We have studied the effect of RE substitution on the structure and the local atomic disorder in REO_0.5F_0.5BiS₂ (RE = rare-earth) to understand their correlation with the bulk superconductivity in these materials.

Structural phase separation in A_xFe_2−ySe₂ system has been studied by different experimental techniques, however, it should be important to know how the electronic uniformity is influenced, on which length scale the electronic phases coexist, and what is their spatial distribution. Here, we have used novel scanning photoelectron microscopy (SPEM) to study the electronic phase separation in K_xFe_2−ySe₂, providing a direct measurement of the topological spatial distribution of the different electronic phases. The SPEM results reveal a peculiar interconnected conducting filamentary phase that is embedded in the insulating texture. The filamentary structure with a particular topological geometry could be important for the high T_c superconductivity in the presence of a phase with a large magnetic moment in A_xFe_2−ySe₂ materials.

Tardigrades can cope with adverse environmental conditions by turning into anhydrobiotes with a characteristic tun shape. Tun formation is an essential morphological adaptation for tardigrade entry into the anhydrobiotic state. The tun cell structure and ultrastructure have rarely been explored in tardigrades in general and never in Hypsibius exemplaris. We used transmission electron microscopy to compare cellular organization and ultrastructures between hydrated and anhydrobiotic H. exemplaris. Despite a globally similar cell organelle structure and a number of cells not significantly different between hydrated and desiccated tardigrades, reductions in the sizes of both cells and mitochondria were detected in dehydrated animals. Moreover, in anhydrobiotes, secretory active cells with a dense endoplasmic reticulum network were observed. Interestingly, these anhydrobiote-specific cells are in a close relationship with a specific extracellular structure surrounding each cell. It is possible that this rampart-like extracellular structure resulted from the accumulation of anhydrobiotic-specific material to protect the cells. Interestingly, after five hours of rehydration, the number of secretory cells decreased, and the specific extracellular structure began to disappear. Twenty-four hours after the beginning of rehydration, the cellular structure and ultrastructure were comparable to those observed in hydrated tardigrades.

We report a photoemission study at room temperature on ${\mathrm{BaFe}}_2{X}_3$ $(X=\mathrm{S}$ and Se) and ${\mathrm{CsFe}}_2{\mathrm{Se}}_3$ in which two-leg ladders are formed by the Fe sites. The Fe $2p$ core-level peaks of ${\mathrm{BaFe}}_2{X}_3$ are broad and exhibit two components, indicating that itinerant and localized Fe $3d$ sites coexist similar to ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$. The Fe $2p$ core-level peak of ${\mathrm{CsFe}}_2{\mathrm{Se}}_3$ is rather sharp and is accompanied by a charge-transfer satellite. The insulating ground state of ${\mathrm{CsFe}}_2{\mathrm{Se}}_3$ can be viewed as a ${\mathrm{Fe}}^{2+}$ Mott insulator in spite of the formal valence of +2.5. The itinerant versus localized behaviors can be associated with the stability of chalcogen $p$ holes in the two-leg ladder structure.

We have studied electronic and magnetic properties of the K${}_{0.8}$Fe${}_{1.6}$Se${}_2$ superconductor by x-ray absorption and emission spectroscopy. Detailed temperature-dependent measurements along with a direct comparison with the binary FeSe system have revealed coexisting electronic phases: a majority phase with a high-spin ${}^{3+}$Fe state and a minority phase with an intermediate-spin ${}^{2+}$Fe state. The effect of high-temperature annealing suggests that the compressed phase with a lower-spin ${}^{2+}$Fe state is directly related to the high-$T_c$ superconductivity in the title system. The results clearly underline the glassy nature of superconductivity in the electronically inhomogeneous K${}_{0.8}$Fe${}_{1.6}$Se${}_2$, similar to the superconductivity in granular phases. DOI: http://dx.doi.org/10.1103/PhysRevB.85.224510 ©2012 American Physical Society