Molecular beam epitaxy growth of SmFeAs(O,F) films with Tc = 55 K using the new fluorine source FeF3

Abstract

REFeAs(O,F) (RE: rare-earth element) has the highest-Tc (∼58 K) among the iron-based superconductors, but a thin-film growth of REFeAs(O,F) is difficult. This is because it is not only a complex compound consisting of five elements but also requires doping of highly reactive fluorine to achieve superconductivity. We have reported in our previous article that fluorine can be supplied to a film by subliming solid-state fluorides such as FeF2 or SmF3. In this article, we report on the growth of SmFeAs(O,F) using FeF3 as an alternative fluorine source. FeF3 is solid at ambient temperatures and decomposes at temperatures as low as 100–200 °C, and releases fluorine-containing gas during the thermal decomposition. With this alternative fluorine source, we have grown SmFeAs(O,F) films with Tc as high as 55 K. This achievement demonstrates that FeF3 has potential as a fluorine source that can be employed ubiquitously for a thin-film growth of any fluorine containing compounds. One problem specific to FeF3 is that the compound is highly hydroscopic and contains a substantial amount of water even in its anhydrous form. In this article, we describe how to overcome this specific problem.

Recent advances in iron-based superconductors toward applications

“…The first 1111-type epitaxial films were grown by pulsed laser deposition (PLD), in which the excitation source of laser ablation was replaced from the usual ultraviolet excimer laser to the second harmonics ( = 532 nm; i.e., visible light) of a Nd:YAG laser to overcome the difficulty of the as-grown phase formation by PLD or FeF 3 [ 134 ]. The current maximum T c reported for Sm1111 thin films grown by MBE is the same as that of bulk samples (T c ≈ 56 K).…”

“…Then, several new F-doping techniques for MBE growth were developed; for example, gallium getter [ 130 ]; diffusion from an overlayer of SmF 3 [ 131 ]; and co-evaporation of SmF 3 [ 132 ], FeF 2 [ 133 ], or FeF 3[ 134 ]. The current maximum T c reported for Sm1111 thin films grown by MBE is the same as that of bulk samples (T c ≈ 56 K).…”

Recent progress in thin-film growth of Fe-based superconductors: superior superconductivity achieved by thin films

“…We describe only the results with SmF 3 in detail and give a brief summary of the results with subliming FeF 2 and with thermally decomposing FeF 3 for comparison. This is because the results of the one-step growth of SmFeAs (O,F) with either FeF 2 or FeF 3 are described in the full-length articles by Sugawara et al (2014) and Sakoda et al (2017).…”

“…4.7. One-step growth of SmFeAs(O,F) assisted by thermally decomposing FeF 3 at TAT In this subsection, the growth of SmFeAs(O,F) films with FeF 3 is briefly described(Sakoda et al 2017). As mentioned above, FeF 3 releases F-containing gas via thermal decomposition at 150 °C-200 °C.…”

Superconducting properties of a mixed anion layered compound, Ca and F co-doped LaFeAsO with T c = 31.5 K

“…Since the discovery of LaFeAsO 1−x F x , 9) MALCs have been considered parent compounds for such novel superconductors [9][10][11][12][13][14][15][16] and several attempts have been made to use them for superconducting wires. [17][18][19][20][21] LnFeAsO (Ln = lanthanide), AeFeAsF (Ae = alkali earth), [22][23][24][25][26][27] and CaFeAsH 28,29) are parent compounds for ironbased superconductors with ZrCuSiAs-type structure (space group P4/nmm) composed of FeAs anti-fluorite-type conducting layers sandwiched between LnO/AeF/CaH fluoritetype insulating layers. Such compounds are classified as "1111-type" compounds and undergo structural transitions from the tetragonal to the orthorhombic phase between 140 and 180 K, accompanied by a transition from a paramagnetic to a stripe-type antiferromagnetic state just below the structural transition temperature.…”