GW quasi-particle spectra from occupied states only

We introduce a method that allows for the calculation of quasi-particle spectra in the GW approximation, yet avoiding any explicit reference to empty one-electron states. This is achieved by expressing the irreducible polarizability operator and the self-energy operator through a set of linear response equations, which are solved using a Lanczos-chain algorithm. We first validate our approach by calculating the vertical ionization energies of the benzene molecule and then show its potential by addressing the spectrum of a large molecule such as free-base tetraphenylporphyrin.Comment: 4 pages, 3 figure

On Electric Fields in Low Temperature Superconductors

The manifestly Lorentz covariant Landau-Ginzburg equations coupled to Maxwell's equations are considered as a possible framework for the effective description of the interactions between low temperature superconductors and magnetic as well as electric fields. A specific experimental set-up, involving a nanoscopic superconductor and only static applied fields whose geometry is crucial however, is described, which should allow to confirm or invalidate the covariant model through the determination of the temperature dependency of the critical magnetic-electric field phase diagram and the identification of some distinctive features it should display.Comment: 14 pages (Latex) + 2 postscript figure

Annular Vortex Solutions to the Landau-Ginzburg Equations in Mesoscopic Superconductors

New vortex solutions to the Landau-Ginzburg equations are described. These configurations, which extend the well known Abrikosov and giant magnetic vortex ones, consist of a succession of ring-like supercurrent vortices organised in a concentric pattern, possibly bound to a giant magnetic vortex then lying at their center. The dynamical and thermodynamic stability of these annular vortices is an important open issue on which hinges the direct experimental observation of such configurations. Nevertheless, annular vortices should affect indirectly specific dynamic properties of mesoscopic superconducting devices amenable to physical observation.Comment: 12 pages, LaTeX, 2 Postscript figure

Configurations de vortex magnétiques dans des cylindres mésoscopiques supraconducteurs

Mesoscopic superconductors are described within the framework of the nonlinear Ginzburg-Landau theory. The two coupled nonlinear equations are solved numerically and we investigate the properties, in particular the order of the transition and the vortex configurations, of cylinders submitted to an external magnetic field. Meissner state, Abrikosov vortices, GiantVortex and MultiVortex solutions are described. The Bean-Livingston barrier in mesoscopic cylinders is also numerically studied. This theoretical work was applied to understand experimental magnetizations of lead nanowires in an array well below the superconducting transition temperature Tc. By freely adjusting the GL phenomenological lengths ? (T) and ? (T), the experimental magnetization curves are reproduced to within a 10% error margin. The Meissner and the Abrikosov state were also experimentally observed in this apparently type-I superconductor. This fact is a consequence of the non-trivial behaviour of the critical boundary ? _c ($=1/?2 in bulk materials) between type-I and type-II phase transition at mesoscopic scales. Beyond the experimental-theoretical agreement, the question whether the GL model remains valid far below Tc is also addressed. The temperature dependence of the adjusted characteristic lengths is compared with different theoretical and empirical laws. The best agreement is achieved for the Gorter-Casimir two-fluid model. A comparison between lead nanowire arrays electrodeposited under constant and pulsed voltage conditions allows us to distinguish both samples in terms of their electronic mean free paths. The characterisation of the latter quantities concurs perfectly with the experimental expectation given the different electrodeposition techniques.Motivées par des données expérimentales sur la magnétisation de réseau de nanofils de plomb, les résolutions numériques des équations stationnaires de Ginzburg-Landau (GL) se sont focalisées sur les géométries à symétrie axiale. L'effet Meissner, les états représentant un vortex d'Abrikosov ou encore des Vortex Géants ('GiantVortex') centrés à l'origine du cylindre ont alors pu être identifiés sous l'hypothèse d'invariance sous rotation selon l'axe de symétrie du cylindre étudié (modèle à une dimension, 1D). En identifiant le type de transition par le caractère continu ou non du paramètre d'ordre autour du changement de phase, une frontière à l'échelle mésoscopique a également pu être identifiée au travers du mod��le 1D. Plus spécifiquement, la limite entre les deux types de transitions décrite par le paramètre phénoménologique ? = ? /? ( =1/?2 à l'échelle macroscopique) devient une fonction non constante dépendant à la fois du rayon normalisé, u=R/?, et de la vorticité L: ? =f(u,L). Les deux longueurs caractéristiques ? et ? représentent respectivement les longueurs de pénétration et de cohérence d'un échantillon supraconducteur. Une comparaison avec les résultats obtenus par Zharkov permet de valider notre démarche numérique employée pour la résolution numérique des équations de GL à une dimension. En employant un modèle à deux dimensions (2D), la symétrie sous rotation des solutions a également été relâchée. Basée sur le principe de moindre action, la résolution propose alors un schéma numérique indépendant du type d'équations du mouvement à solutionner. Les configurations du type MultiVortex ont alors pu être identifiées, et comparées aux solutions du groupe du Professeur F. Peeters. Ces différents accords ont confirmé la démarche développée. Une modélisation de la magnétisation expérimentale d'un réseau de nanofils a également été développée. De par la taille réduite des nanofils, l'interaction magnétique entre ceux-ci a pu être négligée. La magnétisation totale du réseau est alors construite par une sommation incluant la contribution individuelle en magnétisation de chaque fil, pondérée par un poids reflétant une distribution gaussienne pour les rayons des fils constituant le réseau. La magnétisation individuelle est évidemment obtenue par résolution des équations du mouvement de GL précédemment étudiées avec les modèles 1D et 2D. En ajustant les paramètres libres associés à ce modèle décrivant la magnétisation totale du réseau, les données expérimentales ont pu être reproduites endéans 10% de marge d'erreur, l'intervalle d'incertitude caractéristique de la théorie effective de Ginzburg-Landau. [...](PHYS 3) -- UCL, 200

Nanoscopic superconducting slab in static electric and magnetic fields

A space-time covariant extension of the stationary London and Ginzburg-Landau equations is proposed, which naturally entails the coupling of low temperatures superconductors to both electric as well as Magnetic fields. A specific experimental set-up consisting of a slab of nanoscopic thickness submitted to crossed electric and magnetic fields with a specific geometry should allow the confirmation or invalidation of the model. Theoretical considerations and the results of numerical simulations as well as progress towards an actual measuring device are discussed. (C) 2001 Published by Elsevier Science B.V

Vortex matter in lead nanowires

Theoretical and experimental magnetizations of lead nanowire arrays well below the superconducting transition temperature T-c are described. The magnetic response of the array was investigated with a SQUID magnetometer. Hysteretic behaviour and phase transitions have been observed in sweeping up and down the external magnetic field at different temperatures. The Meissner and Abrikosov states were also experimentally observed in this apparently type-I superconductor. This fact brings to the fore the non-trivial behaviour of the critical boundary K-c (= 1/root2 in bulk materials) between type-I and type-II phase transitions at mesoscopic scales. The time-independent Ginzburg-Landau equations particularized to cylindrically symmetric configurations enable one to explain and reproduce the experimental magnetization curves within 10% of error

Temperature dependence of penetration and coherence lengths in lead nanowires

Magnetization curves well below T-c of a lead nanowire array electrodeposited under pulsed voltage conditions are considered. Numerical simulations based on a two-dimensional resolution (2D model) of the Ginzburg-Landau (GL) equations for cylindrical configurations have also been developed. By freely adjusting the GL phenomenological lengths lambda(T) and epsilon(T), the experimental magnetization curves are reproduced to within a 10% error margin. The 2D model also allows us to check the cylindrical symmetry of the obtained configurations. Beyond the experimental-theoretical agreement, the question of whether the GL model remains valid far below T-c is also addressed. The temperature dependence of the adjusted characteristic lengths is compared with different theoretical and empirical laws. The most satisfactory agreement is achieved for the Gorter-Casimir two-fluid model. A comparison with a lead nanowire array electrodeposited under constant voltage conditions allows us to distinguish both samples in terms of their electronic mean free paths. The characterization of the latter quantities concurs perfectly with the experimental expectation given the different electrodeposition techniques

Ring-like vortex solutions to the Landau-Ginzburg equations in superconducting mesoscopic devices

Beyond the well-known Abrikosov and giant vortex configurations, we describe new types of solutions to the Landau-Ginzburg (LG) equations for low temperature superconductivity. These solutions correspond to ring-like current vortices surrounding one another in concentric patterns, which in addition may possibly be bound to a standard L fluxoid configuration in their center. These solutions are possible in mesoscopic devices, due to their finite spatial extent, and generalize the usual vortex configurations for large enough samples. They contribute additional states at a given applied magnetic field, whose metastability still needs to be assessed. Such configurations could add to the richness of the phase transition behavior of mesoscopic superconducting disks and annuli in external magnetic fields, in ways to be explored. The properties and characterization of these new configurations are described, both for mesoscopic disks and annuli. (C) 2000 Elsevier Science B.V. All rights reserved

Superconducting properties of lead nanowires arrays

Mesoscopic superconducting lead nanowires with high aspect ratio and diameter ranging from 40 to 270 nm have been grown by electrodeposition inside nanoporous polycarbonate membranes. Nanowires with a diameter less than 50 nm were insulators due to a poor crystal structure. The others are shown to be type II superconductors because of their small electronic mean free path, instead of being type I which is usual for the bulk form of lead. An increase in the thermodynamic critical field H-c is observed and is attributed to the small transversal dimension leading to an incomplete Meissner effect. Finally, it is demonstrated that this enhancement agrees with numerical simulations based on the Ginzburg-Landau theory. (C) 2002 Elsevier Science B.V. All rights reserved