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We report the synthesis and characterizations of a new FeSe-based compound CsFe

Structural characterization, exploiting X-ray scattering differences at elemental absorption edges, is developed to quantitatively determine crystallographic site-specific metal disorder. We apply this technique to the problem of Zn-Cu chemical disorder in ZnCu(3)(OH)(6)Cl(2). This geometrically frustrated kagomé antiferromagnet is one of the best candidates for a spin-liquid ground state, but chemical disorder has been suggested as a mundane explanation for its magnetic properties. Using anomalous scattering at the Zn and Cu edges, we determine that there is no Zn occupation of the intralayer Cu sites within the kagomé layer; however there is Cu present on the Zn intersite, leading to a structural formula of (Zn(0.85)Cu(0.15))Cu(3)(OH)(6)Cl(2). The lack of Zn mixing onto the kagomé lattice sites lends support to the idea that the electronic ground state in ZnCu(3)(OH)(6)Cl(2) and its relatives is nontrivial.

Pressure has an essential role in the production and control of superconductivity in iron-based superconductors. Substitution of a large cation by a smaller rare-earth ion to simulate the pressure effect has raised the superconducting transition temperature T(c) to a record high of 55 K in these materials. In the same way as T(c) exhibits a bell-shaped curve of dependence on chemical doping, pressure-tuned T(c) typically drops monotonically after passing the optimal pressure. Here we report that in the superconducting iron chalcogenides, a second superconducting phase suddenly re-emerges above 11.5 GPa, after the T(c) drops from the first maximum of 32 K at 1 GPa. The T(c) of the re-emerging superconducting phase is considerably higher than the first maximum, reaching 48.0-48.7 K for Tl(0.6)Rb(0.4)Fe(1.67)Se(2), K(0.8)Fe(1.7)Se(2) and K(0.8)Fe(1.78)Se(2).

This paper addresses the exponential stability problem for discrete-time switched systems with time-varying delay. The average dwell time (ADT) property is used to prove the exponential stability in this paper. A condition to check the exponential stability of discrete-time switched systems is proposed. Finally, an example is provided to show the effectiveness of the proposed results.