报告摘要：The interplay between different ordered phases, such as superconducting, charge or spin ordered phases, is of central interest in condensed matter physics. In the early stage of our study on the FeAs-based systems, we identified a spin-density-wave (SDW) order with a stripe (or collinear) type spin structure for the parent compound based on combined experimental and theoretical investigations. Superconductivity emerges when the SDW order was suppressed by carrier doping. However, the collinear spin structure could also be explained from local super-exchange interactions. Currently, there has been much debate about itinerant or localized approaches to the SDW order. In this talk, I shall address this issue based on our recent optical data of single crystal samples. We found that the undoped compounds are quite metallic with relatively high plasma frequencies at high temperature. Upon entering the SDW ordered state, a large part of the Drude component is removed by the gapping of the Fermi surfaces. Meanwhile, the carrier scattering rate is even more dramatically reduced. Those observations favor an itinerant description for the driving mechanism of SDW instability. Nevertheless, the FeAs-based compounds are not simple metals. A high energy gap-like feature is present even above TSDW, which seems to be linked with the antiferromagnetic spin fluctuations. We also studied the superconducting pairing energy gap, and found that the Ferrell-Glover-Tinkham sum rule is satisfied at a low energy scale.
 
报告人简介：王楠林研究员1991年获得中国科学技术大学物理系博士毕业。曾在德国Darmstadt技术大学固体物理研究所（Humboldt Fellow）、日本国际超电导研究中心（ISTEC Fellow and STA Fellow）、加拿大Simon Fraser大学和McMaster大学物理系开展研究工作。1998年入选中科院“百人计划”，2000年获国家杰出青年基金，现为极端条件物理重点实验室主任。主要从事强关联电子体系和相关复杂电子材料红外光学响应及其它物理性质研究。对这些电子系统的电荷动力学性质、准粒子与其它集体激发模式的耦合、非费米液体行为、Kohn-Overhauser机制电荷密度波、电荷有序、轨道简并与轨道有序、绝缘体-金属转变、赝能隙等方面作出了许多有意义的工作。发表论文一百余篇，包括Phys. Rev. Lett.、Nature、Nature Materials 二十篇。