2013年9月24日（周四）下午4點，物理系colloquium：

報告題目： Possible Evidence for a New Paradigm for Higher Tc 報告人： Prof. Shrinivas R. Kulkarni (Caltech，Director of Caltech Optical Observatories) 報告時間： 9月24日（周二）上午10:00 報告地點：理科樓鄭裕彤講堂摘要： We envision that the holistic multidisciplinary enlightened empirical approach may be most fruitful to higher Tc. It depends on imagination, insight, experience and knowledge from different fields plus courage and luck. Two general steps have been adopted: to discover new compounds guided by experience and insight empirically and to realize novel mechanisms inspired by models theoretically. Until now, successes have come almost exclusively from the former. Of the many theoretical mechanisms proposed, few has led to the discovery of a superconductor with a clear enhanced-Tc. Interfacial mechanism has been one such most explored theoretically and experimentally but without clear evidence for an enhanced Tc. Our recent detection of non-bulk superconductivity with an unexpectedly high onset-Tcs up to 49 K in rare-earth-doped CaFe2As2 (Ca122) single crystals [1] and the report by Xue's group of a Tc up to 50's K in unit-cell FeSe epi-films [2], offer an unusual opportunity for the search for a new paradigm for higher Tc. Through systematic compositional, structural and magnetic investigations, we have shown [3] a doping-level independent Tc, the simultaneous appearance of superparamagnetism and superconductivity, the existence of mesoscopic-2D structures in rare-earth doped Ca122 single crystals and thus provided evidence for the possible interface-enhanced Tc in Fe-based superconductors. Such mesoscopic-structures may be attributed to defects as evident from the presence of superparamagnetism [4] detected in these superconducting samples.
Similar observations were also made in the FeSe thin epi-films from Xue. We have observed in the 3-4 unit-cell FeSe-films Meissner effect below 1 Oe with extensive weak-links up to ~ 20 K; unconnected small superconducting patches up to ~ 40 K; and an unusual dispersion of the magnetic susceptibility with frequency up to 70 K. [5] The weak-links and relaxation of the diamagnetic moment observed in the films suggest that the superconducting state of the FeSe films below 20 K may be a glassy state. The experimental results will be presented and the implications discussed.
*The work on FeSe films was carried out in collaboration with Q. K. Xue's group at Tsinghua; on rare-earth doped Ca122 with J. H. Hoffman's group at Harvard.
References
1. B. Lv et al. PNAS USA 108, 15705 (2011).
2. Q. Y. Wang et al. Chin. Phys. Lett. 29, 037402 (2012).
3. F. Y. Wei et al. arXiv 1308.
4. B. Lv et al. arXiv 1308.3129.
5. L. Z. Deng et al. in preparation.