The magnetic properties of the system were studied in which an intermediate-valence state of Eu is realized at around x = 0.75. It was found that the compounds with show a temperature-induced valence transition below room temperature, while those with are antiferromagnetic with a stable state. A first-order valence transition induced by high magnetic field was observed for . Such valence transitions against temperature or field are discussed on the basis of the interconfigurational fluctuation (ICF) model. It is shown that the observed linear relation between the transition field and the transition temperature is explained by the present model.

(Abstract)We report successful fabrication of multi-and mono-core FeSe wires with high transport critical current density J c using a simple in-situ Fe-diffusion process based on the powder-in-tube (Fe-diffusion PIT) method. The seven-core wire showed transport J c of as high as 1027 A/cm 2 at 4.2 K. The superconducting transition temperature T c zero was observed at 10.5 K in the wire-samples, which is about 2 K higher than that of bulk FeSe. The Fe-diffusion PIT method is suitable for fabricating multi-core wires of the binary FeSe superconductors with superior properties.3

Interacting orbital degrees of freedom in a Mott insulator are essentially directional and frustrated. In this Letter, the effect of dilution in a quantum-orbital system with this kind of interaction is studied by analyzing a minimal orbital model which we call the two-dimensional quantum compass model. We find that the decrease of the ordering temperature due to dilution is stronger than that in spin models, but it is also much weaker than that of the classical model. The difference between the classical and the quantum-orbital systems arises from the enhancement of the effective dimensionality due to quantum fluctuations.

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Dilution effects on the long-range ordered state of the doubly degenerate e(g) orbital are investigated. Quenched impurities without the orbital degree of freedom are introduced in the orbital model where the long-range order is realized by the order-from-disorder mechanism. It is shown by Monte Carlo simulations and the cluster-expansion method that a decrease in the orbital-ordering temperature by dilution is substantially larger than that in the randomly diluted spin models. Tilting of orbital pseudospins around impurities is the essence of this dilution effect. The present theory provides a new viewpoint for the recent resonant x-ray scattering experiments in KCu(1-x)Zn(x)F(3).