Superconductors: Is Japan Ahead?

According to the Office of Technology Assessment (OTA), Japan may pull ahead of the U.S. in the race to bring new, high-temperature superconductors to the marketplace. If the U.S. hopes to compete with Japan, American industry must intensify basic research and work on applications and potential manufacturing processes. In a study, done at the request of several House and Senate committees, the OTA determined that the country lacks a cohesive, focused strategy for developing superconductors and applying them to commercial products. The U.S. may lead on the science front, but this advantage will soon disappear if American companies are not positioned to transform research findings into viable products.
The report is not directly critical of Reagan Administration efforts to promote the field, but indicates that steps taken to date are not adequate. For example, research and development spending on superconductors in Japan in 1988 is virtually equal to the $97 million that U.S. companies will spend, and Japan has 900 people engaged in superconductor research compared with 625 in the U.S.
The OTA suggests that the U.S. drive to understand superconductors and make useful materials is partly flawed because there is no assurance of a long-term commitment by government or industry to fund this research. The OTA report examines three possible strategies that policy-makers may face in trying to shape a sustained and coordinated superconductivity research and development program.
The first is a business-as-usual approach where the Department of Defense pursues processing methods for super conductors to support specialized defense needs. Concurrently, Department of Energy research would be carried out through its ten national laboratories and with industry and the university sector. A second and more aggressive course would increase support for National Science Foundation-funded research and establish a working group on commercialization of high-temperature superconductor research. Industry, the university sector, and various government agencies would be represented on this working group. Finally, the government could establish a federal technology agency or a cabinet-level department of science, which might centralize many fragmented federal science and technology development efforts.
Beyond the federal research sector, there is a need to get industry to conduct more long-term research. Government assistance will be required, according to OTA, but should be less than 50% of any given undertaking. Perhaps most importantly, OTA says that industry must be stimulated to use research results in a timely fashion. Too many firms are taking a wait-and-see approach and could find themselves ill-equipped to compete.

Energy Gaps and Kohn Anomalies in Elemental Superconductors

Experimental data on superconductors can be interpreted using a comprehensive framework based on the Bardeen-Cooper-Schrieffer formulation. Although the discovery of high-temperature superconductivity has challenged this framework, it remains effective in characterizing the physical properties of conventional low-temperature superconductors. However, the transition temperature and the energy gap at the Fermi level, which are two of the most important quantities associated with a superconductor, are still difficult to predict from first principles, because both depend exponentially on material-specific parameters such as the electronic and phononic densities of states and the electron-phonon coupling.
Using resonant spin-echo spectroscopy with neutrons, researchers examined the momentum and temperature dependence of the lifetimes of acoustic phonons in lead (Pb) and niobium (Nb). Lead and niobium are elements with the highest superconducting transition temperatures at 7.2 and 9.3 kelvin, respectively. Because electron-phonon scattering is suppressed for energies lower than the energy gap, the gap can be directly determined in phonon lifetime measurements. The superconducting energy gap in both lead and niobium converges with sharp Kohn anomalies originating from Fermi-surface nesting at low temperatures, implying an unexpected relation between the gap and the geometry of the Fermi surface. For phonon wave vectors connecting nearly parallel segments of the Fermi surface, the electron-phonon scattering probability is enhanced, and Kohn anomalies are often expected to occur. The detection of a low-temperature energy gap coinciding with Kohn anomalies in lead and niobium has not been anticipated by the standard theoretical framework for conventional superconductors.
The results indicate electron many-body correlations beyond the standard theoretical framework for conventional superconductivity. When the temperature is reduced, the spin-echo decay rate, which is proportional to the phonon linewidth and inversely proportional to its lifetime, decreases, indicating that the electron-phonon decay channel is lost in the superconducting state. Dynamical nesting of the Fermi surface is possibly induced by the interplay between superconductivity and spin- or charge-density wave fluctuations.

New Superconductors Come Through

The materials of two new classes of superconductors--one class containing bismuth and the other thallium--lose their resistance to electric current at higher temperatures than any other known substances. This fact has raised hopes that the goal of practical high-temperature superconductivity will soon be within reach. The latest phase in the search for practical high-temperature superconductors has gone on for nearly a 1 1/2 yr., ever since the discovery by Paul Chu at the University of Houston of an yttrium-barium-copper-oxygen mixture that lost all electrical resistance at 90 deg K. But researchers discovered that the high-temperature superconductors had material problems. The bismuth and thallium compounds become superconducting at higher temperatures than the earlier substances and there are signs that they would be easier to work with.
Sandia National Laboratories in Albuquerque, New Mexico, has made a thin film from a thallium-based superconductor that carries much more electric current than similar films made from earlier superconductors. Thin films, which are used in electronics components, are likely to be the first practical application of high-temperature superconductors. The thallium films are impressive because the strength of the links between the crystalline grains can apparently be made weaker or stronger by changing the processing technique, which could allow for tailor-made materials for a desired critical current density.
A group at Stanford University has succeeded in making an exceptionally high-quality fiber out of a bismuth-based superconductor by using a technique called laser-heated pedestal growth. By changing various conditions of the process--such as the composition of the source material, the speed of withdrawing the seed, or the atmosphere in the growth chamber--the group was able to control the composition of the resulting fiber. One of the impressive things about the process is that the fiber is superconducting as it crystallizes, and needs no further processing. Other techniques for making superconducting wires require an extra step to change the material into a superconducting form.