Superfluid density of superconductor-ferromagnet bilayers

We report the first measurements of the effective superfluid density n_S(T) \propto \lambda^{-2}(T) of Superconductor-Ferromagnet (SC/FM) bilayers, where \lambda is the effective magnetic field penetration depth. Thin Nb/Ni bilayers were sputtered in ultrahigh vacuum in quick succession onto oxidized Si substrates. Nb layers are 102 A thick for all samples, while Ni thicknesses vary from 0 to 100 A. T_C determined from \lambda^{-2}(T) decreases rapidly as Ni thickness d_Ni increases from zero to 15 A, then it has a shallow minimum at d_Ni \approx 25 A. \lambda^{-2}(0) behaves similarly, but has a minimum several times deeper. In fact, \lambda^{-2}(0) continues to increase with increasing Ni thickness long after T_C has stopped changing. We argue that this indicates a substantial superfluid density inside the ferromagnetic Ni films.Comment: 13 pages, 2 figures, MMM 2007 proceeding

Quantum critical behaviour in the superfluid density of strongly underdoped ultrathin cuprate films

A central issue in the physics of high temperature superconductors is to understand superconductivity within a single copper-oxide layer or bilayer, the fundamental structural unit in the cuprates, and how it is lost with underdoping. As mobile holes are removed from the CuO_2 planes, the transition temperature T_C and superfluid density n_S decrease in a surprisingly correlated fashion in crystals and thick films. We seek to elucidate the intrinsic physics of bilayers in the strongly underdoped regime, near the critical doping level where superconductivity disappears. We report measurements of n_S(T) in films of Y_{1-x}Ca_xBa_2Cu_3O_{7-\delta} as thin as two copper-oxide bilayers with T_C's as low as 3 K. In addition to seeing the two-dimensional (2D) Kosterlitz-Thouless-Berezinski transition at T_C, we observe a remarkable scaling of T_C with n_S(0) that demonstrates that the disappearance of superconductivity with underdoping is due to quantum fluctuations near a T = 0 2D quantum critical point.Comment: 13 pages, 2 figur

A predictive control scheme for systems with variable time-delay

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Recent developments on the stability of systems with aperiodic sampling: An overview

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A comparison of overapproximation methods for stability analysis of networked control systems

International audienceThe presence of a communication network in a control loop induces many imperfections such as varying transmission delays, varying sampling/transmission intervals and packet loss, which can degrade the control performance significantly and can even lead to instability. Various techniques have been proposed in the literature for stability analysis and con- troller design for these so-called networked control systems. The goal of this paper is to survey discrete-time modeling approaches that are based on polytopic overapproximations of the uncertain NCS model and lead to LMI-based stability conditions. We discuss the advantages and disadvantages of the existing techniques in both qualitative and quantitative manners. In particular, we apply all methods to benchmark examples providing a numerical comparison of the methods with respect to conservatism as well as numerical complex- ity

A comparison of overapproximation methods for stability analysis of networked control systems

International audienceThe presence of a communication network in a control loop induces many imperfections such as varying transmission delays, varying sampling/transmission intervals and packet loss, which can degrade the control performance significantly and can even lead to instability. Various techniques have been proposed in the literature for stability analysis and con- troller design for these so-called networked control systems. The goal of this paper is to survey discrete-time modeling approaches that are based on polytopic overapproximations of the uncertain NCS model and lead to LMI-based stability conditions. We discuss the advantages and disadvantages of the existing techniques in both qualitative and quantitative manners. In particular, we apply all methods to benchmark examples providing a numerical comparison of the methods with respect to conservatism as well as numerical complex- ity