In-situ growth of superconducting SmO1−xFxFeAs thin films by pulsed laser deposition

Abstract

Oxypnictide thin film growth by pulsed laser deposition (PLD) is one of many insufficiently resolved issues in the research of iron-based superconductors. Here we report on the successful realization of superconducting SmO1−xFxFeAs oxypnictide thin film growth by in-situ PLD on CaF2 (fluorite) substrates. CaF2 acts as fluorine supplier by diffusion and thus enables superconducting oxypnictide thin film growth by PLD. Films are grown heteroepitaxially and characteristically have a broad resistive normal-to-superconducting transition. Best films have onset transition temperatures around 40 K. The proposed in-situ PLD film growth offers an alternative and cheap route for the fabrication of iron oxypnictides. PLD becomes now an additional option for iron oxypnictide synthesis.

Comparative Review on Thin Film Growth of Iron-Based Superconductors

“…Although the fluoride CaF 2 is generally thought to be stable, the fluorine in the substrate can diffuse into a film in the film fabrication process. The similar fluorine diffusion was proposed also in the film growth of FeSe 0.5 Te 0.5 [45,46], which was confirmed from these above superconductivity in 1111 films [31,41].…”

“…Therefore, CaF 2 is an appropriate substrate material for this system also in terms of the lattice mismatch. In addition, the F diffusion into the film from the CaF 2 substrate was confirmed [46] and the CaF 2 substrate plays a role as the F dopant [31,41]. Therefore, in terms of both the lattice mismatch and the F dopant, the CaF 2 is suitable as a substrate material for the film growth of 1111.…”

“…However, in thin films of 1111, 122, and 11, the highest T c is commonly obtained in the case of CaF 2 substrates. Tsukada et al firstly reported the effectiveness of the CaF 2 substrate in the 11 film fabrication [40], and then the fabrication of the film with high T c were reported in other systems [31,41,63]. Here, we define the lattice misfit,…”

“…Recently, Haindl et al reported the in situ growth of superconducting Sm1111 films on CaF 2 substrates via PLD by using the F diffusion from the substrate [31]. The SmFeAs(O,F) films fabricated on BaFe 2 As 2 -buffered MgO substrates by using the SmFeAsO 0.9 F 0.1 polycrystalline target had almost the same lattice parameters as those of SmFeAsO bulk samples and showed an anomaly, which is due to a structural transition [39], around 150 K and no superconductivity in the temperature dependence of resistivity.…”

“…In their latter work [30], the authors recognized that the difficulty of thin film growth of LnFePnO (Pn: Pnictogen) may come from the crystal structure containing two different anions, because it is difficult to control their stoichiometry, especially for thin film growth. Indeed, it was only recently that the superconducting 1111 film fabrication via in situ PLD method was reported by using the diffusion of fluorine into the film from the CaF 2 substrate [31].…”