Anisotropic Strong Coupling Calculation of the Local Electromagnetic Response of High-Tc Superconductors

The electromagnetic response of the CuO2-planes is calculated within a strong coupling theory using model tight binding bands and momentum dependent pairing interactions representing spin fluctuations and phonon exchange. The superconducting state resulting from these interactions has d-wave symmetry. With phonon exchange included the order parameter amplitude grows rapidly below Tc at elevated frequencies which leads to improved agreement with the observed temperature dependence of the penetration depth. Good agreement between theory and experiment can only be achieved if it is assumed that the strength of the quasiparticle interaction decreases with temperature in the superconducting state. The amount of this reduction depends sensitively on the momentum dependence of the interactions, the energy dispersion and the position of the Fermi line.Comment: 3 pages, LaTex, to be published in J.Phys.Chem.Solids, 1997, SNS Conf., Cape Co

Funktionale organische Dünnfilme

Organic thin films are used in many technological and engineering applications nowadays. They find use as coatings, sensors, detectors, as matrix materials in nanocomposites, as self-assembled monolayers for surface functionalization, as low-k dielectrics in integrated circuits and in advanced organic electronic applications like organic light emitting diodes, organic field effect transistors and organic photovoltaics (esp. organic solar cells) and many other applications. OLED displays are now commonly implemented in portable multimedia products like mp3-players. And just recently, self-cleaning ultrahydrophobic and ultrahydrophilic surfaces found first applications. Organic thin films can exhibit a large variety of mechanical, electronical and chemical properties depending on their composition and structure. The applications mentioned above use very different properties of these organic thin films. For example, in magnetic nanocomposites for high frequency applications the electric insulating properties of Teflon thin films are used to reduce losses due to eddy currents, while electronic applications utilize very different properties, like a low dielectric number in low-k dielectrics or semiconduction in organic electronics. This work shows how different properties of even the same organic thin film can be used to create different functionalities that alter the macroscopic properties of the coated materials. Due to the huge variety of organic thin films it is impossible to describe their properties and functionalities comprehensively within the framework of a PhD thesis. In order to address the topic, examples from various fields of actual research in materials science were chosen, including organic semiconductors, dielectrics, barrier layers and hydrophobic materials. For organic semiconductors, diffusion of noble metal atoms is examined and correlated with the changes in the electronic properties of the interface between metal contact and the organic semiconductor. Furthermore Teflon-based thin films were tested for their ability to act as capping layer for protection of the organic semiconductor against aging and to control the threshold voltage of organic field effect transistors. Teflon-based thin films were also used to create ultrahydrophobic surfaces and the reflection of water jets on these surfaces was studied for the first time

Asymmetric double-well potential for single atom interferometry

We consider the evolution of a single-atom wavefunction in a time-dependent double-well interferometer in the presence of a spatially asymmetric potential. We examine a case where a single trapping potential is split into an asymmetric double well and then recombined again. The interferometer involves a measurement of the first excited state population as a sensitive measure of the asymmetric potential. Based on a two-mode approximation a Bloch vector model provides a simple and satisfactory description of the dynamical evolution. We discuss the roles of adiabaticity and asymmetry in the double-well interferometer. The Bloch model allows us to account for the effects of asymmetry on the excited state population throughout the interferometric process and to choose the appropriate splitting, holding and recombination periods in order to maximize the output signal. We also compare the outcomes of the Bloch vector model with the results of numerical simulations of the multi-state time-dependent Schroedinger equation.Comment: 9 pages, 6 figure

Ultrasonic attenuation in magnetic fields for superconducting states with line nodes in Sr2RuO4

We calculate the ultrasonic attenuation in magnetic fields for superconducting states with line nodes vertical or horizontal relative to the RuO_2 planes. This theory, which is valid for fields near Hc2 and not too low temperatures, takes into account the effects of supercurrent flow and Andreev scattering by the Abrikosov vortex lattice. For rotating in-plane field H(theta) the attenuation alpha(theta)exhibits variations of fourfold symmetry in the rotation angle theta. In the case of vertical nodes, the transverse T100 sound mode yields the weakest(linear)H and T dependence of alpha, while the longitudinal L100 mode yields a stronger (quadratic) H and T dependence. This is in strong contrast to the case of horizontal line nodes where alpha is the same for the T100 and L100 modes (apart from a shift of pi/4 in field direction) and is roughly a quadratic function of H and T. Thus we conclude that measurements of alpha in in-plane magnetic fields for different in-plane sound modes may be an important tool for probing the nodal structure of the gap in Sr_2RuO_4.Comment: 5 pages, 6 figures, replaced in non-preprint form, to appear in Phys. Rev.