新型准一维铁硫基（FeX4, X = S, Se）超导化合物的制备和高压相变机理研究

The general rule of two-dimensional layered crystal structure for iron-based high-temperature superconducting family has been interrupted by the recent discovery of quasi-one-dimensional iron-sulphur based superconductor which should be the best system for investigating the relationship between crystalline dimensionality with magnetic configuration and superconductivity. Quasi-one-dimensional iron-sulphur based system hosts a large amount of compounds with rich magnetic configurations, which is also helpful for solving key scientific problems like, explorations of new-type high-temperature superconductors and studies of the relationship between superconductivity and magnetism. In this proposal, we will focus on quasi-one-dimensional iron-sulphur based materials, and aim to address the following important scientific issues: (1) By using measurement tools like resistivity, magnetic susceptibility, magnetic torque, and etc., we will study the electrical and magnetic properties for some quasi-one-dimensional iron-sulphur based compounds, and try to make clear of the magnetic configurations and the roles of itinerant and localized spins in the origin of magnetic order; (2) Through application of high pressure and chemical substitution, we will try to tune the ground states and phase transitions for these materials and explore possible new-type superconductors; (3) We will compare the physical properties for both the normal and superconducting states between quasi-one-dimensional and two-dimensional layered iron-based superconductors. And we will emphasis on revealing the correlations among crystalline dimensionality, magnetic configuration, and superconductivity, which will give clues and data support for the understanding of the high-temperature superconductivity mechanism for quasi-one-dimensional iron-based superconductors.

准一维铁硫基超导体的发现打破了铁基高温超导家族具有二维层状晶体结构的一般规律，是研究晶体维度、磁有序结构与超导关系的极佳体系。准一维铁硫化合物中蕴含的丰富磁有序结构，有利于高温超导体的探索，以及超导与磁性关系这些关键科学问题的研究。本项目拟针对新型的准一维铁硫基化合物，着重开展以下几个方面的研究：（1）通过电阻率、磁化率、磁扭矩等实验手段，研究准一维铁硫基化合物在常压下的电、磁性质，探究巡游和局域自旋在该系列化合物磁有序起源上的作用。（2）通过压力和掺杂去调控样品的基态和相变，寻找新型准一维铁基超导体系。（3）对比研究准一维和二维层状铁基超导体在正常态和超导态性质上的异同，从而揭示晶体维度、磁有序结构和超导电性之间的关联，为准一维铁基超导体的高温超导机制问题提供线索和数据支持。

准一维梯子结构BaFe2S3中高压产生的超导电性打破了铁基超导层状晶格的一般规律，为研究晶体维度、磁有序结构、超导电性之间的关联问题提供了一个极好的契机。据报道，该系列化合物中BaFe2S3和BaFe2Se3在磁有序结构上存在着巨大的差异，BaFe2S3表现为stripe型反铁磁序，而BaFe2Se3表现为block型反铁磁序，因此BaFe2(S1-xSex)3中磁有序随掺杂的演化规律，及其与高压超导的关系值得研究。我们合成了BaFe2(S1-xSex)3系列单晶，详细研究了样品的磁性和高压效应，发现了反铁磁序发生突变的临界掺杂量和一个未报导的磁相变，并建立了磁相变的掺杂相图。高压研究表明样品在临界掺杂以下可以观察到超导电性，在超导发生之前样品在低温下出现了类似弱局域效应的绝缘体行为。另外，由于目前存在的准一维铁基化合物较少，我们尝试探索了多个可能的准一维铁基化合物，发现了一个具有一维FeS4链的反铁磁体Ba7Fe6S14，并与BaFe2S3的物性进行了对比。为了对比研究晶体维度对准一维和二维铁基超导体物性的影响规律，我们也对一个层状铁基超导体开展了高压研究，与准一维超导体的绝缘体/半导体行为不同的是，其正常态表现非费米液体行为，并与超导紧密相关；同时，我们对比发现准一维BaFe2S3超导体具有一个相对较小的上临界磁场。上述研究结果一定程度上解决了部分相关领域的争论问题，同时为理解铁基超导体中晶体维度、磁有序、超导之间的关联提供了一些线索和数据支撑。此外，在项目完成的基础上，我们拓展研究了部分重费米化合物、拓扑半金属、非中心对称超导体，等等，也取得了丰富的研究成果。