Why is the T c So High in Fe-Based Pnictide and Chalcogenide Superconductors?
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Why is the Tc So High in Fe-Based Pnictide and Chalcogenide Superconductors? Bing Lv1, Liangzi Deng1, Zheng Wu1, Fengyan Wei1, Kui Zhao1, James K. Meen2, Yu-Yi Xue1, Li-Li Wang3, Xu-Cun Ma3, Qi-Kun Xue3, Ching-Wu Chu1,4 1 Texas Center for Superconductivity (TcSUH) and Department of Physics, University of Houston, Houston, TX 77204-5002, U.S.A. 2 TcSUH and Department of Chemistry, University of Houston, Houston, TX 77204-5003, U.S.A. 3 Department of Physics, Tsinghua University, Beijing, China. 4 Lawrence Berkeley National Laboratory, Berkeley, CA 94720, U.S.A. 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. INTRODUCTION Studies of the Fe-based pnictide and chalcogenide superconductors have accounted for most of the research effort in high temperature superconductivity in the last five years since their discoveries [1,2]. This stems from their relatively high superconducting transition temperatures Tcs in the presence of a large amount of Fe, which is antithetic to superconductivity, and the existence of a large number of Fe-based iso-structural compounds. Their studies have been expected to help reveal the role of magnetism in high temperature superconductivity in the cuprates and the possible discovery of new superconductors with higher Tcs. Indeed, many Febased superconductors have subsequently been found and their electronic structures investigated. They fall into four general structure classes, which consist of the Fe2As2 or Fe2Se2 layers, as: 11 (FeSe) [2]; 111 (AFeAs where A = alkali metal) [3]; 1111 (LnOFeAs where Ln = rare earth) [1]; and 122 (AeFe2As2 where Ae = alkaline earth) [4]. These parent compounds all display a semimetallic behavior and a spin-density-wave (SDW) formation, although the formation of SDW in 11 and 111 remains inconclusive. They become bulk superconducting upon doping and/or applica
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