Rb $_{x}$ Fe $_{2-y}$ Se $_{2}$晶体退火过程中微观结构演变的高分辨表征
High resolution characterisation of microstructural evolution in Rb$_{x}$Fe$_{2-y}$Se$_{2}$ crystals on annealing
摘要：
已知相分离的A $_x$ Fe $_{2-y}$ Se $_2$化合物的超导和磁性取决于生长后的热处理和冷却曲线。本文着重研究了退火过程中微观结构的演变，以及退火对Rb $_x$ Fe $_2-y$ Se $_2$晶体超导性能的影响。我们发现，生长的晶体中的少数相具有增加的晶胞各向异性（c/a比），减少Rb含量和增加Fe含量相比，矩阵。微观结构相当复杂，具有沿{113}惯面排列的两相介观板状特征。少数相在{113}平面上强烈地刻面，我们已经表明通过使作为相变的结果引入的体积应变能最小化来驱动。在488 K退火导致粗化的介观板状特征和第三个不同的相的形成。晶体中少数相的结构和化学性质的微妙差异被认为是超导转变温度变化的原因。此外，扫描光电子显微镜已经清楚地表明，少数相的电子结构具有较高的低结合能Fe 3d轨道的占据态密度，这是表现出超导性的晶体的特征。这表明具有高c/a比的Fe-无空位相与超导相的电子结构特征之间存在明显的相关性。
英文摘要：
The superconducting and magnetic properties of phase-separated A$_x$Fe$_{2-y}$Se$_2$ compounds are known to depend on post-growth heat treatments and cooling profiles. This paper focusses on the evolution of microstructure on annealing, and how this influences the superconducting properties of Rb$_x$Fe$_2-y$Se$_2$ crystals. We find that the minority phase in the as-grown crystal has increased unit cell anisotropy (c/a ratio), reduced Rb content and increased Fe content compared to the matrix. The microstructure is rather complex, with two-phase mesoscopic plate-shaped features aligned along {113} habit planes. The minority phase are strongly facetted on the {113} planes, which we have shown to be driven by minimising the volume strain energy introduced as a result of the phase transformation. Annealing at 488K results in coarsening of the mesoscopic plate-shaped features and the formation of a third distinct phase. The subtle differences in structure and chemistry of the minority phase(s) in the crystals are thought to be responsible for changes in the superconducting transition temperature. In addition, scanning photoemission microscopy has clearly shown that the electronic structure of the minority phase has a higher occupied density of states of the low binding energy Fe3d orbitals, characteristic of crystals that exhibit superconductivity. This demonstrates a clear correlation between the Fe-vacancy-free phase with high c/a ratio and the electronic structure characteristics of the superconducting phase.
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