction of vertex corrections in the many-body self-energy does not modify the results except for a global rigid shift of the many-body corrections.

Positron scattering and annihilation in hydrogenlike ions

Diagrammatic many-body theory is used to calculate the scattering phase shifts, normalized annihilation rates Zeff, and annihilation ? spectra for positron collisions with the hydrogenlike ions He+, Li2+, B4+, and F8+. Short-range electron-positron correlations and longer-range positron-ion correlations are accounted for by evaluating nonlocal corrections to the annihilation vertex and the exact positron self-energy. The numerical calculation of the many-body theory diagrams is performed using B-spline basis sets. To elucidate the role of the positron-ion repulsion, the annihilation rate is also estimated analytically in the Coulomb-Born approximation. It is found that the energy dependence and magnitude of Zeff are governed by the Gamow factor that characterizes the suppression of the positron wave function near the ion. For all of the H-like ions, the correlation enhancement of the annihilation rate is found to be predominantly due to corrections to the annihilation vertex, while the corrections to the positron wave function play only a minor role. Results of the calculations for s-, p-, and d-wave incident positrons of energies up to the positronium-formation threshold are presented. Where comparison is possible, our values are in excellent agreement with the results obtained using other, e.g., variational, methods. The annihilation-vertex enhancement factors obtained in the present calculations are found to scale approximately as 1+(1.6+0.46l)/Zi, where Zi is the net charge of the ion and l is the positron orbital angular momentum. Our results for positron annihilation in H-like ions provide insights into the problem of positron annihilation with core electrons in atoms and condensed matter systems, which have similar binding energies.

Nonlinear optics from an ab initio approach by means of the dynamical Berry phase: Application to second- and third-harmonic generation in semiconductors

We present an ab initio real-time-based computational approach to study nonlinear optical properties in condensed matter systems that is especially suitable for crystalline solids and periodic nanostructures. The equations of motion and the coupling of the electrons with the external electric field are derived from the Berry-phase formulation of the dynamical polarization [Souza et al., Phys. Rev. B 69, 085106 (2004)]. Many-body effects are introduced by adding single-particle operators to the independent-particle Hamiltonian. We add a Hartree operator to account for crystal local effects and a scissor operator to correct the independent particle band structure for quasiparticle effects. We also discuss the possibility of accurately treating excitonic effects by adding a screened Hartree-Fock self-energy operator. The approach is validated by calculating the second-harmonic generation of SiC and AlAs bulk semiconductors: an excellent agreement is obtained with existing ab initio calculations from response theory in frequency domain [Luppi et al., Phys. Rev. B 82, 235201 (2010)]. We finally show applications to the second-harmonic generation of CdTe and the third-harmonic generation of Si. 

An analysis of decoupling procedures in generating thermal Green's functions of O([lambda]â ´) by the Zubarev equation of motion method

The Zubarev equation of motion method has been applied to an anharmonic crystal of O( ,,4). All possible decoupling schemes have been interpreted in order to determine finite temperature expressions for the one phonon Green's function (and self energy) to 0()\4) for a crystal in which every atom is on a site of inversion symmetry. In order to provide a check of these results, the Helmholtz free energy expressions derived from the self energy expressions, have been shown to agree in the high temperature limit with the results obtained from the diagrammatic method. Expressions for the correlation functions that are related to the mean square displacement have been derived to 0(1\4) in the high temperature limit.

Generalization of the BTK Theory to the Case of Finite Quasiparticle Lifetimes

A generalization to the BTK theory is developed based on the fact that the quasiparticle lifetime is finite as a result of the damping caused by the interactions. For this purpose, appropriate self-energy expressions and wave functions are inserted into the strong coupling version of the Bogoliubov equations and subsequently, the coherence factors are computed. By applying the suitable boundary conditions to the case of a normal-superconducting interface, the probability current densities for the Andreev reflection, the normal reflection, the transmission without branch crossing and the transmission with branch crossing are determined. Accordingly the electric current and the differential conductance curves are calculated numerically for Nb, Pb, and Pb0.9Bi0.1 alloy. The generalization of the BTK theory by including the phenomenological damping parameter is critically examined. The observed differences between our approach and the phenomenological approach are investigated by the numerical analysis.

Calculs ab initio de structures électroniques et de leur dépendance en température avec la méthode GW

Cette thèse porte sur le calcul de structures électroniques dans les solides. À l'aide de la théorie de la fonctionnelle de densité, puis de la théorie des perturbations à N-corps, on cherche à calculer la structure de bandes des matériaux de façon aussi précise et efficace que possible. Dans un premier temps, les développements théoriques ayant mené à la théorie de la fonctionnelle de densité (DFT), puis aux équations de Hedin sont présentés. On montre que l'approximation GW constitue une méthode pratique pour calculer la self-énergie, dont les résultats améliorent l'accord de la structure de bandes avec l'expérience par rapport aux calculs DFT. On analyse ensuite la performance des calculs GW dans différents oxydes transparents, soit le ZnO, le SnO2 et le SiO2. Une attention particulière est portée aux modèles de pôle de plasmon, qui permettent d'accélérer grandement les calculs GW en modélisant la matrice diélectrique inverse. Parmi les différents modèles de pôle de plasmon existants, celui de Godby et Needs s'avère être celui qui reproduit le plus fidèlement le calcul complet de la matrice diélectrique inverse dans les matériaux étudiés. La seconde partie de la thèse se concentre sur l'interaction entre les vibrations des atomes du réseau cristallin et les états électroniques. Il est d'abord montré comment le couplage électron-phonon affecte la structure de bandes à température finie et à température nulle, ce qu'on nomme la renormalisation du point zéro (ZPR). On applique ensuite la méthode GW au calcul du couplage électron-phonon dans le diamant. Le ZPR s'avère être fortement amplifié par rapport aux calculs DFT lorsque les corrections GW sont appliquées, améliorant l'accord avec les observations expérimentales.

Entropy of a correlated system of nucleons

Realistic nucleon-nucleon interactions induce correlations to the nuclear many-body system, which lead to a fragmentation of the single-particle strength over a wide range of energies and momenta. We address the question of how this fragmentation affects the thermodynamical properties of nuclear matter. In particular, we show that the entropy can be computed with the help of a spectral function, which can be evaluated in terms of the self-energy obtained in the self-consistent Green's function approach. Results for the density and temperature dependences of the entropy per particle for symmetric nuclear matter are presented and compared to the results of lowest order finite-temperature Brueckner-Hartree-Fock calculations. The effects of correlations on the calculated entropy are small, if the appropriate quasiparticle approximation is used. The results demonstrate the thermodynamical consistency of the self-consistent T-matrix approximation for the evaluation of the Green's functions.

Mesonic fluctuations in a nonlocal Nambu–Jona-Lasinio model

The effects of meson fluctuations are studied in a nonlocal generalization of the Nambu–Jona-Lasinio model, by including terms of next-to-leading order (NLO) in 1/Nc. In the model with only scalar and pseudoscalar interactions NLO contributions to the quark condensate are found to be very small. This is a result of cancellation between virtual mesons and Fock terms, which occurs for the parameter sets of most interest. In the quark self-energy, similar cancellations arise in the tadpole diagrams, although not in other NLO pieces which contribute at the 25% level. The effects on pion properties are also found to be small. NLO contributions from real pi-pi intermediate states increase the sigma meson mass by 30%. In an extended model with vector and axial interactions, there are indications that NLO effects could be larger.

Firms in international trade: importers’ and exporters’ heterogeneity in Italian manufacturing industry

Combining data on structural characteristics and economic performance for a large sample of Italian firms with data on exporting and importing activity, we uncover evidence supporting recent theories on firm heterogeneity and international trade, together with some new facts. In particular, we find that importing is associated with substantial firm heterogeneity. First, we document that trade is more concentrated than employment and sales, and show that importing is even more concentrated than exporting both within sectors and along the sector- and country-extensive margins. Second, while supporting the fact that firms involved in both are the best performers, we also find that firms involved only in importing activities perform better than those involved only in exporting. Our evidence suggests there is a strong self-selection effect in the case of importers and the performance premia of internationalised firms correlate relatively more with the degree of geographical and sectoral diversification of imports.