铁硒超导体单晶制备和物性研究

Although there are various crystal structures in the family of iron-based superconductors, most of them exhibit similar electronic phase diagram, i.e., with chemical doping or increasing external pressure, the long-range antiferromagnetic as well as structural phase transitions appearing in mother compounds are suppressed gradually, accompanying with the emergence of superconductivity. For FeSex, however, it shows the entirely different characteristics which is opposite to above universal phase diagram. Although there is a structural phase transition in FeSex at ambient pressure, but the long-range antiferromagnetic transition is absent. In contrast, superconductivity appears at low temperature. On the other hand, superconducting and antiferromagnetic transition is enhanced with pressure. Because of these anomalies, it is certainly worth studying the physical properties of FeSex in detail. However, the difficulty of growing (00l)-oriented FeSe single crystals of tetragonal phase severely impedes research progress in related area. Based on the previous study of iron-chalcogenide superconductors, we plan to grow high quality and large size (00l)-oriented FeSe single crystals of tetragonal phase using halide flux and temperature gradient furnace. Then, we will study the influences of changing the Fe content, doping 3d transition metals as well as exerting external pressure on the evolution of physical properties of FeSex. Through current study, we try to reveal the intrinsic electronic phase diagram and the relationship of structure, magnetism and superconductivity in FeSex. Furthermore, these studies are also expected to shed light not only on the superconducting mechanisms of FeSex as well as other iron-based superconductors but also on the electron correlation effects in these materials.

已有研究表明，大多数具有不同的晶体结构铁基超导材料都展现出了相似的电子相图，即随着化学掺杂或者压力的增加，母体中反铁磁长程序和结构相变逐渐被压制，超导电性开始出现。但是FeSex却展现出了与这一普适相图完全不同的特点：FeSex在常压下具有结构相变但却没有反铁磁长程序而是在低温出现了超导电性；另一方面，超导和反铁磁转变温度同时随着压力的增加而升高。这些反常都促使人们对其结构和物性进行深入地研究。而难于获得高质量四方结构(00l)取向FeSex单晶严重阻碍了研究的进展。在前期对FeSe基超导体研究的基础上，我们计划采用卤化物助熔剂结合温度梯度场的方法制备高质量、大尺寸四方结构(00l)取向FeSex单晶，并系统地研究Fe含量变化、3d过渡金属元素掺杂和压力对其物性的影响。以揭示其本征的电子相图以及结构、磁性与超导电性之间的关系，促进对FeSex和其他铁基超导体超导机制和电子关联效应的认识。

对铁硒基超导材料的研究是当前铁基超导体研究领域的前沿热点之一。虽然铁基超导材料表现出了相似的电子相图，但是FeSex却表现出了不同于多数铁基超导体的特征。为了深入地理解铁硒基超导体的这些特征及其超导机制，在获得高质量FeSex单晶的基础上，我们研究了高压下的FeSex的磁结构，重电子掺杂FeSe单晶的物性以及3d过渡金属元素掺杂的FeSe单晶物性。首先使用卤化物助熔剂结合温度梯度场的方法制备获得了高质量晶体。其次通过核磁共振研究了压力诱导的FeSex单晶的磁结构，并发现高压下向列序、磁有序和超导之间的相互关系。第三，通过低温液氨的方法制备出了一系列碱金属与氨共插层的重电子掺杂的FeSe基超导体，并对其常压和高压下的物性开展了详细的研究。第四，通过Cu原子对FeSe中Fe原子的替代，得到一系列不同掺杂浓度的FeSe单晶，并对其物性和相图进行了研究。在该项目执行期间，发表研究论文43篇。这些结果不仅有助于我们理解FeSe基超导体中的结构、磁性、电子结构与超导电性之间的关系，促进对于FeSe基以及其他铁基超导体超导机制和电子关联效应的认识，而且也为探索新型超导材料提供了研究思路。