High- T c and high- J c SmFeAs(O,F) films on fluoride substrates grown by molecular beam epitaxy
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High-Tc and high-Jc SmFeAs(O,F) films on fluoride substrates grown by molecular beam epitaxy Michio Naito,1,2 Shinya Ueda,1,2 Soichiro Takeda,1,2 Shiro Takano,1,2 and Akiyasu Yamamoto3 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 Applied Chemistry, University of Tokyo, Hongo, Tokyo 113-8656, Japan ABSTRACT We report on our growth of superconducting SmFeAs(O,F) films by F diffusion. In our process, F-free SmFeAsO films were grown by molecular beam epitaxy (MBE) first, and subsequently F was introduced into the films via F diffusion from an overlayer of SmF3. We performed a detailed comparison of the growth conditions and also the properties of resultant films for fluoride and oxide substrates. The best films on CaF2 exhibited a high transition temperature, Tcon (Tcend) = 57.8 K (56.4 K) at highest, which may exceed the highest Tc ever reported for bulk samples. Furthermore the films on CaF2 also showed high critical current density over 1 MA/cm2 in self-field at 5 K. INTRODUCTION Four years after the recent discovery of high-Tc iron pnictides [1], worldwide intensive efforts have been made for film growth of these superconductors. At the beginning, epitaxial thin films of the highest-Tc REFeAs(O,F) (RE-1111, RE = rare-earth element) phase appeared to be hard to produce, but at present high-quality epitaxial films of REFeAs(O,F) have already been produced using molecular beam epitaxy (MBE) [2-6]. Employing CaF2 as a substrate, Tc over 55 K has been achieved [4,5]. In this presentation, we report our MBE growth of high-Tc and highJc SmFeAs(O,F) films. In our process, non-superconducting F-free SmFeAsO films were first prepared and subsequently converted to superconducting films by F diffusion from an overlayer of SmF3. By this simple process, the films on CaF2 showed record high Tc, namely, Tcon (Tcend) = 57.8 K (56.4 K). Furthermore these films showed high critical current density over 1 MA/cm2 in the self-field at 5 K. On the other hand, the films grown on LaAlO3(001) showed Tcon (Tcend) = 52 K (48.5 K), slightly lower than the bulk Tc. The difference in the structure and superconductivity between SmFeAs(O,F) films grown on fluoride and oxide substrates is discussed. EXPERIMENTAL All the films were grown in the custom-designed MBE chamber (base pressure of ≈ 1 x 10-9 Torr) equipped with the precise control systems for atomic beam fluxes. The details of our MBE growth of Fe-based superconductors is described in ref. [5]. In brief, first, F-free SmFeAsO films were grown by coevaporating Sm, Fe, and As in oxygen atmosphere. The oxygen atmosphere was set by introducing 0.1-0.5 sccm of O2 through a stainless steel tube directed at a
substrate. The As flux was controlled by the cell temperature and monitored by an ionization gauge (IG) and a quadrupole mass spectrometer (Q-mass). The typical growth temperature was 650°C. Subsequently F was diffused to the Sm
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