Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan.
Institute of Physics, Academia Sinica, Taipei 11529, Taiwan.
Proc Natl Acad Sci U S A. 2019 Jan 22;116(4):1104-1109. doi: 10.1073/pnas.1815237116. Epub 2019 Jan 7.
The exact superconducting phase of K Fe Se has so far not been conclusively decided since its discovery due to its intrinsic multiphase in early material. In an attempt to resolve this mystery, we have carried out systematic structural studies on a set of well-controlled samples with exact chemical stoichiometry K Fe Se ( = 0-0.3) that are heat-treated at different temperatures. Using high-resolution synchrotron radiation X-ray diffraction, our investigations have determined the superconducting transition by focusing on the detailed temperature evolution of the crystalline phases. Our results show that superconductivity appears only in those samples that have been treated at high-enough temperature and then quenched to room temperature. The volume fraction of superconducting transition strongly depends on the annealing temperature used. The most striking result is the observation of a clear contrast in crystalline phase between the nonsuperconducting parent compound KFeSe and the superconducting K Fe Se samples. The X-ray diffraction patterned can be well indexed to the phase with I4/m symmetry in all temperatures investigated. However, we need two phases with similar I4/m symmetry but different parameters to best fit the data at a temperature below the Fe vacancy order temperature. The results strongly suggest that superconductivity in K Fe Se critically depends on the occupation of Fe atoms on the originally empty 4d site.
由于其早期材料的固有多相性，KFeSe 的精确超导相迄今为止尚未得到明确决定。为了解决这个谜团，我们对一组具有精确化学计量比 KFeSe（=0-0.3）的、经过不同温度热处理的、得到良好控制的样品进行了系统的结构研究。利用高分辨率同步辐射 X 射线衍射，我们通过聚焦于晶体相的详细温度演化，确定了超导转变。我们的研究结果表明，超导性仅出现在那些经过足够高温度处理并随后淬火至室温的样品中。超导转变的体积分数强烈依赖于所使用的退火温度。最引人注目的结果是观察到非超导母体化合物 KFeSe 和超导 KFeSe 样品之间在晶体相上存在明显的对比。在所有研究的温度下，X 射线衍射模式都可以很好地用 I4/m 对称相来指数化。然而，我们需要两个具有相似 I4/m 对称但不同参数的相来最好地拟合低于 Fe 空位有序温度下的数据。结果强烈表明，KFeSe 中的超导性与 Fe 原子在原始空的 4d 位上的占据密切相关。
