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The Supergiant X-ray binary Vela X-1 represents one of the best astrophysical sources to investigate the wind environment of a O/B star irradiated by an accreting neutron star. Previous studies and hydrodynamic simulations of the system revealed a clumpy environment and the presence of two wakes: an accretion wake surrounding the compact object and a photoionisation wake trailing it along the orbit. Our goal is to conduct, for the first time, high-resolution spectroscopy on Chandra/HETG data at the orbital phase $\varphi_\mathrm{orb} \approx 0.75$, when the line of sight is crossing the photoionisation wake. We aim to conduct plasma diagnostics, inferring the structure and the geometry of the wind. We perform a blind search employing a Bayesian Block algorithm to find discrete spectral features and identify them thanks to the most recent laboratory results or through atomic databases. Plasma properties are inferred both with empirical techniques and with photoionisation models within CLOUDY and SPEX. We detect and identify five narrow radiative recombination continua (Mg XI-XII, Ne IX-X, O VIII) and several emission lines from Fe, S, Si, Mg, Ne, Al, and Na, including four He-like triplets (S XV, Si XIII, Mg XI, and Ne IX). Photoionisation models well reproduce the overall spectrum, except for the near-neutral fluorescence lines of Fe, S, and Si. We conclude that the plasma is mainly photoionised, but more than one component is most likely present, consistent with a multi-phase plasma scenario, where denser and colder clumps of matter are embedded in the hot, photoionised wind of the companion star. Simulations with the future X-ray satellites Athena and XRISM show that a few hundred seconds of exposure will be sufficient to disentangle the lines of the Fe K$\alpha$ doublet and the He-like Fe XXV, improving, in general, the determination of the plasma parameters., Comment: 13 pages, 14 figures. Accepted for publication on A&A

The energy gap for electronic excitations is one of the most important characteristics of the superconducting state, as it directly refects the pairing of electrons. In the copper-oxide high temperature superconductors (HTSCs), a strongly anisotropic energy gap, which vanishes along high symmetry directions, is a clear manifestation of the d-wave symmetry of the pairing. There is, however, a dramatic change in the form of the gap anisotropy with reduced carrier concentration (underdoping). Although the vanishing of the gap along the diagonal to the square Cu-O bond directions is robust, the doping dependence of the large gap along the Cu-O directions suggests that its origin might be different from pairing. It is thus tempting to associate the large gap with a second order parameter distinct from superconductivity. We use angle-resolved photoemission spectroscopy (ARPES) to show that the two-gap behavior, and the destruction of well defined electronic excitations, are not universal features of HTSCs, and depend sensitively on how the underdoped materials are prepared. Depending on cation substitution, underdoped samples either show two-gap behavior or not. In contrast, many other characteristics of HTSCs, such as the domelike dependence of Tc on doping, long-lived excitations along the diagonals to the Cu-O bonds, energy gap at the antinode (crossing of the underlying Fermi surface and the (pi, 0)-(pi, pi) line) decreasing monotonically with doping, while persisting above Tc (the pseudogap), are present in all samples, irrespective of whether they exhibit two-gap behavior or not. Our results imply that universal aspects of high Tc superconductivity are relatively insensitive to differences in the electronic states along the Cu-O bond directions., Comment: Accepted for publication at PNAS

In order to understand the origin of high-temperature superconductivity in copper oxides, we must understand the normal state from which it emerges. Here, we examine the evolution of the normal state electronic excitations with temperature and carrier concentration in Bi2Sr2CaCu2O8 using angle-resolved photoemission. In contrast to conventional superconductors, where there is a single temperature scale Tc separating the normal from the superconducting state, the high- temperature superconductors exhibit two additional temperature scales. One is the pseudogap scale T*, below which electronic excitations exhibit an energy gap. The second is the coherence scale Tcoh, below which sharp spectral features appear due to increased lifetime of the excitations. We find that T* and Tcoh are strongly doping dependent and cross each other near optimal doping. Thus the highest superconducting Tc emerges from an unusual normal state that is characterized by coherent excitations with an energy gap., Comment: 5 pages, 4 figures. Published in Proc. Nat. Acad. Sci. vol. 108 pp 9346 (2011). Free to download

We use angle resolved photoemission spectroscopy to probe the electronic excitations of the non-superconducting state that exists between the antiferromagnetic Mott insulator at zero doping and the superconducting state at larger dopings in Bi_2Sr_2CaCu_2O_{8+\delta}. We find that this state is a nodal liquid whose excitation gap becomes zero only at points in momentum space. Despite exhibiting a resistivity characteristic of an insulator and the absence of coherent quasiparticle peaks, this material has the same gap structure as the d-wave superconductor. We observe a smooth evolution of the spectrum across the insulator-to-superconductor transition, which suggests that high temperature superconductivity emerges when quantum phase coherence is established in a non-superconducting nodal liquid., Comment: 16 pages, 5 figures

An overview of Alcoa's wind energy program is given with emphasis on the the development of a low cost, reliable Darrieus Vertical Axis Wind Turbine System. The design layouts and drawings for fabrication are now complete, while fabrication and installation to utilize the design are expected to begin shortly.