A. M. Zhang, T. L. Xia, W. Tong, Z. R. Yang, Q. M. Zhang
We have grown A$_{x}$Fe$_{2-y}$Se$_2$ crystals with a small amount of excess Fe as starting materials. Besides a superconducting (SC) transition at 30 K, we observe a sharp drop at 44 K in both resistivity and susceptibility. By combining thermodynamic measurements with electron spin resonance (ESR), we prove that this is a new SC transition. Energy Dispersed X-ray spectroscopy (EDX), Raman scattering and X-ray diffraction (XRD) unambiguously reveal two phases existing in the crystals: the "$\sqrt{5}\times \sqrt{5}$" phase and a phase with Fe and Se content ratio of $\sim$1:1, which are responsible for the SC transitions at 30 and 44 K, respectively. The structural experiments consistently point to the conclusion that the 44K-SC phase is close to an ideal 122 structure, but with an unexpectedly large lattice parameter along c-axis of 18.10 $\AA$. We further point out a novel monotonic dependence between maximum Tc and the distance between neighboring FeSe layers, which sheds light on the search for FeSe-based SCs with higher Tcs.
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http://arxiv.org/abs/1203.1533
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