Impurity states in multiband s-wave superconductors: analysis of iron pnictides

We examine the effect of a single, non-magnetic impurity in a multiband, extended s-wave superconductor allowing for anisotropy of the gaps on the Fermi surfaces. We derive analytic expressions for the Green's functions in the continuum and analyse the conditions for the existence of sharp impurity-induced resonant states. Underlying band structure is more relevant for the multiband than for single band case, and mismatch between the bands generically makes the formation of the impurity states less likely in the physical regime of parameters. We confirm these conclusions by numerically solving the impurity problem in a tight-binding parameterization of the bands relevant to pnictide superconductors.Comment: minor corrections, published versio

LPM Interference and Cherenkov-like Gluon Bremsstrahlung in Dense Matter

Gluon bremsstrahlung induced by multiple parton scattering in a finite dense medium has a unique angular distribution with respect to the initial parton direction. A dead-cone structure with an opening angle $\theta^2_0 \approx 2(1-z)/(zLE)$ for gluons with fractional energy $z$ arises from the Landau-Pomeranchuck-Migdal (LPM) interference. In a medium where the gluon's dielectric constant is $\epsilon >1$, the LPM interference pattern is shown to become Cherenkov-like with an increased opening angle determined by the dielectric constant $\cos^2\theta_c=z+(1-z)/\epsilon$. For a large dielectric constant $\epsilon \gg 1+2/z^2LE$, the corresponding total radiative parton energy loss is about twice that from normal gluon bremsstrahlung. Implications of this Cherenkov-like gluon bremsstrahlung to the jet correlation pattern in high-energy heavy-ion collisions is discussed.Comment: 4 pages in RevTEx with 1 postscript figur

Robust and Resilient State Dependent Control of Discrete-Time Nonlinear Systems with General Performance Criteria

A novel state dependent control approach for discrete-time nonlinear systems with general performance criteria is presented. This controller is robust for unstructured model uncertainties, resilient against bounded feedback control gain perturbations in achieving optimality for general performance criteria to secure quadratic optimality with inherent asymptotic stability property together with quadratic dissipative type of disturbance reduction. For the system model, unstructured uncertainty description is assumed, which incorporates commonly used types of uncertainties, such as norm-bounded and positive real uncertainties as special cases. By solving a state dependent linear matrix inequality at each time step, sufficient condition for the control solution can be found which satisfies the general performance criteria. The results of this paper unify existing results on nonlinear quadratic regulator, H∞ and positive real control to provide a novel robust control design. The effectiveness of the proposed technique is demonstrated by simulation of the control of inverted pendulum

Comment on 'Note on the dog-and-rabbit chase problem in introductory kinematics'

We comment on the recent paper by Yuan Qing-Xin and Du Yin-Xiao (Eur. J. Phys. 29 (2008) N43-N45).Comment: 2 pages, no figure

Exciton Condensate Modulation in Electron-Hole Bilayers: A Real-Space Visualization

We study the texture of the exciton condensate at low temperatures in an independently gated electron-hole bilayer system. A model Hamiltonian is solved in real space within a mean-field approximation. It is found that, with increased electron-hole density polarization, the system experiences phase transformations from the zero center-of-mass momentum superfluid state, through one- and two-dimensional exciton pair modulated states, into the normal state. At weak density polarization, the modulating state resembles the Larkin-Ovchinikov state in superconductors in the presence of an exchange field in the weak-coupling BCS limit, and becomes stripe-like in the strong coupling BEC limit. In the one-dimensional modulated phase, the density of states exhibits low-energy intra-gap resonance quasiparticle states, which are localized in the nodal region.Comment: 4+ pages, 6 eps figure

Fitting the Viking lander surface pressure cycle with a Mars General Circulation Model

We present a systematic attempt to fit the Viking lander surface pressure cycle using a Mars General Circulation Model, MarsWRF. Following the earlier study by Wood and Paige (1992) using a one-dimensional model, high-precision fitting was achieved by tuning five time-independent parameters: the albedo and emissivity of the seasonal caps of the two hemispheres and the total CO_2 inventory in the atmosphere frost system. We used a linear iterative method to derive the best fit parameters: albedo of the northern cap = 0.795, emissivity of the northern cap = 0.485, albedo of the southern cap = 0.461, emissivity of the southern cap = 0.785, and total CO_2 mass = 2.83 × 10^(16) kg. If these parameters are used in MarsWRF, the smoothed surface pressure residual at the VL1 site is always smaller than several Pascal through a year. As in other similar studies, the best fit parameters do not match well with the current estimation of the seasonal cap radiative properties, suggesting that important physics contributing to the energy balance not explicitly included in MarsWRF have been effectively aliased into the derived parameters. One such effect is likely the variation of thermal conductivity with depth in the regolith due to the presence of water ice. Including such a parameterization in the fitting process improves the reasonableness of the best fit cap properties, mostly improving the emissivities. The conductivities required in the north to provide the best fit are higher than those required in the south. A completely physically reasonable set of fit parameters could still not be attained. Like all prior published GCM simulations, none of the cases considered are capable of predicting a residual southern CO_2 cap