Thin films of Fe-based superconductors (FBS) have been utilized in a plethora of different experiments for a fundamental understanding of the superconductivity in these materials, as well as for understanding and improving the materials with regard to possible applications. The developments and progress in thin film growth of these materials in the past 10 years are reviewed with a focus on the two main deposition techniques used so far for FBS (pulsed laser deposition, and molecular beam epitaxy). Possible choices of substrates or buffer systems are motivated and explained with regard to misfit, thermal expansion, chemical stability, etc. In the second part of this review, investigations on tuning the superconducting properties, especially critical temperature T _c and critical current density J _c , in FBS thin films are reviewed.

MoTe ₂ is a Weyl semimetal, which exhibits unique non-saturating magnetoresistance and strongly reinforced superconductivity under pressure. Here, we demonstrate that a novel mesoscopic superconductivity at ambient pressure arises on the surface of MoTe ₂ with a critical temperature up to 5 K significantly exceeding the bulk T _c = 0.1 K. We measured the derivatives of I – V curves for hetero-contacts of MoTe ₂ with Ag or Cu, homo-contacts of MoTe ₂ as well as ‘soft’ point contacts (PCs). Large number of these hetero-contacts exhibit a dV / dI dependence, which is characteristic for Andreev reflection. It allows us to determine the superconducting gap Δ. The average gap values are 2Δ = 1.30 ± 0.15 meV with a 2Δ/ k _B T _c ratio of 3.7 ± 0.4, which slightly exceeds the standard BCS value of 3.52. Furthermore, the temperature dependence of the gap follows a BCS-like behavior, which points to a nodeless superconducting order parameter with some strong-coupling renormalization. Remarkably, the observation of a ‘gapless-like’ single minimum in the dV / dI of ‘soft’ PCs may indicate a topological superconducting state of the MoTe ₂ surface as these contacts probe mainly the interface and avoid additional pressure effect. Therefore, MoTe ₂ might be a suitable material to study new forms of topological superconductivity.

Recent efforts in the development of YBa ₂ Cu ₃ O _{7− x} (YBCO) coated conductors are devoted to the increase of the critical current I _c in magnetic fields. This is typically realized by growing thicker YBCO layers as well as by the incorporation of artificial pinning centers. We studied the growth of doped YBCO layers with a thickness of up to 7 μ m using pulsed laser deposition with a growth rate of about 1.2 nm s ⁻¹ . Industrially fabricated ion-beam textured YSZ templates based on metal tapes were used as substrates for this study. The incorporation of BaHfO ₃ (BHO) or Ba ₂ Y(Nb _0.5 Ta _0.5 )O ₆ (BYNTO) secondary phase additions leads to a denser microstructure compared to undoped films. A purely c -axis-oriented YBCO growth is preserved up to a thickness of about 4 μ m, whereas misoriented texture components were observed in thicker films. The critical temperature is slightly reduced compared to undoped films and independent of film thickness. The critical current density J _c of the BHO- and BYNTO-doped YBCO layers is lower at 77 K and self-field compared to pure YBCO layers; however, I _c increases up to a thickness of 5 μ m. A comparison between films with a thickness of 1.3 μ m revealed that the anisotropy of the critical current density J _c ( θ ) strongly depends on the incorporated pinning centers. Whereas BHO nanorods lead to a strong B ∣∣ c -axis peak, the overall anisotropy is significantly reduced by the incorporation of BYNTO forming a mixture of short c -axis-oriented nanorods and small ( a – b )-oriented platelets. As a result, the J _c values of the doped films outperform the undoped samples at higher fields and lower temperatures for most magnetic field directions.

Revealing the universal behaviors of iron-based superconductors (FBS) is important to elucidate the microscopic theory of superconductivity. In this work, we investigate the effect of in-plane strain on the slope of the upper critical field H _c2 at the superconducting transition temperature T _c (i.e. −dH _c2/dT) for FeSe_0.7Te_0.3 thin films. The in-plane strain tunes T _c in a broad range, while the composition and disorder are almost unchanged. We show that −dH _c2/dT scales linearly with T _c, indicating that FeSe_0.7Te_0.3 follows the same universal behavior as observed for pnictide FBS. The observed behavior is consistent with a multiband superconductivity paired by interband interaction such as sign change s _± superconductivity.