[en] The BCS-model has proved to be a powerful approach for describing key aspects of low- and high-temperature superconductivity as well as of superfluidity. The theory is brilliant in its success and has been understood in fantastic detail. Emphasizing the power of BCS, we shall show that the BCS-theory predicts the electron systems of the high-temperature superconductors to display extraordinary and surprising collective phenomena which are as yet unknown in superconductivity. We will also discuss current and possibly future trends in superconductivity which push the physics far beyond the BCS-limits and open vast, new ranges. The work to be presented was performed together with Y. S. Barash, G. Hammerl, F. Loder, A. Kampf, T. Kopp, and C. W. Schneider. (orig.)

[en] It is shown that a boundary region with a depressed order parameter is intrinsically created at the interface between two d_x²-y² wave superconductors, which form a contact at a tilt angle. The contact is a weak link or a Josephson junction, with properties determined by the misalignment angle. Complementing existing theories that discuss the influence of d_x²-y² symmetry on the charge transfer across junctions, the ideas presented provide insights into the origin of the junction formed in such configurations. The mechanism discussed is of relevance for the understanding of the electronic properties of grain boundaries in superconductors, including the angular dependence of the critical current density. (orig.)

[en] An introduction to critical currents in high-T_c superconductors is given which includes - Measurement techniques; - Limiting mechanisms; - Overview of the state of the art, strategies to raise J_c. (orig.)

[en] The distribution of the maximum Josephson current in two-dimensional tunnel junctions is investigated both theoretically and experimentally. Measurements of the Josephson current density are performed using low-temperature scanning electron microscopy. Josephson current densities are calculated to means of a numerical iteration procedure, which is presented in detail. These theoretical calculations are found to be essential for the correct interpretation of the measured Josephson current densities. Combining these experimental and theoretical methods, we investigate trapped transverse vortices. Further, the strength of the elementary pinning centers acting upon the trapped flux quanta is measured by recording the shift of the vortex position as a function of an applied Lorentz force. Ringlike vortices are also observed, and the influence of different current feed configurations on the Josephson current distribution is investigated

[en] A variety of potential devices based on the electric field effect of high-T_c superconductors will be presented, and their fundamental limitations will be discussed. (orig.)

[en] It has been shown that the symmetry of the order parameter in the high-T_c cuprates is dominated by a d_x²-y²-component. In this contribution, the implications of the unconventional symmetry on devices and on large-scale applications of high-T_c superconductors are discussed. (author)

[en] Fabrication and transport properties of three-terminal Josephson junctions are described. The devices are excellent Josephson junctions which can be switched and tuned with an applied gate voltage

[en] The normal state transport properties of grain boundaries in high-T_c superconductors of central importance for a large number of electronic applications, still lack a thorough explanation. In this contribution we consider the implications of grain boundaries on the electronic band structures of high-T_c cuprates. It is shown that the normal state properties observed may be caused by depletion layers formed at the interfaces. (orig.)

[en] The magnetic control of the filamentary current flow during avalanche breakdown in a semiconductor at low temperatures represents an interesting new principle of a magnetic field-effect transistor. The power losses in such a device are minimized by using superconducting lines for the generation of the magnetic control field and for the interconnections. For such a hybrid concept, implementing semiconducting and superconducting components, the important performance characteristics are evaluated

[en] Is superconductivity necessary for zero-resistance currents? We investigate the possibility of designing wires with non-superconducting, but nevertheless dissipation-free current flow at room temperature. Guided by a set of constraints for states carrying currents in thermodynamic equilibrium, we suggest using quantum mechanical interference and collapse processes to produce devices that yield the desired function. Two proposals for such devices are presented as examples and their function is discussed also with respect to the second law of thermodynamics.