Reduction of discrete-time two-channel delayed systems

In this letter, the reduction method is extended to time-delay systems affected by two mismatched input delays. To this end, the intrinsic feedback structure of the retarded dynamics is exploited to deduce a reduced dynamics which is free of delays. Moreover, among other possibilities, an Immersion and Invariance feedback over the reduced dynamics is designed for achieving stabilization of the original systems. A chained sampled-data dynamics is used to show the effectiveness of the proposed control strategy through simulations

Discrete port-controlled Hamiltonian dynamics and average passivation

The paper discusses the modeling and control of port-controlled Hamiltonian dynamics in a pure discrete-time domain. The main result stands in a novel differential-difference representation of discrete port-controlled Hamiltonian systems using the discrete gradient. In these terms, a passive output map is exhibited as well as a passivity based damping controller underlying the natural involvement of discrete-time average passivity

Nonlinear discrete-time systems with delayed control: a reduction

In this work, the notion of reduction is introduced for discrete-time nonlinear input-delayed systems. The retarded dynamics is reduced to a new system which is free of delays and equivalent (in terms of stabilizability) to the original one. Different stabilizing strategies are proposed over the reduced model. Connections with existing predictor-based methods are discussed. The methodology is also worked out over particular classes of time-delay systems as sampled-data dynamics affected by an entire input delay

Lyapunov stabilization of discrete-time feedforward dynamics

The paper discusses stabilization of nonlinear discrete-time dynamics in feedforward form. First it is shown how to define a Lyapunov function for the uncontrolled dynamics via the construction of a suitable cross-term. Then, stabilization is achieved in terms of u-average passivity. Several constructive cases are analyzed

Integer filling metal insulator transitions in the degenerate Hubbard model

We obtain exact numerical solutions of the degenerate Hubbard model in the limit of large dimensions (or large lattice connectivity). Successive Mott-Hubbard metal insulator transitions at integer fillings occur at intermediate values of the interaction and low enough temperature in the paramagnetic phase. The results are relevant for transition metal oxides with partially filled narrow degenerate bands.Comment: 4 pages + 4 figures (in 5 ps-files), revte

The Mott transition in V_2 O_3 and Ni Se_x S_{2-x}: insights from dynamical mean field theory

We discuss some aspects of the pressure (or interaction) driven Mott transition, in three dimensional transition metal oxides by means of dynami cal mean field theory. We isolate the universal properties of the transition from the aspects which depend more on the detailed chemistry of the compounds. In this light we can understand the main differences and the remarkable similarities between these systems. Both theory and experiment converge on the transfer of spectral weight from low energies to high energies as the universal mechanism underlying the Mott transition, and we comment on the possible relevance of these ideas to other metal to non metal transitions.Comment: Talk presented at SCES 9

Planar spin exchange in LiNiO_2

We study the planar spin exchange couplings in LiNiO2 using a perturbative approach. We show that the inclusion of the trigonal crystal field splitting at the Oxygen sites leads to the appearance of antiferromagnetic exchange integrals in deviation from the Goodenough-Kanamori-Anderson rules for this 90 degree bond. That gives a microscopic foundation for the recently observed coexistence of ferromagnetic and antiferromagnetic couplings in the orbitally-frustrated state of LiNiO2. (F. Reynaud et al, Phys. Rev. Lett. 86, 3638 (2001))Comment: latex, revtex4, 6 pages, 3 figure

Sur la gestion échantillonnée de l'énergie d'un système - pile à combustible-super condensateurs

International audienceIl s'agit d'illustrer les performances d'une commande non lin eaire echantillonn ee pour la gestion de l' energie d'un syst eme compos e d'une pile a combustible associ ee a un super condensateur - P aC-SC. L'objectif est de piloter l' echange energ etique entre les deux sources pour solliciter dans un premier temps le super condensateur et dans un deuxi eme temps la pile. Deux strat egies non lin eaires sont etudi ees. Une premi ere approche fait r ef erence a la m ethode de fa connement de l' energie et utilise une conception de type Lyapunov reposant sur la notion de passivit e. La deuxi eme exploite la structure a deux echelles de temps des deux sources de courant (pile et super condensateur) et propose une strat egie aux perturbations singuli eres. Les param etres choisis sont ceux d'un banc d'essai exp erimental en pr evision de l'implantation. Les r esultats sont compar es aux performances d'un sch ema continu dit id eal et a une implantation classique en termes de bloqueur d'ordre z ero

Interface superconductivity in La1.48_{1.48}Nd0.4_{0.4}Sr0.12_{0.12}CuO4_{4}/La1.84_{1.84}Sr0.16_{0.16}CuO4_{4} bilayers

We identify a distinct superconducting phase at the interface of a La$_{1.48}$Nd$_{0.4}$Sr$_{0.12}$CuO$_4$ (LNSCO)/La$_{1.84}$Sr$_{0.16}$CuO$_4$ (LSCO) epitaxial bilayer system using ac screening measurements. A model based on inter-diffusion of quasiparticles and condensate at the interface yields a thickness of $\sim$ 25 nm for the interfacial layer. Two-dimensional superconductivity of the interface layer appears to be governed by Kosterlitz-Thouless-Berezinskii transition. A parallel magnetic field suppresses the superconducting transition temperature of this layer with a pair breaking parameter $\alpha$ varying as $H^2$