铁砷基超导薄膜物理性质研究

Pocessing the second highest transition temperature (56 K), longer coherence length, symmetric order parameter (S+- wave), and much smaller anisotropy, the iron pnictide superconductor (IPSC) thin films are much more suitable than cuprate for superconducting electronic applications. Furthermore, it has been observed very recently that the IPSC thin films exibit very unique physical properties in comparison with their bulk counterparts, e.g., non-superconducting parent compound BaFe2As2 becomes superconducting in the form of thin films because of the strain effects. 6-years scientific research practices told us that how to obtain the high-quality thin films is still the top priority on the agenda of the IPSC thin film research. In our own pre-research, we studied on the new methodology for deposition of such hard-to-be-synthesized and easy-to-deteriorate thin films, and built a new equipment named molecular beam assisted pulsed laser deposition (MBAPLD). Based on this newly studied methodology and newly built equipment, we obtained preliminarily the high-quality thin films of the IPSC 122-phase as well as high-quality thin films of the recently found new dilute magnetic semiconductor (DMS) 122-phase. Supported by this NSFC project, we plan to solve the key technical problems the deposition of the IPSC thin film (especially the 1111-phase thin film) are facing, and to study systematically a set of distinguishing characteristics of the thin films, including the strain effects, the interfacial superconductivity, the interlayer coupling in multilayers, the spin injection and the Andreev reflection at the interface of the IPSC/DMS heterostructure, the Josephson effect, and the votex physics such as flux flow and the magnetic phase diagram, etc. We also plan to measure and accumulate a set of intrinsic paramters of the high-quality IPSC thin films, which can serve as the guide for future electronics applications. Publish 8-10 SCI papers.

铁砷基超导薄膜具有第二高的超导转变温度（56 K）、较长的相干长度、对称的超导波函数、较小的各向异性，因此比铜氧化物更适合于超导电子学应用。另外，铁砷基超导薄膜表现出不同于体材料的、特殊的物理性质，例如，不超导的母相在薄膜状态下由于应力效应出现超导。六年来的实践表明，如何制备高质量的薄膜仍然是亟待解决的首要问题。在前期研究工作中，我们研制了新型分子束辅助的脉冲激光沉积装备，初步得到了高质量的铁砷基122相超导薄膜、锌锰基122相稀磁半导体（DMS）薄膜。通过本项目，突破铁砷基超导、尤其是1111相超导薄膜制备的关键技术，开展一些以薄膜为特色的实验，包括应力效应、界面超导电性、多层膜的层间耦合、超导与DMS异质结的超导/铁磁界面的自旋注入和Andreev反射、约瑟夫森效应、磁通钉扎和磁通运动，等。测量一批与薄膜应用密切相关的本征的基本物理参数，为超导电子学应用打下基础。SCI文章8-10篇。

铁基超导体主要包括铁砷基超导体和铁硒基超导体。在我们前期的FeSe、FeTe、Fe(Se,Te)薄膜研究的基础上，本基金项目针对近年来超导研究领域的重点和难点，开展了铁砷基超导体薄膜的制备和物理性质研究。首先，我们优化了薄膜制备工艺，在多种不同的单晶衬底上制备了单相、单向外延的，高质量的Ba(Fe,Co)2As2薄膜。系统开展了不同掺杂条件下的薄膜的电学、磁学性质的研究。其次，在此基础上，重点研究了薄膜厚度低至数个元胞层厚度的超薄膜的薄膜生长特性、晶体结构特征，以及输运性质。观察发现到了一些有趣的物理现象：（1）在SrTiO3（STO）基片上制备了一系列不同厚度的Ba(Fe,Co)2As2超薄膜（3 ~ 50 nm），探讨了界面效应、应力效应对超薄膜的晶体结构和超导电性的影响作用。（2）运用掠入射条件下的X-射线倒空间绘图方法，研究了超薄膜的晶体结构特征，发现厚度低于约12纳米的薄膜受基片的约束表现为赝型生长。（3）赝型生长的超薄膜的超导电性被抑制，并出现铁磁性。（4）系统研究了赝型生长的超薄膜的电、磁输运性质，发现超导电性的消失、铁磁性的出现，伴随着电子载流子的逐步减少以及消失，空穴载流子的出现以及逐步成为主导。最后，开展了与铁基超导体具有相同的四方ThCr2Si2结构、相近的面内晶格常数的新型磁性半导体薄膜的制备和物理性质研究的探索。我们通过不断优化沉积工艺，已经能够制备出高质量的磁性半导体/铁基超导体异质结。综上，本基金项目研究了铁砷基超导体薄膜，尤其是超薄膜的制备和物理性质，发现了在赝型生长的超薄膜中超导电性被抑制以及铁磁性的出现，并进一步发现铁磁性的出现、超导电性的消失伴随着空穴载流子的出现及其所起到的主导作用；研究了磁性半导体/铁基超导体异质结。