Ba1-xKxFe2As2单晶（Tc=38.5 K）磁通钉扎力与钉扎机理研究∗

摘要

铁基超导体是在2008年由Hosono发现的一种新型超导材料,由于其具有上临界场高、各向异性小、临界电流密度大等优点,在世界范围内引起了广泛关注.以Ba1−xKxFe2As2为代表的FeAs-122系超导体具有结构简单、合成温度低、单晶容易制备等优点,是物理学家和材料学家关注的焦点.本工作在获得最优化掺杂的Ba1−xKxFe2As2单晶(Tc =38.5 K)基础上,通过分析其在不同磁场条件下电阻温度变化关系、不同温度条件下的磁滞回线等数据,系统的研究了Ba1−xKxFe2As2单晶磁通钉扎力和磁通钉扎机理.研究发现Ba1−xKxFe2As2超导体具有非常高的磁通钉扎势,其中9 T的外场条件下,其在H//c轴和H//ab面的钉扎势分别为5800 K和8100 K,展示出良好的应用前景；通过进一步分析发现,其磁通钉扎机理应是由于晶格内部的小尺寸缺陷引起的电子平均自由程变化而导致的δl钉扎.%The discovery of superconductivity in iron-based superconductors by Professor Hosono in Japan in 2008 has triggered off an enormous group of researches the world wide. The iron-based superconductors are regarded as another kind of high-Tc superconductors, which possess lots of merits, such as very high upper critical field (Hc2), high critical current density (Jc), and small crystal anisotropy (γ), are promising for high field applications. Ba1−xKxFe2As2, as a typical FeAs-122 superconductor, is focused on by both theoretical physicists and material scientists since its discovery. In this paper, we first successfully fabricate Ba1−xKxFe2As2 single crystal. It has an onset transition temperature up to 38.5 K, while its zero resistivity temperature reaches 37.2 K. Both the R-T and M-T data of it show very sharp superconducting transition, and its critical current density at 5 K and self field is over 106 A·cm−2 and almost field independent. The flux pinning force and the relative pinning mechanisms in Ba1−xKxFe2As2 are discussed by analyzing the data obtained from the measurements about the R-T and M-H under different conditions. Results indicate that the Ba1−xKxFe2As2 superconductors have very strong intrinsic vortex pinning force, and the vortex potentials (U0) under 9 T field are 5800 K and 8100 K for the H//c and H//ab, respectively. Furthermore, the vortex pinning mechanism is also investigated by analyzing the relationship Jc-B. According to the present results, the flux pinning mechanism should beδ(l) pinning because of the change of mean free path for electrons induced by nano-size crystallographic defects in Ba1−xKxFe2As2.