Dynamics of the 2D two-component plasma near the Kosterlitz-Thouless transition

We study the dynamics of a classical, two-component plasma in two dimensions, in the vicinity of the Kosterlitz-Thouless (KT) transition where the system passes from a dielectric low-temperature phase (consisting of bound pairs) to a conducting phase. We use two ``complementary'' analytical approaches and compare to simulations. The conventional, ``intuitive'' approach is built on the KT picture of independently relaxing, bound pairs. A more formal approach, working with Mori projected dynamic correlation functions, avoids to assume the pair picture from the start. We discuss successes and failures of both approaches, and suggest a way to combine the advantages of both.Comment: 4 pages, LaTeX, 4 eps figures included; to appear in the proceedings of the ``1999 International Conference on Strongly Coupled Coulomb Systems'', 4-11 Sept 1999 in Saint-Malo (France); some typos correcte

Pseudogap in the one-electron spectral functions of the attractive Hubbard model

We calculate the one-electron Green's function of the 2D attractive Hubbard model by coupling the electrons to pair fluctuations. The latter are approximated by homogeneous amplitude fluctuations and phase correlations corresponding to the XY-model. The electronic density of states shows a pseudogap at temperatures well above the transition temperature $T_{C}$. For a quasi-3D system, a superconducting gap emerges out of the pseudogap below $T_{C}$.Comment: 2 pages, one major typo corrected, one reference adde

One-Electron Spectral Functions of the Attractive Hubbard Model for Intermediate Coupling

We calculate the one-electron spectral function of the attractive (negative-$U$) Hubbard model. We work in the intermediate coupling and low density regime and obtain the self-energy in an approximate analytical form. The excitation spectrum is found to consist of three branches. The results are obtained in a framework, based on the self-consistent T-matrix approximation, which is compatible with the Mermin-Wagner theorem.Comment: 7 pages, revtex, 7 figures in postscript format, submitted to PR

Marché électrique libéralisé en Suisse: Vers de nouvelles stratégies de coopération entre collectivités publiques et distributeurs d'électricité?

L’entrée en vigueur de la loi sur l’approvisionnement en électricité (LApEl) au début 2008 et l’ouverture du marché électrique sont en passe de recomposer le paysage énergétique suisse et ses acteurs principaux: les collectivités publiques et les entreprises de production et de distribution. Un séminaire organisé le 18 septembre dernier à l’EPFL par le Centre de Recherches Energétiques et Municipales (CREM), l’Energy Center de l’EPFL et SuisseEnergie pour les communes a tâché d’explorer les différentes opportunités, ainsi que les risques en matière de stratégies de coopération entre les multiples acteurs du secteur électrique suisse.Liberalisierung des Schweizer Strommarkts: hin zu neuen Strategien für die Zusammenarbeit zwischen dem Gemeinwesen und Stromanbietern? Das Stromversorgungsgesetz (StromVG), das Anfang 2008 in Kraft getreten ist, und die Öffnung des Strommarkts werden die Schweizer Energielandschaft und deren Hauptakteure stark prägen: das Gemeinwesen sowie die Stromhersteller und -anbieter. Am 18. September 2008 fand an der ETH Lausanne ein vom Centre de Recherches Energétiques et Municipales (CREM), vom EnergyCenter der ETHL und von Energie Schweiz für die Gemeinden organisiertes Seminar statt, an dem die diversen Chancen, aber auch die Risiken bezüglich der Strategien für die Zusammenarbeit der vielen Akteure des Schweizer Strommarktes erörtert wurden

One-electron spectral functions of the attractive Hubbard model at intermediate coupling

We calculate the one-electron spectral function of the attractive-U Hubbard model in two dimensions. We work in the intermediate coupling and low density regime and evaluate analytically the self-energy. The results are obtained in a framework based on the self-consistent T-matrix approximation. We also calculate the chemical potential of the bound pairs as a function of temperature. On the basis of this calculation we analyze the low-temperature resistivity and specific heat in the normal state of this system. We compare our results with recent beautiful tunneling experiments in the underdoped regime of HTSC-materials.Comment: 2 pages, LT22 Conference paper, phbauth and elsart style files include

Excitation spectrum of the two-dimensional attractive Hubbard model

We calculate the one-particle spectral functions above the superconducting transition temperature $T_{C}$, in the framework of a functional integral approach. The coupling of the electronic self-energy to pair fluctuations, which are treated by means of a time-dependent Ginzburg-Landau equation, yields a double-peak structure, around the Fermi wavenumber. The peak separation is essentially temperature-independent, but the structure sharpens when $T_{C}$ is approached.Comment: 3 pages, revtex, 3 embedded figures (eps), submitted to Physica

Internal structure of fluctuating Cooper pairs

Abstract.: In order to obtain information about the internal structure of fluctuating Cooper pairs in the pseudogap state and below the transition temperature of high Tc superconductors, we solve the Bethe-Salpeter equation for the two-electron propagator in order to calculate a "pair structure function” $g_{P}({\mathbf{P}},\pmb{\rho})$ that depends on the internal distance $\pmb{\rho}$ between the partners and on the center of mass momentum P of the pair. We use an attractive Hubbard model with a local potential for s-wave and a separable potential for d-wave symmetry. The amplitude of gP for small ρ depends on temperature, chemical potential and interaction symmetry, but the ρ dependence itself is rather insensitive to the interaction strength. Asymptotically gP decreases as an inverse power of ρ for weak coupling, but exponentially when a pseudogap develops for stronger interaction. Some possibilities of observing the pair structure experimentally are mentione

Screening effects in superconductors

The partition function of the Hubbard model with local attraction and long range Coulomb repulsion between electrons is written as a functional integral with an action $A$ involving a pairing field $\Delta$ and a local potential $V$. After integration over $V$ and over fluctuations in $|\Delta|^{2}$, the final form of $A$ involves a Josephson coupling between the local phases of $\Delta$ and a "kinetic energy" term, representing the screened Coulomb interaction between charge fluctuations. The competition between Josephson coupling and charging energy allows to understand the relation between $T_{C}$ and composition in high-$T_{C}$ materials, in particular superlattices, alloys and bulk systems of low doping.Comment: 4 pages, revtex, no figures, submitted to Physica B (Proceedings of SCES '96 International Conference, held in Zurich from 19th to 21st of August

Persistence of pseudogap formation in quasi-2D systems with arbitrary carrier density

The existence of a pseudogap above the critical temperature has been widely used to explain the anomalous behaviour of the normal state of high-temperature superconductors. In two dimensions the existence of a pseudogap phase has already been demonstrated in a simple model. It can now be shown that the pseudogap phase persists even for the more realistic case where coherent interlayer tunneling is taken into account. The effective anisotropy is surprisingly large and even increases with increasing carrier density.Comment: 17 pages, LaTeX, 1 EMTeX figure; extended versio