新BiS超导体的探索研究

Through long-term effort, one has learned two basic rational routes for searching new superconducting materials. The first one is to carry out chemical doping in a so-called parent compound which is potentially superconducting. The other is to synthesize a new compound that contains low-dimensional superconductive layers via structural design. Our interests are mainly focused on the BiS2 layered superconducting compounds, which contain the EuF or ThN layer as the carrier stocking layer. We have synthesized two novel self-doping BiS2-based superconductors, EuBiS2F and Eu3Bi2S4F4. I plan to explore new superconductors by doping rare-earth elements into these two compounds and study the relationship between carrier doping level with Tc, Eu valence and magnetism. The discovery of new iron based superconductor, ThFeAsN, provides a novel carrier stocking layer. We try to design and synthesize two BiS2 superconductors ThBiS2N and Th3Bi2S4N4 which share the same structure with EuBiS2F and Eu3Bi2S4F4, respectively.

在合适的母体化合物中进行掺杂和以低维超导层辅以载流子层构造新的化合物是探索新超导体的两种基本方法。我们主要的研究兴趣是以这两种手段探索分别以EuF层和ThN层为载流子层的新BiS超导体。我们成功合成了EuBiS2F和Eu3F4Bi2S4两个自掺杂超导体，本项目拟在前期工作的基础上通过不同的掺杂手段提高载流子浓度，研究其超导转变温度以及Eu的磁性之间的关系并绘制相关相图。ThFeAsN超导样品的发现为我们提供了一个新的载流子层。我们拟尝试合成分别与EuBiS2F和Eu3Bi2S4F4相同结构ThBiS2N和Th3Bi2S4N4样品，为BiS超导家族增加新的成员。

铜氧超导和铁基超导之后BiS超导体系的发现引起了广泛的关注，其母体是大多是类似铜氧超导体的绝缘体或半导体，其超导性质和铁基超导又有很多相似之处。新的BiS超导层的发现为研究铜氧超导和铁基超导架起了一座桥梁。我们合成了EuBiS2F（Eu1121）和Eu3Bi4S2F4两个新超导体。区别于其他BiS超导体母体的半导体行为，这两个样品由于Eu的混合价态，正常态行为表现为本征的金属导电性，并在0.3和1.5 K出现了超导。但是由于由混合价态引入的电子有限，其超导转变温度过低。我们在此项目中对Eu1121体系进行了La系稀土的系列掺杂研究。由于La系稀土对Eu的替换，成功提高了载流子浓度并提高了超导转变温度。由于杂质散射的原因正常态行为变为半导体行为，CDW异常也被压制，低温下出现了4f电子的磁性和超导的共存。除了这些成功的尝试，本项目还对ThN层为载流子层的BiS超导体进行了尝试，遗憾的是ThBiS2N（Eu1121）和Th3Bi4S2N4都没有成功合成。