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Abstract
We present direct evidence for phase separation and chemical inhomogeneity in Fe1+yTexSe1−x single crystals from scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS) data. In STEM, images recorded using a high-angle annular dark field (HAADF) detector show characteristic nanometer-scale patterns of phase separation from the Z-dependent image contrast. The separation was observed both in non-superconducting samples with excess iron and in superconducting samples. Using EELS, we determined ∼20% (or lower) fluctuation in Te concentration from the average local compositions by integrating the intensity of the Te-M4,5 edge. The energy-loss near-edge structure (ELNES) of the Fe-L2,3 edge changes with varying composition, especially the L3/L2 white-line intensity ratio, which is sensitive to the d-state occupancy of the Fe. The results suggest a miscibility gap in the Fe1+yTexSe1−x system and changes in the d-electron states at the nanometer scale from the separated phases.
Highlights
Single crystals of Fe1+yTexSe1-x were grown by a unidirectional solidification method with slow cool down[9]
The appropriate interpretation on the contrast in annular dark field (ADF) images shown in Fig. 2 would be that the brighter regions contain higher Te concentration than darker regions, since ADF image intensity is proportional to atomic numbers
We noticed that the contrast remained similar when the sample was tilted by as much as 20°, and the thickness of samples we investigated ranges from 20 to 40 nm as estimated by the energy loss spectroscopy (EELS) log-ratio method[11], indicating that the Te rich or poor regions do not persist in the entire sample thickness, otherwise tilting angle of 20° would cause some of these regions to overlap in projection
Summary
Single crystals of Fe1+yTexSe1-x were grown by a unidirectional solidification method with slow cool down[9]. To obtain TEM a specimen with uniform thickness in a relative large area (~ 100 nm), a method based on water dissolved tape is introduced.
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