The Exotic Barium Bismuthates

We review the remarkable properties, including superconductivity, charge-density-wave ordering, and metal-insulator transitions, of lead- and potassium-doped barium bismuthate. We discuss some of the early theoretical studies of these systems. Our recent theoretical work, on the negative-U\/, extended-Hubbard model for these systems, is also described. Both the large- and intermediate-U\/ regimes of this model are examined, using mean-field and random-phase approximations, particularly with a view to fitting various experimental properties of these bismuthates. On the basis of our studies, we point out possibilities for exotic physics in these systems. We also emphasize the different consequences of electronic and phonon-mediated mechanisms for the negative U.\/ We show that, for an electronic mechanism, the \secin \,\,phases of these bismuthates must be unique, with their transport properties {\it dominated by charge $\pm 2e$ Cooperon bound states}. This can explain the observed difference between the optical and transport gaps. We propose other experimental tests for this novel mechanism of charge transport and comment on the effects of disorder.Comment: UUencoded LaTex file, 122 pages, figures available on request To appear in Int. J. Mod. Phys. B as a review articl

Gunnery Surface Target

The functional and technical requirements of the Gunnery surface target for Naval firing practice are given. The design evolved, and the raw materials required for its various components are described and discussed in detail. The target designed can with stand the sea state of seven in all operating conditions while being towed at a speed of 15 knots in a wind speed of 25 knots. It is visible upto a distance of 18 km and has radar responsive characteristics upto 22 km

Nonequilibrium Phase Transitions in a Driven Sandpile Model

We construct a driven sandpile slope model and study it by numerical simulations in one dimension. The model is specified by a threshold slope \sigma_c\/, a parameter \alpha\/, governing the local current-slope relation (beyond threshold), and $j_{\rm in}$, the mean input current of sand. A nonequilibrium phase diagram is obtained in the \alpha\, -\, j_{\rm in}\/ plane. We find an infinity of phases, characterized by different mean slopes and separated by continuous or first-order boundaries, some of which we obtain analytically. Extensions to two dimensions are discussed.Comment: 11 pages, RevTeX (preprint format), 4 figures available upon requs

Negative-U extended hubbard model for doped barium bismuthates

We present detailed mean-field and random-phase-approximation studies of the negative-U, extended Hubbard model with a view to understanding the properties of the doped barium bismuthates. In particular, we obtain the phase diagram, the excitation spectrum, and the optical conductivity in the semiconducting phase of the bismuthates. We show by explicit calculations how this model leads to a natural explanation for the two, well-separated transport and optical gaps observed in the semiconducting phases of the bismuthates. We fix the parameters in our model by fitting these experimentally observed gaps; and with these parameter values we compute other properties of these systems. We also show how metallic screening and disorder can decrease the superconducting Tc dramatically. Our theory leads to an exotic charge-transport mechanism, dominated by charge ±2e bosons (cooperons), in the semiconducting phases of these systems

Superfluid, Mott-Insulator, and Mass-Density-Wave Phases in the One-Dimensional Extended Bose-Hubbard Model

We use the finite-size density-matrix-renormalization-group (FSDMRG) method to obtain the phase diagram of the one-dimensional ($d = 1$) extended Bose-Hubbard model for density $\rho = 1$ in the $U-V$ plane, where $U$ and $V$ are, respectively, onsite and nearest-neighbor interactions. The phase diagram comprises three phases: Superfluid (SF), Mott Insulator (MI) and Mass Density Wave (MDW). For small values of $U$ and $V$, we get a reentrant SF-MI-SF phase transition. For intermediate values of interactions the SF phase is sandwiched between MI and MDW phases with continuous SF-MI and SF-MDW transitions. We show, by a detailed finite-size scaling analysis, that the MI-SF transition is of Kosterlitz-Thouless (KT) type whereas the MDW-SF transition has both KT and two-dimensional-Ising characters. For large values of $U$ and $V$ we get a direct, first-order, MI-MDW transition. The MI-SF, MDW-SF and MI-MDW phase boundaries join at a bicritical point at ($U, V) = (8.5 \pm 0.05, 4.75 \pm 0.05)$.Comment: 10 pages, 15 figure

Operational Variables for improving industrial wind turbine Yaw Misalignment early fault detection capabilities using data-driven techniques

This is the author accepted manuscript. The final version is available from the Institute of Electrical and Electronics Engineers via the DOI in this recordOffshore wind turbines are complex pieces of engineering and are, generally, exposed to harsh environmental conditions that are making them to susceptible unexpected and potentially catastrophic damage. This results in significant down time, and high maintenance costs. Therefore, early detection of major failures is important to improve availability, boost power production and reduce maintenance costs. This paper proposes a SCADA data based Gaussian Process (GP) (a data-driven, machine learning approach) fault detection algorithm where additional model inputs, called operational variables (pitch angle and rotor speed) are used. Firstly, comparative studies of these operational variables are carried out to establish whether the parameter leads to improved early fault detection capability; it is then used to construct an improved GP fault detection algorithm. The developed model is then validated against existing methods in terms of capability to detect in advance (and by how much) signs of failure with a low false positive rate. Failure due to yaw misalignment results in significant down time and a reduction in power production was found to be a useful case study to demonstrate the effectiveness of the proposed algorithms. Historical SCADA 10-minute data obtained from pitch-regulated turbines were used for models training and validation purposes. Results show that (i) the additional model inputs were able to improve the accuracy of GP power curve models with rotor speed responsible for a significant improvement in performance; (ii) the inclusion of rotor speed enhanced early failure detection without any false positives, in contrast to the other methods investigated.U.S. Department of Commerc

Investigation of complete and incomplete fusion in 7^{7}Li+124^{124}Sn reaction around Coulomb barrier energies

The complete and incomplete fusion cross sections for $^{7}$Li+$^{124}$Sn reaction were measured using online and offline characteristic $\gamma$-ray detection techniques. The complete fusion (CF) cross sections at energies above the Coulomb barrier were found to be suppressed by $\sim$ 26 \% compared to the coupled channel calculations. This suppression observed in complete fusion cross sections is found to be commensurate with the measured total incomplete fusion (ICF) cross sections. There is a distinct feature observed in the ICF cross sections, i.e., $\textit{t}$-capture is found to be dominant than $\alpha$-capture at all the measured energies. A simultaneous explanation of complete, incomplete and total fusion (TF) data was also obtained from the calculations based on Continuum Discretized Coupled Channel method with short range imaginary potentials. The cross section ratios of CF/TF and ICF/TF obtained from the data as well as the calculations showed the dominance of ICF at below barrier energies and CF at above barrier energies.Comment: 9 pages, 8 figure