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Molecular beam epitaxy growth of Sr1-xKxFe2As2 and Ba1-xKxFe2As2

Michio Naito1,2, Shinya Ueda1,2, Soichiro Takeda1,2, Shiro Takano1,2, and Akihiro Mitsuda3 1

Department of Applied Physics, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan 2 TRIP, Japan Science and Technology Agency (JST), Chiyoda, Tokyo 102-0075, Japan 3 Department of Physics, Kyushu University, Hakozaki, Fukuoka 812-8581, Japan ABSTRACT Single-crystalline films of superconducting Sr1-xKxFe2As and Ba1-xKxFe2As2 were grown by molecular beam epitaxy (MBE). The most crucial problem in MBE growth of these compounds is the high volatility of elemental K. The key to incorporating K into films is low-temperature growth (≤ 350 ºC) in reduced As flux. We performed a systematic study of the doping dependence of Tc in Ba1-xKxFe2As2 for x = 0.0 to 1.0. The highest Tcon (Tcend) so far attained for Ba1-xKxFe2As2 is 38.3 K (35.5 K) at x ~ 0.3. INTRODUCTION Since the discovery of Fe-based superconductors [1], great efforts have been taken for the synthesis of thin films of these compounds. High-quality epitaxial thin films are required to explore the intrinsic properties of Fe-based superconductors. The AEFe2As2 family (FeAs-122, AE = alkaline-earth element) with the ThCr2Si2 structure show relatively high Tc, reaching ~ 38 K for Ba1-xKxFe2As2 and ~ 37 K for Sr1-xCsxFe2As2 [2,3]. The AEFe2As2 family provides a good opportunity for systematic doping studies on the superconducting properties, which is an advantage over the REFeAsO family (FeAs-1111, RE = rare-earth element). The doping dependence of the crystal structure and superconductivity has been intensively studied on REFeAsO1-x and REFeAsO1-xFx. However, the exact oxygen or fluorine content in doped REFeAsO is not easily determined in most cases, and the doping level is limited to x ≤ 0.2. Moreover, the changes in the lattice parameters are small (≤ 0.01 Å in the a-axis lattice constant (a0) and ≤ 0.03 Å in the c-axis lattice constant (c0)) and their significance is often doubtful. By contrast, in the AEFe2As2 family, the doping level, x, of the alkaline elements can be varied from 0.0 to 1.0, and the structure and physical properties are regulated systematically by the doping level. It enables us to see the correlation of superconductivity and magnetism to hole doping to the FeAs layers. Several articles have already been published on the pulsed laser deposition of FeAs-122 films, but almost all are on the growth of Co-doped 122 films (Sr(Fe,Co)2As2 or Ba(Fe,Co)2As2) with rather low Tc (≤ 25 K). There has been only one report on the growth of (Ba,K)Fe2As2 with Tc ~ 40 K, in which Ba-Fe-As amorphous precursor films were annealed at 700ºC under the vapor of K in a quartz tube [4]. In in situ growth of (Sr,K)Fe2As2 and (Ba,K)Fe2As2, however, the volatility of K presents a serious concern. In this presentation, we report our in situ MBE growth of high-quality single-crystalline films of superconducting (Sr,K)Fe2As2 and (Ba,K)Fe2As2. The key to growing these K-containing films is low