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Thursday, September 18

What Relevance Do the Oxypnictides Have to
Future Large-Scale Superconducting Applications?

D. C. Larbalestier, Florida State University, Tallahassee
Materials Science and Technology Division Seminar
3:00 PM, Central Laboratory and Office Building (8600),
Iran Thomas Auditorium (Room A-103)
Contact: Rongying Jin (jinr@ornl.gov), 865.574.5495

Abstract

A great beauty of working in superconductivity is that discoveries of new superconducting materials systems occur quite frequently and often make considerable impact on applications. A major part of applications interest for many years has been composed of work on the cuprates and MgB2. Since the remarkable discovery of superconductivity in the La-Fe arsenides in the group of Professor Hosono, the Tc of the RE-arsenides has doubled to ~55K and it is now well established that Hc2 even in the least favorable orientation is clearly well over 50 T. Our own interests in the materials were stimulated by samples from the group of David Mandrus and Rongying Jin at ORNL in which we immediately found strong evidence for anomalous upturn in the Hc2 that was suggestive of two band superconductivity in the system. This two band effect was first seen in MgB2 but is much stronger in the La, Nd, and Sm and double layer Ba arsenides that we have examined. In this way Hc2 becomes almost twice as large as would be predicted by one-band s-wave predictions of WHH. Of enormous importance for applications is the electronic anisotropy g = (mc/mab)0.5. Since, scientifically, HTS applications are controlled by the need for g not to be too high and by disruption of superconductivity at grain boundaries, it was natural for us to investigate these two parameters. For the oxypnictides there appears to be good news on both counts. Hc2 anisotropy measurements show that g is smaller than or similar to YBa2Cu3O7-x (5-7) in several of the compounds thanks to Sm and Nd arsenide samples from the groups of Zhongxian Zhao and Haihu Wen in Beijing. We find that transport across grain boundaries in present polycrystals is obstructed. However, careful evaluation of the inter- and intra-grain components of the current density leads to the conclusion that a dominant obstruction to current flow in the far-from-single phase state of present polycrystalline samples, especially due to wetting grain boundary phase, rather than to the major intrinsic obstruction seen in cuprate grain boundaries. All in all the oxypnictides seem to offer much of interest to the applications community with respect to understanding the key roles that anisotropy and carrier density play in controlling the vortex phase diagram of complex layered superconductors. If, as well, Tc can be raised then the much discussed and wished-for combination of MgB2 and YBa2Cu3O7-x might emerge from this serendipitous discovery.