Control of Vortex Pinning in YBCO Thin Films by Incorporating APCs Through Surface Modified Target Approach

The transport of electrical currents in superconductors with much higher efficiency and without any dissipation is considered as the “energy superhighway.” After the discovery of YBa2Cu3O7-δ (YBCO), a high temperature superconductor (HTS), the prospect of using superconducting materials in practical technological applications became very prominent. With much higher Tc (～ 92 K) than conventional low temperature superconductors (LTS), YBCO was considered very promising due to cheaper cooling requirements. The evolution of critical current density (Jc), however, took long time for the material to become useful in practical applications. This was achieved through continuous modification of the processing parameters, deposition of highly oriented thin films on single crystal and buffered metallic substrates and use of artificial pinning centers (APCs) for strong pinning of quantized magnetic vortices. Pulsed laser deposition (PLD) technique is one of the most common and highly efficient techniques for depositing highly oriented YBCO thin films on single crystal and buffered metallic substrates. Using PLD technique, APCs are incorporated into YBCO thin films by many methods which include premixed target method, alternating target method and surface modified target method. In this chapter, the use of surface modified target method to introduce different kinds of APCs into YBCO thin films is presented. These APCs are effective in improving the vortex pinning properties of YBCO thin films for different range of applied magnetic field and its orientation depending upon their geometry and density.Chapter

The Role of Surface Chemistry in Adhesion and Wetting of Gecko Toe Pads

An array of micron-sized setal hairs offers geckos a unique ability to walk on vertical surfaces using van der Waals interactions. Although many studies have focused on the role of surface morphology of the hairs, very little is known about the role of surface chemistry on wetting and adhesion. We expect that both surface chemistry and morphology are important, not only to achieve optimum dry adhesion but also for increased efficiency in self-cleaning of water and adhesion under wet conditions. Here, we used a plasma-based vapor deposition process to coat the hairy patterns on gecko toe pad sheds with polar and non-polar coatings without significantly perturbing the setal morphology. By a comparison of wetting across treatments, we show that the intrinsic surface of gecko setae has a water contact angle between 70–90°. As expected, under wet conditions, adhesion on a hydrophilic surface (glass) was lower than that on a hydrophobic surface (alkyl-silane monolayer on glass). Surprisingly under wet and dry conditions the adhesion was comparable on the hydrophobic surface, independent of the surface chemistry of the setal hairs. This work highlights the need to utilize morphology and surface chemistry in developing successful synthetic adhesives with desirable adhesion and self-cleaning properties

Communication over Multipath Fading Channels: A Time-Frequency Perspective

Dynamics of multipath fading have a major effect on the performance of mobile wireless communication systems. The inherently time-varying nature of the mobile wireless channel makes nonstationary signal processing techniques particularly attractive for system design. Time-frequency representations are powerful tools for timevarying signal processing, and in this paper, we present a time-frequency view of wireless communication over multipath channels. Our discussion is anchored on a fundamental finite-dimensional time-frequency representation of the wireless channel that facilitates diversity signaling by exploiting multipath and Doppler shifts. The substantially higher level of diversity afforded by time-frequency processing over conventional techniques translates into significant gains in virtually all aspects of system performance. We illustrate the utility of the time-frequency framework via novel signaling and receiver structures, and multiuser acquisition and interfere..