Séminaire Théorique du Vendredi

Attention : changement de salle du 26 février au 26 mars (HERCULES) Les participants ne possédant pas de badges permettant l’accès au site ILL/ESRF sont priés d’envoyer une demande d’entrée sur le site à Alison Mader (mader@ill.fr)

Salle 106, Bâtiment Commun ILL/ESRF

Vendredi 12 mars 2010 – 11h00

 Evgeny BUROVSKIY

Université Paris-Sud

We consider two-component one-dimensional quantum gases with a density imbalance. While generically such fluids are two-component Luttinger liquids, we show that if the ratio of the densities is a rational number, p/q, and mass asymmetry between components is sufficiently strong, one of the two eigenmodes acquires a gap. The gapped phase corresponds to (algebraic) ordering of (p+q)-particle composites. In particular, for attractive mixtures, this implies that the superconducting correlations are destroyed. We illustrate our predictions by numerical simulations of the fermionic Hubbard model with hopping asymmetry.

Séminaire théorique du vendredi

Attention : changement de salle du 26 février au 26 mars (HERCULES)

Les participants ne possédant pas de badges permettant l’accès au site ILL/ESRF sont priés d’envoyer une demande d’entrée sur le site à Alison Mader (mader@ill.fr)

Salle 106, Bâtiment Commun ILL/ESRF

Vendredi 26 février 2010 – 11h00

Alberto ROSSO

Université Paris-Sud

In this talk we explore the deep connections between the statistics of the extremes (maxima and minima) of a large set of random variables and the physics of disordered systems. In particular, a simple property of the statistics of the minima allows to determine if the system undergoes a Freezing transition. As an example we study in detail the case of log-correlated Gaussian variables.  

Séminaire théorique du vendredi

organisé par le Centre Théorique pour la Physique Grenobloise

Salle de conférences - Maison des Magistères CNRS, Grenoble

Vendredi 12 février 2010 – 11h00 (à partir de 10h30: café & cookies)

Harold U. BARANGER

Duke University

Quantum error correction (QEC) is one of the primary ways to protect quantum information from decoherence by the environment. To shed light on how well it works, I shall discuss the long time behavior of a quantum computer running a QEC code in the presence of a correlated environment. Starting from a Hamiltonian formulation of noise models, we find formal expressions for the probability of a given syndrome history and the associated residual decoherence encoded in the reduced density matrix. With certain simplifying assumptions, an explicit calculation can be carried out for, e.g., a spin-boson model. The key result is a dimensional criterion: If the correlations decay sufficiently fast, the system evolves toward a stochastic error model for which the "threshold theorem" of fault-tolerant quantum computation holds. In the absence of the simplifying assumptions, the form of the propagator for a logical qubit at very long times can still be obtained, showing how quantum information is ultimately lost.

Séminaire Théorique du Vendredi

Bâtiment Commun ILL/ESRF – Salle 106

Vendredi 5 février 2010 – 11h00

Giovanni SORDI

Universite de Sherbrooke

The metal-insulator transition induced by interactions, the Mott transition, is well understood in dimensions larger than two. Many materials of current interest however, such as the high-temperature superconductors, are layered. In layered materials, the effect of short-range spin correlations has to be taken into account. This is achieved by solving the cluster dynamical mean-field equations for the Hubbard model on a 2x2 plaquette with continuous-time quantum Monte Carlo. The phase diagram as a function of temperature T, interaction strength U and filling n reveals that the metal that first appears upon doping does so continuously but, for all U, has some insulating regions in momentum space. For intermediate values of U, that metal undergoes a first-order transition to a more ordinary metal at finite doping. At larger U this transition loses its first-order character and becomes continuous. A competition between charge and spin fluctuations is at the origin of the transition and of a coincident maximum in the scattering rate as a function of doping. That maximum occurs near what would be called optimal doping in high-temperature superconductors.