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

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

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 $H^2$

Approximate transverse feedback linearization under digital control

Thanks to a suitable redesign of the maps involved in the continuous-time solution, a digital design procedure preserving transverse feedback linearization up to a prefixed order of approximation in the sampling period is described. Simulated examples illustrate the results

Energy-Balance PBC of nonlinear dynamics under sampling and delays

The paper provides a new class of passivity-based controllers (PBCs) for stabilizing sampled-data input-delayed dynamics at a desired equilibrium via energy-balancing (EB) and reduction. Given a nonlinear dynamics under piecewise constant and retarded input, we first exhibit a new dynamics (the reduced dynamics) that is free of delays and equivalent to the original one. Accordingly, we design the digital controller assigning a suitable energetic behaviour to the reduced delay-free model with a stable target equilibrium. Then, it is proved that such a controller solves the EB-PBC problem on the original retarded system. The results are illustrated over a simple mechanical system

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

A new connection protocol for multi-consensus of discrete-time systems

In this paper, a new connection protocol for consensus of multi-agent discrete-time systems under a general communication graph is proposed. In particular, the coupling is realized based on the outputs making each agent passive in the u-average sense so guaranteeing convergence to the agreement steady-state, with no need of mitigating the coupling gain, as typically done in concerned literature. The proposed connection rule is shown to apply for network dynamics under aperiodic sampling when the sampling sequence is known to all agents

Virtual Holonomic Constraints for Euler-Lagrange systems under sampling

In this paper, we consider the problem of imposing Virtual Holonomic Constraints to mechanical systems in Euler-Lagrangian form under sampling. An exact solution based on multi-rate sampling of order two over each input channel is described. The results are applied to orbital stabilization of the pendubot with illustrative simulations

Robust half-metallic antiferromagnets LaAAVOsO6_6 and LaAAMoYYO6_6 (AA = Ca, Sr, Ba; YY = Re, Tc) from first-principles calculations

We have theoretically designed three families of the half-metallic (HM) antiferromagnets (AFM), namely, La$A$VOsO$_6$, La$A$MoTcO$_6$ and La$A$MoReO$_6$ ($A$ = Ca, Sr, Ba), based on a systematic {\it ab initio} study of the ordered double perovskites La$ABB'$O$_6$ with the possible $B$ and $B'$ pairs from all the 3$d$, 4$d$ and 5$d$ transtion metal elements being considered. Electronic structure calculations based on first-principles density-functional theory with generalized gradient approximation (GGA) for more than sixty double perovskites LaCa$BB'$O$_6$ have been performed using the all-electron full-potential linearized augmented-plane-wave method. The found HM-AFM state in these materials survives the full {\it ab initio} lattice constant and atomic position optimizations which were carried out using frozen-core full potential projector augmented wave method. It is found that the HM-AFM properties predicted previously in some of the double perovskites would disappear after the full structural optimizations. The AFM is attributed to both the superexchange mechanism and the generalized double exchange mechanism via the $B$ ($t_{2g}$) - O (2$p_{\pi}$) - $B'$ ($t_{2g}$) coupling and the latter is also believed to be the origin of the HM. Finally, in our search for the HM-AFMs, we find La$A$CrTcO$_6$ and La$A$CrReO$_6$ to be AFM insulators of an unconventional type in the sense that the two antiferromagnetic coupled ions consist of two different elements and that the two spin-resolved densities of states are no longer the same.Comment: To appear in Phys. Rev.

Electric instability in superconductor-normal conductor ring

Non-linear electrodynamics of a ring-shaped Andreev interferometer (superconductor-normal conductor-superconductor hybrid structure) inductively coupled to a circuit of the dissipative current is investigated. The current-voltage characteristics (CVC) is demonstrated to be a series of loops with several branches intersecting in the CVC origin. The sensitivity of the transport current to a change of the applied external magnetic flux can be comparable to the one of the conventional SQUID's. Spontaneous arising of coupled non-linear oscillations of the transport current, the Josephson current and the magnetic flux in Andreev interferometers are also predicted and investigated. The frequency of these oscillations can be varied in a wide range, while the maximal frequency can reach $\omega_{max} \sim 10^{12}$ $sec^{-1}$.Comment: 4 pages, 4 figure