铁基超导体角分辨光电子能谱及电子拉曼光谱的研究

Fe-based superconductivity is possibly the first important topic in modern condensed matter physics for which China is widely recognized abroad as the main World leader. This field is of strategic importance since the understanding of this new family of high-temperature superconductors is believed to lead to the synthesis of compounds with higher critical temperature that will pave the way for a revolution in electronic devices that may equal the revolution once introduced by semi-conductors. The electronic properties of the Fe-based superconductors are dominated by the electronic states in the vicinity of the Fermi level. These states are not localized and disperse in the momentum space. To characterize them completely, it is thus necessary to use probes sensitive to the momentum space. With its capability to resolve the electronic structure of materials in the momentum space, Angle-Resolved Photoemission Spectroscopy (ARPES) is a powerful toll that allows direct momentum characterization of such dispersive electronic states. In particular, it is suitable to determine the Fermi surface topology of materials and the momentum profile of superconducting gaps, which characterize the superconducting pairing mechanism. However, ARPES is mainly a surface sensitive technique, and it is often necessary to support ARPES data with other experimental techniques. By taking advantage of optical selection rules and the polarization flexibility of lasers, electronic Raman scattering allows one to measure superconducting gaps in the bulk of materials with some momentum information. It is thus well suited to support ARPES data, in addition of providing precious information on the lattice itself...In this project, we propose to combine ARPES and Raman scattering to characterize the Fe-based superconductors, with emphasis on their superconducting gaps, whose momentum dependence is directly shaped by the superconducting pairing mechanism. After developing a systematic procedure to correlate features observed from both techniques, we will apply these two spectroscopic tools to the investigation of possible nodes in the superconducting gap functions of Fe-based superconductors. Our research is believed to reveal precious information necessary to elucidate the superconducting pairing mechanism in these materials.

铁基超导体研究是现代凝聚态物理领域的重要课题，对其深入了解可以带来电子器件的革命。铁基超导体令人感兴趣的电子性质在于它们的电子能带结构，特别是费米能级附近的电子态，这些电子态是非局域化的，因此迫切需要对铁基超导体动量空间的电子结构开展全面的研究。尽管角分辨光电子能谱(ARPES)是一种能够直接获取费米面拓扑结构，超导能隙大小和对称性，以及k空间分辨的能带结构的有力工具，然而，ARPES是一种表面敏感的分析技术，通常需要其他实验技术得到的结果作为参照。我们拟结合ARPES和拉曼散射(Raman)两种技术来表征铁基半导体，着重表征超导能隙，超导能隙能直接反映配对机制。得益于Raman光学选择定则和激光的偏振可调节性，我们可以获得带有部分动量信息的超导材料体相的超导能隙性质，从而为ARPES数据提供很好的佐证。我们深信获得的大量的数据能够进一步探讨和求证这些材料中的超导电子配对机制。

此项目中，我们利用角分辨光电子能谱（ARPES）和拉曼散射来研究铁基超导体，特别是这些化合物的超导能隙以及电子关联可能起到的作用。. 根据我们获得的可靠的ARPES数据表明，短程相互作用导致在绝对动量空间的能隙函数，并负责超导电子的配对。我们发现在个别情况下的节点行为是由作用于电子结构的低能部分的竞争序引起的。我们还表征了超导能隙相位的符号。. 通过比较ARPES测量的电子结构与理论计算，我们可以对电子关联的作用有一个好的理解。研究表明，在存在大的洪特耦合的情况下，通过对3d壳层的电子填充可以调节电子的关联作用；同时，超交换，而不是正常交换作用，起着重要作用。. 拉曼散射有助于研究铁基超导体的向列相和向列相的涨落。在提交本提案后，这个课题成为一个很热的的主题。我们的研究结果指出样品应力对这个问题的重要性，这些结果质疑了基于其他技术的实验结果。. 除了铁基超导体，我们也积极研究了其他量子材料尤其是Weyl半金属。Weyl半金属的发现被广泛地认为是2015年物理学的世界重大突破之一。除了观察这些体系被Weyl性质调制的表面态，我们课题组第一次在体电子结构中直接观察到Weyl节点，这为表面态的自旋结构的确定了方向；我们还用拉曼散射光谱表征了Weyl半金属的晶格动力学性质。 . 在过去的几年里，我们发表和本项目相关的47篇文章包括1篇Nature Physics, 1篇 Nature Communications, 3篇Physical Review Letters 和 4 篇Physical Review X。这些文章被引用了700多次。从2014年到2016年，我受邀在4个有影响力的国际会议和1个国内会议上发表这些工作。毫无疑问，我们的研究工作将对铁基超导体和Weyl半金属领域保持深刻的、持久的影响。