Will higher T c superconductors be useful? Fundamental issues from the real world
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Introduction The search for higher temperature superconductors is a tough business. It has been largely carried out by a small segment of the superconducting materials community whose struggles are legendary. The last time there was a broad, concerted effort to search for higher temperature superconductors was in the decade following the development of the BCS (Bardeen-Cooper-Schrieffer) theory in the late 1950s. Many interesting theoretical suggestions (including new mechanisms) were put forth at that time, which in turn motivated various experimental searches. These ideas had a large and lasting impact on condensed matter physics. Alas, no really higher Tc materials were discovered.1 Recently, however, there has been renewed interest in the search for higher temperature superconductors, as evidenced by the results of two workshops: “The Road to Room Temperature Superconductivity” organized by the U.S. Air Force Office of Scientific Research, and a workshop sponsored by the U.S. Department of Energy that led to the report, “Basic Research Needs in Superconductivity.”2 Based on these meetings, both agencies introduced new programs explicitly aimed (wholly or in part) at the search for higher temperature superconductors. There are several driving motivations for these developments: • The strong sense in the superconducting materials community that the proliferation of higher temperature superconductors in recent decades demonstrates great opportunity and researchers should search broadly.
• The developing understanding of the high-critical-temperature cuprate and Fe-based superconductors, although still incomplete, can usefully guide searches for higher temperature superconductors in related materials. Even for conventional superconductors that depend on the electron-phonon interaction, some would argue that the theory and associated computational tools are well-enough developed that materials by design (or at least specific computationally derived guidance) in the search for higher transition temperatures may be possible. • The simplification of cryogenic refrigeration afforded by high-temperature superconductors significantly increases the likelihood of their adoption in practice. • A documented need (e.g., in the DOE report mentioned previously) for a new high-temperature superconductor if there are to be electric power applications of superconductivity operating above liquid nitrogen temperatures. Much has been said about the first three points. The fourth has received less attention and is the focus of this article. We discuss the ways in which the present high transition temperature cuprate superconductors are inadequate for electric power applications above liquid nitrogen temperatures and why. Going down this path uncovers a seemingly fundamental competition between the material characteristics needed for a high-temperature superconductor to be useful and those believed to be favorable for high transition temperatures in and of themselves.
M.R. Beasley, Geballe Laboratory for Advanced Materials, Stanford Univers
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