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Why is the Tc So High in Fe-Based Pnictide and Chalcogenide Superconductors?

  • Bing Lv (a1), Liangzi Deng (a1), Zheng Wu (a1), Fengyan Wei (a1), Kui Zhao (a1), James K. Meen (a2), Yu-Yi Xue (a1), Li-Li Wang (a3), Xu-Cun Ma (a3), Qi-Kun Xue (a3) and Ching-Wu Chu (a1) (a4)...

Abstract

Recently, the detection of non-bulk superconductivity with unexpectedly high onset-Tcs up to 49 K in Pr-doped CaFe2As2 [(Ca,Pr)122] single crystals and the report of a Tc up to 65 K in one-unit-cell (1UC) FeSe epi-films, offer an unusual opportunity to seek an answer to the question posed in the title. Through systematic compositional, structural, resistive, and magnetic investigations on (Ca,Pr)122 single crystals, we have observed a doping-level-independent Tc, the simultaneous appearance of superparamagnetism and superconductivity, large magnetic anisotropy, and the existence of mesoscopic-2D structures in these crystals, thus providing clear evidence consistent with the proposed interface-enhanced Tc in these naturally occurring rareearth-doped Fe-based superconductors, (Ca,R)122. Similar resistive and magnetic measurements were also made on the 3–4UC FeSe ultrathin epi-films. We have detected weak links in the Meissner state below 20 K, weakly coupled small superconducting patches between 20–45 K, and collective excitations of spin and/or superconducting nature between 45–80 K. The unusual frequency dependences of the diamagnetic moment observed in the films in different temperature ranges will be presented and their implications discussed.

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