We report on the synthesis of large single crystals of a new FeSe layer superconductor Cs(0.8)(FeSe(0.98))(2). X-ray powder diffraction, neutron powder diffraction and magnetization measurements have been used to compare the crystal structure and the magnetic properties of Cs(0.8)(FeSe(0.98))(2) with those of the recently discovered potassium intercalated system K(x)Fe(2)Se(2). The new compound, Cs(0.8)(FeSe(0.98))(2), shows a slightly lower superconducting transition temperature (T(c) = 27.4 K) in comparison to 29.5 in (K(0.8)(FeSe(0.98))(2)). The volume of the crystal unit cell increases by replacing K by Cs-the c parameter grows from 14.1353(13) to 15.2846(11) Å. For the alkali metal intercalated layered compounds known so far, (K(0.8)Fe(2)Se(2) and Cs(0.8)(FeSe(0.98))(2)), the T(c) dependence on the anion height (distance between Fe layers and Se layers) was found to be analogous to those reported for As-containing Fe superconductors and Fe(Se(1 - x)Ch(x)), where Ch = Te, S.

We report on muon-spin rotation and relaxation ($\mu \mathrm{SR}$), electrical resistivity, magnetization and differential scanning calorimetry measurements performed on a high-quality single crystal of ${\mathrm{Cs}}_{0.8}({\mathrm{FeSe}}_{0.98}{)}_2$. Whereas our transport and magnetization data confirm the bulk character of the superconducting state below $T_c=29.6(2)\mathrm{K}$, the $\mu \mathrm{SR}$ data indicate that the system is magnetic below $T_N=478.5(3)\mathrm{K}$, where a first-order transition occurs. The first-order character of the magnetic transition is confirmed by differential scanning calorimetry data. Taken all together, these data indicate in ${\mathrm{Cs}}_{0.8}({\mathrm{FeSe}}_{0.98}{)}_2$ a microscopic coexistence between the superconducting phase and a strong magnetic phase. The observed $T_N$ is the highest reported to date for a magnetic superconductor.

We report on a new iron selenide superconductor with a T(c) onset of 45 K and the nominal composition Li(x)(C(5)H(5)N)(y)Fe(2-z)Se(2), synthesized via intercalation of dissolved alkaline metal in anhydrous pyridine at room temperature. This superconductor exhibits a broad transition, reaching zero resistance at 10 K. Magnetization measurements reveal a superconducting shielding fraction of approximately 30%. Analogous phases intercalated with Na, K and Rb were also synthesized and characterized. The superconducting transition temperature of Li(x)(C(5)H(5)N)(y)Fe(2-z)Se(2) is clearly enhanced in comparison to those of the known superconductors FeSe(0.98) (T(c) ~ 8 K) and A(x)Fe(2-y)Se(2) (T(c) ~ 27-32 K) and is in close agreement with critical temperatures recently reported for Li(x)(NH(3))(y)Fe(2-z)Se(2). Post-annealing of intercalated material (Li(x)(C(5)H(5)N)(y)Fe(2-z)Se(2)) at elevated temperatures drastically enlarges the c-parameter of the unit cell (~44%) and increases the superconducting shielding fraction to nearly 100%. Our findings indicate a new synthesis route leading to possibly even higher critical temperatures for materials in this class: by intercalation of organic compounds between Fe-Se layers.

Neutron and x-ray powder and single crystal synchrotron diffraction of ${\mathrm{Cs}}_{\text{y}}{\mathrm{Fe}}_{2-\text{x}}{\mathrm{Se}}_2$ show the presence of superstructure reflections with propagation vector $k=[\frac{2}{5},\frac{1}{5},1]$ with respect to the average crystal structure $I4/\mathit{mmm}$ ($a~4,c~15$ Å). The propagation vector star corresponds to the 5 times bigger unit cell given by transformation $\mathbf{A}=2\mathbf{a}+\mathbf{b}$, $\mathbf{B}=-\mathbf{a}+2\mathbf{b}$, $\mathbf{C}=\mathbf{c}$. A solution for the atomic structure is found in the space group $I4/m$ with an ordered pattern of iron vacancies corresponding to the iron deficiency $x=0.29$ and Cs stoichiometry $y=0.83$. The superstructure satellites are more pronounced in the neutron diffraction patterns suggesting that they can have some magnetic contribution. We have sorted out all possible symmetry adapted magnetic configurations and found that the presence of antiferromagnetic ordering with the ordered magnetic moment of Fe with $\simeq$ $2{\mu }_B$ does not contradict the experimental data. However, the solutions space is highly degenerate and we cannot choose a specific solution. Instead we propose possible magnetic configurations with the Fe magnetic moments in $(\mathit{ab})$ plane or along $c$ axis. The superstructure is destroyed above $T_s\simeq 500$ K by a first-order-like transition.

We report on the superconducting properties of $A_x$Fe${}_{2-y}$Se${}_2$ ($A$ $=$Rb, K) single crystals studied with the muon spin relaxation or rotation ($\mu$SR) technique. At low temperatures, close to 90$\%$ of the sample volumes exhibit large-moment magnetic order which impedes the investigation of their superconducting properties by $\mu$SR. On the other hand, about 10$\%$ of the sample volumes remain paramagnetic and clearly show a superconducting response. The temperature dependence of the superconducting carrier density was analyzed within the framework of a single $s$-wave gap scenario. The zero-temperature values of the in-plane magnetic penetration depths ${\lambda }_{\mathrm{ab}}\left(0\right)=258\left(2\right)$ and 225(2) nm and the superconducting gaps $\Delta \left(0\right)=7.7\left(2\right)$ and 6.3(2) meV have been determined for $A$ $=$ Rb and K, respectively. The microscopic coexistence and/or phase separation of superconductivity and magnetism is discussed.

In this study, xanthan gum is used as a model exopolymer to demonstrate potential effects of non-Newtonian properties of natural aquatic systems on settling dynamics of particles. Rheological measurements combined with settling experiments using visualization methods revealed that instantaneous velocity fluctuations and a flow pattern formed around a particle are the effects of solution viscoelasticity and shear-thinning properties and that the average settling velocity depends on the exopolymer concentration and particle size. Our study showed that in the considered conditions a disk-shaped particle settles preferably in vertical position with a negative wake behind. The understanding of these processes is essential in technology and engineering and is necessary to improve prediction accuracy of large-scale sedimentation processes and biogeochemical cycles in the ocean involving settling of minerals, marine snow, microplastics, and locomotion of microorganisms.