Hartmut Bielefeldt

Enhanced supercurrent density in polycrystalline YBa2Cu3O7-δ at 77 K from calcium doping of grain boundaries

With the discovery of high-temperature superconductivity, it seemed that the vision of superconducting power cables operating at the boiling point of liquid nitrogen (77 K) was close to realization. But it was soon found that the critical current density Jc of the supercurrents that can pass through these polycrystalline materials without destroying superconductivity is remarkably small. In many materials, Jc is suppressed at grain boundaries, by phenomena such as interface charging and bending of the electronic band structure. Partial replacement (‘doping’) of the yttrium in YBa2Cu3O7-δ with calcium has been used to increase grain-boundary Jc values substantially, but only at temperatures much lower than 77 K (ref. 9). Here we show that preferentially overdoping the grain boundaries, relative to the grains themselves, yields values of J c at 77 K that far exceed previously published values. Our results indicate that grain-boundary doping is a viable approach for producing a practical, cost-effective superconducting power cable operating at liquid-nitrogen temperatures.

Calculation of the optimal imaging parameters for frequency modulation atomic force microscopy

True atomic resolution of conductors and insulators is now routinely obtained in vacuum by frequency modulation atomic force microscopy. So far, the imaging parameters (i.e., eigenfrequency, stiffness and oscillation amplitude of the cantilever, frequency shift) which result in optimal spatial resolution for a given cantilever and sample have been found empirically. Here, we calculate the optimal set of parameters from first principles as a function of the tip–sample system. The result shows that the either the acquisition rate or the signal-to-noise ratio could be increased by up to two orders of magnitude by using stiffer cantilevers and smaller amplitudes than are in use today.

Design and realization of an all d-wave dc π-superconducting quantum interference device

The predominantly dx2−y2-pairing symmetry in most high-Tc superconductors provides the opportunity to fabricate Josephson junction circuits in which part of the junctions are biased by a phase difference of the superconducting order parameter of π. We present fabrication and measurements of an all high-Tc dc superconducting quantum interference device (dc SQUID) realized with thin-film technology, of which the Josephson junctions consist of one standard junction and one junction with a π-phase shift. The characteristics of the π-SQUID are compared with the properties of a standard high-Tc SQUID.

Imaging of atomic orbitals with the Atomic Force Microscope - experiments and simulations

Atomic force microscopy (AFM) is a mechanical profiling technique that allows to image surfaces with atomic resolution. Recent progress in reducing the noise of this technique has led to a resolution level where previously undetectable symmetries of the images of single atoms are observed. These symmetries are related to the nature of the interatomic forces. The Si(111)-(7 × 7) surface is studied by AFM with various tips and AFM images are simulated with chemical and electrostatic model forces. The calculation of images from the tip-sample forces is explained in detail and the implications of the imaging parameters are discussed. Because the structure of the Si(111)-(7 × 7) surface is known very well, the shape of the adatom images is used to determine the tip structure. The observability of atomic orbitals by AFM and scanning tunneling microscopy is discussed.

A simplified but intuitive analytical model for intermittent-contact-mode force microscopy based on Hertzian mechanics

The forces acting on the substrate in intermittent-contact-mode (IC mode, tapping mode) atomic force microscopy are not accessible to a direct measurement. For an estimation of these forces, a simple analytical model is developed by considering only the shift of the cantilever resonance frequency caused by Hertzian (contact) forces. Based on the relationship between frequency shift and tip–sample force for large-amplitude frequency-modulation atomic force microscopy, amplitude and phase versus distance curves are calculated for the intermittent contact mode, and the forces on the substrate are calculated. The results show a qualitative agreement with numerical calculations, yielding typical maximal forces of 50–150 nN. When working above the unperturbed resonance, forces are found to be significantly larger than below the resonance.

Doping induced enhancement of the critical currents of grain boundaries in high-Tc superconductors

Abstract Appropriate doping of the grains provides a means to optimize the transport properties of grain boundaries in high- T c superconductors. This is demonstrated for the exemplary case of grain boundaries in bicrystalline Ca- and Co-doped YBa 2 Cu 3 O 7−δ films. By Ca-doping the critical current density is strongly increased and the normal state resistivity significantly reduced as compared to the values obtained for equivalent junctions in undoped films.

Tailoring of high-Tc Josephson junctions by doping their electrodes

Appropriate doping of the electrodes of high-Tc Josephson junctions provides a means to systematically adjust the junctions’ electronic properties. This is demonstrated for the exemplary case of grain boundary junctions in bicrystalline Ca-doped YBa2Cu3O7−δ films. It is found that the critical current density is strongly increased and the normal state resistivity significantly reduced in comparison with the values obtained for equivalent junctions in undoped films.

Large grain boundary area superconductors

Abstract:For many applications of polycrystalline high-Tc superconductors the small critical currents of the grain boundaries pose a severe problem. To solve this problem, we derive novel designs for the microstructure of coated conductors.

Doping-induced enhancement of grain boundary critical currents

The critical-current density of grain boundaries in high-T/sub c/ superconductors was enhanced to values exceeding the previously known limits both at 4.2 K and at 77 K. Noting the importance of space-charge layers and of the d(x/sup 2/-y/sup 2/)-wave pairing symmetry on grain-boundary transport, we have established a model that provides a comprehensive description of the grain boundaries and proposes ways for their improvement, such as overdoping of the grains and of their boundaries. Exploring as example the effects of overdoping of YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// with Ca, we enhanced significantly the critical current densities and decreased the normal-state resistivities of grain boundaries to unprecedented values. By introducing doping heterostructures to overdope grain boundaries selectively over a few nanometers by benefiting from grain boundary diffusion, the enhancement of the critical-current density is achieved at all temperatures up to T/sub c/. At 77 K, critical current densities are obtained which before had been found only at 4.2 K. This concept is proposed as a practical and cost-effective route to enhance the performance of high-T/sub c/ coated conductors fabricated by ion beam assisted deposition (IBAD) or by the rolling assisted biaxially aligned substrate process (RABITS).

Coated conductors containing grains with big aspect ratios

It is shown that the critical currents of high-Tc superconducting tapes fabricated by the coated conductor technologies are enhanced considerably if grain arrangements with large effective grain boundary areas are used. Increasing the aspect ratios of the grains reduces the deleterious effects of the grain boundaries. A practical road to competitive high-Tc cables is proposed.