Fe-based superconducting thin films on metallic substrates: growth, characteristics and relevant properties
Kazumasa Iida Jens Hänisch Chiara Tarantini




 


iida@mp.pse.nagoya-u.ac.jp
Department of Materials Physics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan

Institute for Technical Physics, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany

Applied Superconductivity Center, National High Field Laboratory, Florida State University, Tallahassee, FL, 32310, USA





The discovery of Fe-based superconductors (FBS) as the second class of high-temperature superconducting transition (high–T_ c) materials after the cuprates generated a significant impact on the community of fundamental and applied superconductivity research. Whenever a new class of high–T_ c superconducting materials is discovered, a lot of effort is devoted to growing single crystals and epitaxial thin films for exploring basic physical quantities. Although several properties of FBS are similar to the cuprates (large upper critical fields, as a consequence short coherence lengths, and small carrier density), others are distinctly different. For instance, in FBS the symmetry of the superconducting order parameter is most likely not a d–wave but an s±–wave, depending on the stoichiometry, crystallographic system, and doping level. Additionally, the critical current densities of FBS are less sensitive to the presence of grain boundaries (GBs) than those of the high–T_ c cuprates. These features are highly beneficial for the realization of cheaper conductors for high-field magnets at low temperatures. Indeed, several groups have demonstrated FBS thin films on technical metallic substrates and powder-in-tube processed FBS wires as proof-of-principle studies for conductor applications. FBS on technical substrates also give many opportunities for studying how GB networks affect the critical current and how uniaxial strain impacts the superconducting properties. In this article, we review FBS thin films, especially on technical metallic substrates, and focus on application-relevant properties like pinning improvement by natural and artificial defects as well as the transparency of grain boundaries and GB networks. The recent development of FBS thin films on technical substrates and their superconducting properties  are presented and the performance gap with respect to films on single crystals is discussed.


Fe-based superconducting thin films on metallic substrates: growth, characteristics and relevant properties
    Chiara Tarantini
    December 30, 2023
===========================================================================================================





§ INTRODUCTION

Since the first report of superconductivity in LaFeAs(O,F) 10 years ago by Hosono's group,<cit.> a lot of progress in the growth and synthesis of Fe-based superconducting (FBS) single crystals, bulks, wires, and thin films has been made. Now, high-quality thin films [e.g., FeSe, Fe(Se,Te), doped and undoped AEFe_2As_2, where AE being an alkali earth element, and LnFeAs(O,F), where Ln is a lanthanoide element, named respectively 11, 122 and 1111] are available on a large variety of single-crystalline and technical substrates. 
Hence fundamental as well as application-oriented research have been remarkably developed. In this article, we review FBS thin films, especially on technical metallic substrates, and focus on application-related properties like pinning improvement by natural and artificial defects as well as the transparency of grain boundaries (GBs) and GB networks. Many excellent review articles focussing on the growth of FBS thin films on single crystals and their physical properties have already been published<cit.> and the interested readers may also refer to them.
The structure of this article is as follows: Naturally grown pinning centers are summarized, followed by artificial pinning centers. Afterwards, grain boundary experiments on Fe(Se,Te), doped BaFe_2As_2 (Ba-122) and NdFeAs(O,F) will be reviewed. Finally, FBS thin films on technical substrates will be described and discussed.



§ PINNING IMPROVEMENTS




One of the most important properties for wire and tape applications is the critical current density J_ c, which can be improved by microstructural engineering. Similar to the high–T_ c cuprates, the coherence length of FBS is in the order of a few nm.<cit.> Hence nano-sized precipitates and defects can strongly pin the vortices in the mixed state. When transport currents flow in FBS in the mixed state, the Lorentz force acts on the vortices. If the pinning force is weaker than the Lorentz force, the vortices start to move, resulting in dissipation of energy. Hence the stronger the pinning force density, the higher the critical current density. Another effect which depins vortices is thermal fluctuations. This is quantified by the Ginzburg number (Gi) defined as the ratio of the minimal condensation energy within the coherence volume and the thermal energy at the critical temperature (k_ BT_ c, where k_ B is the Boltzmann constant). Thermal fluctuations are a serious problem for high–T_ c superconductors (HTS), for instance in YBa_2Cu_3O_7 (YBCO) with a high Gi of around 10^-3 (table <ref>). For anisotropic superconductors, Gi number in SI unit is evaluated by the following equation,<cit.>



    Gi = 1/2(k_ BT_ c/4πμ_0H_ c^2(0)ϵξ^3(0))^2 = πμ_0^2k_ B^2/ϕ_0^3κ^4T_ c^2/μ_0H_ c2(0)ϵ^2= 1.07×10^-13κ^4T_ c^2/μ_0H_ c2(0)ϵ^2


where μ_0H_ c(0)=μ_0H_ c2(0)/√(2)κ is the thermodynamic critical field at 0 K, μ_0H_ c2(0) is the upper critical field at 0 K, ϵ is the anisotropy parameter defined as ξ_ c(0)/ξ_ ab(0), ξ_ ab(0)=√(ϕ_0/2πμ_0H_ c2(0)) is the in-plane coherence length at 0 K, κ is the Ginzburg-Landau parameter at 0 K, and the right hand side is valid for basic SI units. It is noted that the μ_0H_ c2(0) is calculated by using the WHH formula. By using eq. (1), Gi for various FBS is evaluated (table <ref>). Only for 122 and FeSe, Gi is smaller than for YBCO and, therefore, the flux creep at the probable operating temperatures (minimum flux creep rate is expressed by S∼ Gi^1/2(T/T_ c))<cit.> due to thermal fluctuations is not a serious problem. Different is the case of the most anisotropic NdFeAs(O,F) and Fe(Se,Te) that could more strongly suffer from thermal fluctuations.




In general, thin films contain a higher density of natural defects than single crystals. In low magnetic fields, J_ c for thin films is higher than those for single crystals (fig. <ref>), whereas in high fields they appear to merge toward similar values. It is also noted that for single crystals J_ c has a non-monotonic field dependence due to the fish tail effect, whereas such an effect is absent in thin films. Considering the good performance already obtained in clean thin films, improvements in the in-field behavior are achievable by the introduction of artificial pinning centers with an appropriate control of their dimensionality (1D: columnar defects and dislocations, 2D: grain boundaries and anti-phase boundaries, and 3D: particles<cit.>) and their density. 

 
In this section, the naturally grown pinning centers and artificial pinning centers in FBS are reviewed, and how these pinning centers influence the in-field J_ c is discussed. 



 §.§ Naturally grown pinning centers
     


  §.§.§ Ba-Fe-O nanopillars




S. Lee et al. implemented SrTiO_3 as buffer layer to grow epitaxial Co-doped Ba-122 on 001-oriented substrates
((LaAlO_3)_0.3(Sr_2TaAlO_6)_0.7 (LSAT), GdScO_3, LaAlO_3 and Si) by pulsed laser deposition 
(PLD, KrF excimer laser).<cit.> The nominal Co doping level of their PLD target was x=0.08 in Ba(Fe_1-xCo_x)_2As_2 with a small As excess to compensate the loss of As during the deposition. They concluded that the use of a SrTiO_3 template widened the range of possible substrates for high-quality Co-doped Ba-122 thin films. They also observed the formation of a high density of Ba-Fe-O nanopillars (∼8 vol.%) that grew vertically from SrTiO_3 to the film surface (fig. <ref>). As a result, the critical current density (J_ c) for H∥ c was higher than that for H∥ ab, which differs from what is expected by the mass anisotropy. The mean distance between nanopillars was 16-17 nm, which corresponds to a matching field of 7–8 T.<cit.> Indeed, the pinning force density showed a peak at around 8 T for 
H∥ c. Later it was found that the oxygen content of the PLD targets strongly affects the Ba-Fe-O nanopillar density.<cit.> 
Most importantly, this compound can accept a high density of Ba-Fe-O of up to 20 vol.% without compromising T_ c.


The detailed microstructural analysis by TEM identified the Ba-Fe-O phase as BaFeO_2, which is an isostructure of tetragonal SrFeO_2 (a=0.3991 nm and c=0.3474 nm).<cit.> The authors explained that BaFeO_2 has a>0.3991 nm due to a larger ionic radius of Ba compared to Sr and is coherently strained by the Ba-122 matrix.




  §.§.§ Threading dislocations




c-axis-correlated defects were also reported in Fe(Se,Te) thin films fabricated on SrTiO_3 (001) substrates by PLD (KrF excimer laser) in Genova. In fig. <ref>, a high density of such defects with 2 nm diameter, comparable to the coherence length, was observed.<cit.>
Bellingeri et al. speculated those defects were threading dislocations. Owing to these defects, J_ c for H∥ c was higher than for H∥ ab at all temperatures, which is similar to the observation reported for nanopillars in Co-doped Ba-122.<cit.> The mean defect separation was about 10 nm, corresponding to a matching field of 20 T. Consequently, J_ c for H∥ c has a weaker field dependence than that for H∥ ab up to the maximum measured field of 9 T and in the temperature range below 0.65 T_ c. The activation energy U_0 for vortex motion was raised by a factor 1.5 in these samples compared to a film on LaAlO_3(100) substrate, where interestingly this kind of defects was absent.



The introduction of threading dislocations in a controlled manner by tuning the growth conditions was reported for P-doped Ba-122 thin films by Sato et al.<cit.> Shown in fig. <ref>(a) and (b) are the cross sectional views of two P-doped Ba-122 thin films grown on MgO substrates by PLD (Nd:YAG laser with 2nd harmonic) with different growth rates. Relatively short defects which are almost parallel to the Ba-122 c-axis were observed for a growth rate of 2.2 Å/s and identified as threading dislocations (fig. <ref>(a)). On the other hand, tilted, long defects of relatively low density were seen for the film grown at 3.9 Å/s and identified as domain boundaries due to lateral growth (fig. <ref>(b)). The film grown at 2.2 Å/s with threading dislocations showed the highest J_ c in the full angular range among the films measured at 12 K and 3 T (fig. <ref>(c)).




  §.§.§ Fe precipitates
     
Hänisch et al. investigated Co-doped Ba-122 thin films containing 45^∘ rotated grains grown by PLD (KrF excimer laser).<cit.> They fabricated Co-doped Ba-122 thin films on LSAT(001) at a relatively low deposition temperature of 675^∘C in ultra-high vacuum (UHV) condition. As a result, round and c-axis elongated Fe precipitates were observed near or within the grain boundaries (fig. <ref>). These Fe particles, together with the threading dislocations in the GBs lead to a c-axis peak in the J_ c anisotropy.



For P-doped Ba-122 thin films grown by molecular beam epitaxy (MBE), Sakagami et al. reported a maximum self-field J_ c of 10 MA/cm^2 (4.2 K) for Fe-rich films (Fe/ Ba=2.4) on MgO(001) substrates measured by a magnetization method.<cit.> Although no microstructural investigations have been reported, the authors speculated two plausible reasons: one possibility is that nanoparticles of Fe or Fe-based compounds, which were not detectable by x-ray diffraction (XRD), are dispersed in the matrix of such Fe-rich P-doped Ba-122 films and act as pining centers. The other possibility is that an Fe layer is formed at the interface with the substrate and acts
as a buffer layer that improves the crystallinity. 




 §.§ Artificial pinning centers
  


  §.§.§ BaZrO_3 nanoparticles in PLD-grown Co- and P-doped Ba-122
 
Using the same strategy for pinning enhancement of YBa_2Cu_3O_7 films,<cit.> BaZrO_3-added BaFe_2(As_1-xP_x)_2 <cit.> and Ba(Fe_1-xCo_x)_2As_2 <cit.> sintered pellets were prepared by two groups for use as PLD targets. The former target was ablated by a 2nd harmonic Nd:YAG laser whereas the latter was ablated by a KrF excimer laser.





In P-doped Ba-122 films on MgO(001) substrate, nano-sized BaZrO_3 particles with average diameter of 5–10 nm were finely distributed in the Ba-122 matrix.<cit.> As a result, an almost isotropic J_ c of 1.5 MA/cm^2 at 15 K (0.6T_ c) and 1 T was obtained. In Co-doped Ba-122 films on CaF_2(001) substrate, short and c-axis-oriented BaZrO_3 nanorods with ∼ 4 nm diameter, slightly larger than in YBCO (2∼ 3 nm),<cit.> were formed in the Ba-122 matrix.<cit.> The mean separation of BaZrO_3 nanorods was 10–11 nm, corresponding to a matching field of 17–20 T. The optimized Co-doped Ba-122 with 2 mol% BaZrO_3 showed an in-field J_ c of 1.3 MA/cm^2 at 13 T and 4.2 K. 


Figure <ref> compares the reduction of T_ c in P-doped Ba-122 and Co-doped Ba-122. Up to 3 mol% (2.5 vol.%) of BaZrO_3, the initial suppression rate of T_ c for both films is around -1 K/mol% (-1.3 K/vol.%), which is high compared to common values for GdBCO <cit.> and YBCO <cit.>. The difference in T_ c reduction against volume fraction is not as severe because of the smaller molar volume of the Ba-122 phase. 


Compared to BaFeO_2 nanopillars (see section Ba-Fe-O), the reduction of T_ c is significant, which may be ascribed to the different lattice mismatch.



  §.§.§ Superlattices and multilayers



To date, FBS superlattices for Ba-122 <cit.> and Fe(Se,Te) <cit.> have been reported by the Univ. Wisconsin and the Univ. of Tokyo, respectively. S. Lee at al. investigated the effect of SrTiO_3 and undoped Ba-122 insertion layers on the structural and superconducting properties of Co-doped Ba-122 thin films. The cross-sectional TEM images of the (100) projection clearly showed the 24 bilayers of SrTiO_3/Co-doped Ba-122 (fig. <ref>(a)). On the other hand, the insertion layer of undoped Ba-122 grew discontinuously in the lateral direction (fig. <ref>(b)). Similarly to the single layer reported by the same group, the undoped oxygen-rich Ba-122/Co-doped Ba-122 superlattice possessed vertical defects, presumably BaFeO_2, which act as effective pinning centers along the crystallographic c-direction. Combining the c-axis and ab-planar pinning contributions improved in-field J_ c properties considerably, as shown in fig. <ref>(c). 


Nabeshima et al. deposited FeSe/FeTe superlattices on CaF_2 and LaAlO_3.<cit.> From the microstructural investigations, some degree of Te/Se interdiffusion was observed. Because of the interdiffusion, the superconducting transition temperature of the FeSe/FeTe superlattice was improved compared to FeSe (i.e., from 12 K for FeSe on CaF_2 to 15.8 K for FeSe/FeTe superlattice on CaF_2).  No information on critical current densities was reported on these films.


Besides introducing artificial pinning centers, the multilayer approach can improve the superconducting properties.<cit.> Tarantini et al. reported Co-doped, BaO-added Ba-122/Co-doped Ba-122 multilayer fabricated on CaF_2(001) substrate for which the superconducting transition temperature was increased by compressive strain.<cit.> Pinning force densities as high as 84 and 70 GN/m^3 for H∥ ab and ∥ c at 4.2 K were achieved by a combination of short c-axis nanorods, round nanoparticles and ab-arranged precipitates.


Engelmann et al. fabricated
Co:Ba-122 (20 nm)/Fe (4 nm)/Co:Ba-122 (20 nm)/Fe (30 nm) multilayers on MgAl_2O_4, which showed a high T_ c of 29.4 K due probably to an As-deficiency.<cit.> Chen et al. reported the enhancement of in-field performance for an Fe(Se,Te) double layer with a CeO_2 insertion layer on SrTiO_3.<cit.> The improvement is ascribed to the additional interfacial defects introduced by the insertion layer.




  §.§.§ Irradiation



Although many irradiation experiments on FBS single crystals have been reported, only few studies except for 11 report on the effects of irradiation in thin films (see table <ref>). An excellent review article on radiation effects in FBS has recently been published by Eisterer.<cit.> Here, we will briefly summarize the available data on FBS thin films.


The defects created by fast neutron irradiation on Fe(Se,Te) thin films produced only a small impact on the superconducting properties
 (T_ c, upper critical field, H_ c2, irreversibility field, H_ irr and J_ c).<cit.> On the other hand, low-energy proton irradiation created nanoscale compressive strain, resulting in an increase of T_ c by 0.5 K.<cit.> Additionally, cascade defects combined with nanoscale strain worked as strong vortex pinning centers, and consequently J_ c increased by one order of magnitude at 12 K and by a factor of 2 at 4.2 K at fields around and above 15 T. Ozaki et al. also investigated the effect of Au irradiation on the superconducting properties of Fe(Se,Te) thin films.<cit.> The Au-irradiated film showed a slight decrease in T_ c by 0.5 K. However, in-field J_ c properties were improved  for all field orientations at 4.2 K. Furthermore, the J_ c enhancement was nearly 70% compared to the pristine film at 10 K and 9 T. Ahmad et al. reported the effect of proton irradiation effect on the superconducting properties of FeSe_1-xTe_x (x=0.4, 0.55) thin films.<cit.> The reduction of superconducting properties caused by the local heating, which may accelerate the displacement of atoms and create vacancies. Sylva et al. pointed out the importance of the position where the proton is implanted: the improvement of J_ c up to 50% at 12 K and 7 T was observed when protons were accumulated far from the film/substrate interface. Simultaneously, T_ c and H_ c2 were unaltered.<cit.> On the other hand, superconducting properties were degraded when the proton implanted layer was close to the interface.   
 
 
Non-superconducting FeTe thin films were irradiated by oxygen ions.<cit.> At the maximum dose of 1× 10^17  cm^-2, the FeTe film was amorphous.


The effect of 3 MeV proton irradiation on the superconducting properties of Co-doped Ba-122 was reported by Maiorov et al.<cit.>
After the irradiation (total dose of 2×10^16 cm^-2), the T_ c decreased by only 1 K. Overall J_ c for the irradiated film was decreased in low field.
At high field and low temperatures, J_ c was increased, particularly for J_ c (H∥ ab) due to a clear anisotropic contribution from the point-like defects.

The effect of proton irradiation on Co-doped Ba-122 thin films was also reported by Schilling et al. who investigated the disorder-induced symmetry change from s± to s++ by THz spectroscopy.<cit.> On the assumption that the electron-electron coupling constant is positive, disorder induces a transition between s± to s++ states.<cit.> In this report they observed such a transition: the small gap was closed at a certain amount of disorder and re-opened again by further increase of disorder. These results suggested that the superconducting order parameter in the pristine film has s± symmetry. However, no information on H_ c2, H_ irr and J_ c was reported.


Daghero et al. reported on the effect of irradiation with 250-MeV Au ions on the superconducting properties of optimally P-doped Ba-122 thin films fabricated by MBE.<cit.> The residual resistivity at T=0  K (ρ_0) increased with dose level, whilst the decrease in T_ c was only 0.8 K for a maximum dose of 7.26× 10^11  cm^-2. Such a small reduction in T_ c would indicate s± symmetry. The authors also considered the role of the substrate in the radiation effect: Au ion irradiation and heating in the substrate may have caused additional damage.


Unlike in NdFeAs(O,F) single crystals,<cit.> α-particle irradiation on NdFeAs(O,F) thin films did not create Kondo-like defects,<cit.> suggesting that the type of defects induced by this type of irradiation is also strongly affected by other factors, the most likely being the initial composition. In the irradiated NdFeAs(O,F) films an increase of both H_ c2 and H_ irr were observed. However, only for H∥ c a moderate improvement in J_ c  was found, whereas for H∥ ab point-defects compete with intrinsic pinning, reducing J_ c.





§ GRAIN BOUNDARIES

There are two main purposes for experiments on FBS thin films fabricated on bicrystal substrates having a single well-defined grain boundary.
One is to realize Josephson junctions with two FBS electrodes for fundamental research and assessing potential of electronics application. The other goal is to check the J_ c transparency across the grain boundary, which is vital information for wire applications. In the former experiments, thin films are prepared on bicrystal substrates with relatively large misorientation angle θ_ GB to realize an SNS 
(superconductor–normal–superconductor) or SIS (superconductor–insulator–superconductor) junction. It is worth mentioning that all bicrystal experiments on FBS reported to date have been conducted using symmetric [001]-tilt bicrystal substrates (SrTiO_3, MgO and LSAT). Hence grain boundary experiments using different types of bicrystals (i.e., valley- or roof-type [010]-tilt and [100]-twist) are necessary to obtain deeper knowledge of grain boundary characteristics of FBS, since inter-grain J_ c (J_ c^ GB) depends in general on the GB type, as shown for YBa_2Cu_3O_7
deposited on various types of bicrystals.<cit.> 
In general, different results are expected in case of different order parameter symmetry (e.g. s–wave or d–wave) as well as strain sensitivity and charge density.


To date, for FBS there have been only a few systematic and extended bicrystal experiments on Fe(Se,Te) and Co-doped Ba-122 films. Only singular experiments on P-doped Ba-122 <cit.> and recently on NdFeAs(O,F) <cit.> have been reported so far, table <ref>. Reviews on GB properties in FBS bulk and film samples in comparison to
high-T_ c cuprate superconductors have been given by Deutscher in 2010 <cit.> and Durrell et al. in 2011.<cit.> In the following subsections, we review all reported bicrystal experiments on FBS concerning Josephson junctions followed by inter-grain J_ c reduction due to weak-link behaviour. Some of the experiments mentioned below have been conducted for both purposes of Josephson junctions and investigation of the J_ c transparency across the grain boundary. Therefore, occasionally the same experiments are referred to in the subsections Josephson junctions with two FBS electrodes and Misorientation dependence of inter-grain J_ c.






 §.§ Josephson junctions with two FBS electrodes



  §.§.§ Fe(Se,Te)

To date, four bicrystal experiments on Fe(Se,Te) have been published. First bicrystal experiments on Fe(Se,Te) were reported by Sarnelli et al. who fabricated 150 nm thick Fe(Se,Te) films on [001]-tilt SrTiO_3 bicrystals with a misorientation angle (θ_ GB) of 45^∘ by PLD using a Nd:YAG laser with a wave length of 1024 nm.<cit.> The authors made microbridges (2.6–20 μm wide and 20 μm long) on Fe(Se,Te) bicrystals by optical lithography and ion beam milling. To avoid possible damage during the process, the sample holder was cooled to -40^∘C. The I`-V characteristics at 4.2 K showed a resistively shunted junction (RSJ) behavior with a low normal-state resistance R_ N in the range 40≤ R_ N≤ 270  mΩ, which is typical for SNS junctions. Compared to the cuprates, those values are two orders of magnitude lower. As a result, the I_ cR_ N product was 18∼33 μV, where I_ c is the Josephson critical current. Later, Sarnelli et al. fabricated dc superconducting quantum interference devices (dc-SQUIDs) using 100-nm thick Fe(Se,Te) bicrystal with θ_ GB=24^∘.<cit.> The resultant device showed a clear voltage modulation under magnetic field with a maximum amplitude of 3.8 μV at 4.2 K. 


Si et al. fabricated 100-nm thick Fe(Se,Te) thin films on CeO_2-buffered SrTiO_3 bicrystal substrates (θ_ GB=4^∘, 7^∘, 15^∘ and 24^∘) by PLD.<cit.> Microbridges with 300 μm length and 20–25 μm width were fabricated by laser cutting for transport measurements. The ρ`-T curve of the bridge with θ_ GB=24^∘ showed a double superconducting transition at 20 K and 17.5 K with ρ=0 at around 12 K. On the other hand, the bridge with θ_ GB=4^∘ had one transition at 20 K similar to the intra-grain bridge. Using the bridge with θ_ GB=24^∘, the authors observed  multiple Josephson current modulations by magnetic field. The magnitude of the I_ c modulation was around 80%.
The I`-V characteristics at 2 K roughly followed the RSJ model with a normal state resistance of R_ N=28 mΩ.
Such a low value of R_ N is due to the metallic nature of the normal state of the Fe(Se,Te) bicrystal grain boundary junction. The I_ cR_ N product of their junctions was estimated to around 22 μV at 2 K.



  §.§.§ Co- and P-doped Ba-122

Katase et al. reported on Josephson junctions and dc-SQUIDs in PLD-processed Co-doped Ba-122 (T_ c=21.5–22.6 K) using LSAT bicrystal substrates with θ_ GB=30^∘.<cit.> For single Josephson junctions, the authors fabricated microbridges (10 μm wide and 300 μm long) by photolithography and Ar ion milling. The I`-V characteristics of the resultant junction at 4.2 K showed RSJ behavior. The normal state resistance R_ N of the junction was around 12 mΩ, resulting in a low I_ cR_ N product of ∼50 μV. For dc-SQUIDs, bridges with a smaller width of 3 μm (area of SQUID loop was 18×8 μm^2) were made to reduce the critical current. The resultant device showed a clear voltage modulation with an amplitude of 1.4 μV at 14 K. However, the shape of V`-Φ deviated from the ideal sinusoidal function. Additionally, the level of flux noise at 1 Hz was ∼4.3×10^-4 Φ_0Hz^-1/2, which was one order of magnitude higher than for cuprates. From those junction experiments, 
the barrier layer is of metallic nature in Co-doped Ba-122. Indeed, the inter-grain J_ c^ GB showed a  quadratic temperature dependence, indicative of SNS rather than SIS junctions.<cit.> Hence, for realizing practical SQUIDs it is necessary to increase the junction resistance R_ N by an artificial insulating barrier.


Schmidt et al. also reported on PLD-grown Co-doped Ba-122 grain boundary junctions using SrTiO_3 with θ_ GB=30^∘.<cit.> To suppress oxygen diffusion from SrTiO_3, a 10 nm thick MgAl_2O_4 layer was deposited on the SrTiO_3 bicrystal. Fe-buffer layer followed by Co-doped Ba-122 were deposited on this template. All layers were epitaxially grown as confirmed by XRD. In their junctions (T_ c=24 K), a high excess current (I_ ex=645 μA) was observed, and therefore a small I_ cR_ N product of 6.5 μV at 4.2 K was obtained.


Sakagami et al. prepared P-doped Ba-122 by MBE on [001]-tilt MgO bicrystals with θ_ GB=24^∘.<cit.>
The I`-V characteristics of their device (T_ c=29.5 K, bridge width of 30 μm) at 2 K was described by a flux-flow extension of the RSJ model, which gave a normal state resistance of R_ N=4.4 mΩ and a Josephson critical current of I_ c=8.5 mA. As a consequence, a small I_ cR_ N product of 37 μV was obtained. Such a low normal state resistance of the junction is due to the metallic nature of the grain boundary. The same group also fabricated P-doped Ba-122 thin films on LSAT bicrystals with θ_ GB=45^∘ <cit.>. The I`-V characteristics of the resultant junction can again be fitted by a RSJ model with flux flow current. Their device had an I_ cR_ N product of 45 μV at 4.2 K. However, this value was corrected to 11 μV by taking into account the excess current.   




  §.§.§ Summary of Josephson junctions with two FBS electrodes

All bicrystal junctions of Fe(Se,Te), Co-doped, and P-doped Ba-122 have the same low level of I_ cR_ N (tens of μV) compared to that of YBCO (in the order of mV at 4.2 K). The main reason for such low I_ cR_ N is that the barrier layer is of metallic nature, which leads to a low junction resistance R_ N. Therefore, the dc-SQUIDs made from Fe(Se,Te) <cit.> and Co-doped Ba-122 <cit.> showed high flux noise. The properties mentioned above are not suitable for electronics applications, however, are highly beneficial to wire applications. To realize electronics applications like SQUIDs, one would have to increase the R_ N by an artificial insulating barrier. Interested readers for Josephson effects in FBS may refer to Ref. Seidel-1.




 §.§ Misorientation dependence of inter-grain J_ c



  §.§.§ Fe(Se,Te)

 
The inter-grain J_ c of Fe(Se,Te) across a θ_ GB=45^∘ grain boundary without magnetic field was around ∼10^4 A/cm^2 at 4.2 K, one order of magnitude lower than the intra-grain J_ c (J_ c^ GB).<cit.> These results suggested that the decay of J_ c with GB angle may not be as strong as for the cuprates, where at this angle the suppression is significant (4 orders of magnitude). Later, Sarnelli et al. have reported J_ c^ GB/J_ c^ Grain as a function of θ_ GB.<cit.> It was found that the θ_ GB dependence of J_ c^ GB/J_ c^ Grain is almost similar to the result reported by Si et al.<cit.> Additionally, the level of J_ c^ GB was almost constant for θ_ GB=24^∘ and θ_ GB=45^∘, which will be discussed later.


Si et al. found that J_ c^ GB for θ_ GB=4^∘ and 7^∘ maintained ∼10^5 A/cm^2 even in the presence of large 
applied magnetic fields (μ_0H=10  T, H∥ c) at 4.2 K, indicative of strong links in this θ_ GB range.<cit.> On the other hand, J_ c^ GB for θ_ GB>15^∘ was significantly suppressed by a small magnetic field: J_ c^ GB for θ_ GB=24^∘, e.g., was reduced from 4×10^4 to 7×10^3 A/cm^2 even by a small magnetic field of μ_0H=0.5  mT. Based on the experimental results above, the authors concluded that the critical angle θ_ c was around 9^∘.




  §.§.§ Co- and P-doped Ba-122

Already in the early stage of thin film research of pnictide superconductors, S. Lee et al. reported on the weak-link behavior of [001] tilt GBs in Co-doped Ba-122.<cit.> They fabricated 350-nm thick films (T_ c over 20 K) on SrTiO_3 bicrystals (θ_ GB=3^∘, 6^∘, 9^∘ and 24^∘) by PLD. Low-temperature laser scanning microscopy (LTLSM) and magneto-optical imaging (MOI) on these Co-doped Ba-122 bicrystal junctions with θ_ GB=6^∘ and 9^∘ revealed the weak-link behavior also for low-angle GBs (fig. <ref>). As can be seen in fig. <ref>(a), a large dissipation across the 6^∘ grain boundary was detected by LTLSM. For the 9^∘ grain boundary sample, two clear roof-top shapes separated by the grain boundary are observed by MOI (fig. <ref>(b)). These results indicated that the critical angle θ_ c may be below 6^∘, which differs from results reported two years later by Katase et al.<cit.> We speculate that oxygen impurities may have limited the J_ c^ GB for these Co-doped Ba-122 films on SrTiO_3. In fact, it is likely that the nanorods (see subsection BaFeO_3 nanopillars) tend to segregate along the GB reducing the effective cross-section and consequently J_ c. Detailed microstructural investigations are necessary to identify such differences. 





Later Katase,  Hiramatsu et al. reported a θ_ c as large as 9^∘ from their detailed bicrystal experiments on Co-doped Ba-122.<cit.> 
Here the respective T_ c of Co-doped Ba-122 were 20.7 K on MgO and 21.6 K on LSAT bicrystals. Shown in fig. <ref> is the θ_ GB dependence of inter-grain J_ c measured at 4 K without magnetic field. Clearly, the inter-grain J_ c remained at 1 MA/cm^2 up to θ_ GB=9^∘. Beyond this angle, the decay of inter-grain J_ c sets in. Another interesting feature is that J_ c^ GB was almost constant for θ_ GB=30^∘ and θ_ GB=45^∘.






A high inter-grain J_ c over 1 MA/cm^2 even at θ_ GB=24^∘ and 4 K was reported by Sakagami et al.<cit.> for a P-doped Ba-122 thin film grown by molecular beam epitaxy (MBE) on a MgO bicyrstal substrate. Although the number of data points is not sufficient, P-doped Ba-122 most likely also has a higher critical angle than the cuprates.


It was reported that polycrystalline K-doped Ba-122 bulk samples showed a large self-field J_ c of ∼0.1  MA/cm^2.<cit.> Microstructural analysis revealed that the grain boundaries were clean without segregation of impurities. Later, atom probe tomography performed on both K- and Co-doped Ba-122 polycrystals revealed that traces of oxygen impurities were present at the GBs, in particular for the Co-doped sample (fig. <ref>).<cit.> Such oxidized regions may hide the intrinsic characteristics of the grain boundary. Although the microstructures of Co-doped and K-doped Ba-122 polycrystalline samples are almost identical, the in-field J_ c performance of Co-doped Ba-122 is inferior to that of K-doped Ba-122 at 4.2 K.<cit.> These results indicate that grain boundary properties depend on the kind of doping elements (i.e., K, Co and P) and perhaps AE in AEBa_2As_2 too. Therefore, grain boundary experiments on K-doped Ba- and Sr-122 are highly desirable and also important for on-going development of powder-in-tube processed wires and bulk magnets.<cit.>




  §.§.§ NdFeAs(O,F)



LnFeAs(O,F) (Ln= Sm and Nd) compounds have the highest T_ c among the FBS. Hence, a lot of effort was devoted to producing LnFeAs(O,F) wires. However, the self-field J_ c of polycrystalline SmFeAs(O,F) was only ∼4×10^4  A/cm^2 at 4.2 K,<cit.> which is one order of magnitude lower than for K-doped Ba-122, indicative of possible weak-link behavior. Hence bicrystal experiments on LnFeAs(O,F) have been long desired to elucidate this question.


Omura et al. have recently reported on the GB angle dependence of transport properties for NdFeAs(O,F) thin films fabricated by MBE.<cit.> For θ_ GB=6^∘ and 12^∘, the resistivity in the superconducting state was below the instrumental limitation(fig. <ref>). For θ_ GB≥24^∘, in contrast a finite resistivity below T_ c was observed and hence the inter-grain J_ c was zero. The inter-grain J_ c for θ_ GB=6^∘ was reduced by nearly 30% compared to the intra-grain J_ c. Hence the critical angle for these NdFeAs(O,F) films was less than 6^∘. However, the authors stated that the results did not reflect the intrinsic grain boundary properties due to the erosion of the grain boundaries by fluorine diffusion.




  §.§.§ Summary of the misorientation dependence of inter-grain J_ c



Shown in fig. <ref> is a summary of θ_ GB dependence of inter-grain J_ c for various FBS bicrystals. Clearly, P-doped Ba-122 showed a high inter-grain J_ c of 1 MA/cm^2 at θ_ GB=24^∘ (fig. <ref>(a)), which is highly beneficial for wire applications. It is also noted that J_ c^ GB for Ba-122 is higher than those for YBCO beyond θ_ GB=24^∘.
Although intra-grain and inter-grain J_ c are high for P-doped Ba-122, the θ_ GB dependence of J_ c^ GB is similar to that of Co-doped Ba-122 (fig. <ref>(b)). For all FBS except for NdFeAs(O,F), the critical angle θ_ c is around 9^∘, which indicates that in-plane misorientation alignments of FBS grains less than 9^∘ usually do not impede the critical current flow. Hence one can expect that
grain boundary networks whose in-plane misorientation is less than 9^∘ work as flux pinning centers. Indeed, the pinning force density analysis of P-doped Ba-122 on technical substrates revealed that the GB pinning contribution is dominant for H∥ c,<cit.> which will be discussed later.
 

The initial slope for the J_ c decrease with angle for FBS is almost similar to that of YBCO, fig. <ref>(b), which is different from the common belief.
A distinct difference between FBS and YBCO is recognized above θ_ GB=24^∘: for YBCO J_ c^ GB continuously decreases exponentially with θ_ GB, whilst J_ c^ GB remains constant for FBS. This may be due to the different symmetry: d-wave for YBCO and sign reversal s-wave for FBS. Additionally, the grain boundaries for both Ba-122 and Fe(Se,Te) are of metallic nature. Another plausible reason may be 45^∘ [001]-tilt GBs in Ba-122 and Fe(Se,Te) compounds being special,
low-Σ or facetted GBs with a narrower structurally and electronically disturbed region than for GBs with smaller θ_ GB as discussed for high–T_ c cuprates.<cit.>


It is reported that inter-grain J_ c for YBCO is ameliorated by Ca-doping for which the depletion of the carrier density at GBs is compensated.<cit.>
Later, electron energy loss spectroscopy (EELS) revealed high concentrations of Ca at the tensile regions of the GBs.<cit.>
On the assumption that the same mechanism could work for overdoped Ba-122, Durrell et al. fabricated Co-doped Ba-122 bicrystals with Co concentration from 8% (optimal) to 10% (overdoped).<cit.> However, no improvement of inter-grain J_ c was observed.
More of such experiments should be conducted to tackle the GB problem for FBS.  




§ FBS THIN FILMS ON TECHNICAL SUBSTRATES




As described in the previous section, grain boundaries are less critical an issue for FBS than for cuprates, and powder-in-tube-processed wires based on 11, 122 and 1111 have been fabricated. Among them, the in-field J_ c of 122 wires has already reached the practical level of 10^5  A/cm^2,<cit.> and a 120 m long tape of K-doped Sr-122 has been realized.<cit.> However, biaxially textured films on single-crystalline substrates and also single crystals <cit.> showed superior J_ c`-H performance to polycrystalline samples. To date, seven groups have published results on FBS films on technical substrates as prototype coated conductor samples, as summarized in Table <ref>. 
In this section, various FBS thin films on technical substrates are reviewed.  Most of these studies were done on IBAD-MgO templates (textured MgO templates are prepared on Hastelloy by ion beam assisted deposition), supplied by iBeam inc., USA and Shanghai Creative Supercond. Technol. Co. Ltd, China. Three studies on FeSe and Fe(Se,Te) used RABiTS (rolling-assisted biaxially textured substrate) template for which two of them were supplied by ENEA (Italian National Agency for New Technologies, Energy and Sustainable Economic Development) and evico GmbH, Germany. The other has been fabricated by in-house R&D collaboration between Brookhaven National Laboratory and Oak Ridge National Laboratory. 




 §.§ FeSe and Fe(Se,Te)

Despite the large lattice mismatch between Fe(Se,Te) (a_ Fe(Se,Te)=3.81 Å) and MgO (a_ MgO=4.21 Å), epitaxial Fe(Se,Te) thin films were successfully
fabricated on hastelloy tapes, on which epitaxial MgO templates were deposited by IBAD.<cit.> The respective in-plane and out-of-plane full width at half maximum of Fe(Se,Te) were Δϕ=4.5^∘ and Δω=3.5^∘. Clearly, Δϕ=4.5^∘ is less than the critical angle θ_ c for [001]-tilt Fe(Se,Te) grain boundaries.
Although a critical angle for [010]-tilt Fe(Se,Te) grain boundary junctions has not been reported, it is expected that [001]-tilt and [010]-tilt GBs in Fe(Se,Te) have a similar critical angle due to the small anisotropy and likely the sign reversal s-wave type symmetry. Therefore, weak-link behavior is absent in these Fe(Se,Te) films on IBAD-MgO. The lower T_ c (T_ c,R=0=11 K) of that film compared to the film on LaAlO_3 substrate (∼15 K) is mostly ascribed to the large lattice misfit. The Fe(Se,Te) film showed an almost isotropic J_ c over 10^4 A/cm^2 at 25 T and 4.2 K.
The pinning force analysis revealed that point defect pinning governs the J_ c`-H properties.



To further improve J_ c`-H properties of Fe(Se,Te) on technical substrate, Si et al. have fabricated Fe(Se,Te) thin films on CeO_2-buffered RABiTS.<cit.> The lattice parameter of CeO_2 is around 3.82 Å(∼5.41/√(2)), which almost perfectly matches the in-plane lattice parameter of Fe(Se,Te). As a result, Fe(Se,Te) on RABiTS showed a high onset T_ c of over 20 K with a sharp transition of 1 K width. The structural characterization by X-ray diffraction revealed an in-plane FWHM Δϕ_ Fe(Se,Te)=6^∘. Despite the larger in-plane texture spread, the Fe(Se,Te) film was capable of carrying large current densities (fig. <ref>). A nearly isotropic
J_ c∼0.1 MA/cm^2 at 4.2 K and 30 T was observed. The pinning mechanism was the same as for the Fe(Se,Te) film on IBAD-MgO. The authors concluded that the use of
CeO_2 buffer layer is the key for achieving high T_ c. Additionally, the substrates containing low angle grain boundaries up to 7^∘ do not impede the critical current flow, which is different from REBa_2Cu_3O_7.


Quite recently, the group of IEE-CAS reported Fe(Se,Te) thin films on IBAD-MgO with LaMnO_3 (LMO) buffer layers.<cit.> The Fe(Se,Te) was grown on LMO with cube-on-cube configuration. The respective out-of-plane and in-plane full width at half maximum values of Fe(Se,Te) are Δω=3.4^∘ and Δϕ=7.8^∘. The T_ c of the sample is 16.8 K, which is slightly higher than that of bulk due to the compressive strain. Albeit the large Δϕ of 7.8^∘, an in-field J_ c above 0.35 MA/cm^2 up to 9 T was recorded at 4.2 K. Additionally, the anisotropy of J_ c was relatively small.


The group of CNR has been recently working on an attempt to decrease the number of necessary buffer layers or exchanging the metallic substrate to less expensive materials. Both solutions could reduce the fabrication costs of such Fe(Se,Te) coated conductors.<cit.> The films on Ni5W RABiTS covered with a single CeO_2 buffer layer (produced by ENEA) showed a T_ c of ∼18 K and isotropic J_ c above 2×10^4 A/cm^2 at 4.2 K in fields up to 18 T. On biaxially textured invar (nickel-iron alloy with 36 mass% of Ni) tape, epitaxial Fe(Se,Te) layers of the same texture quality as the tape itself (FWHM∼ 9^∘) were grown without any buffer layer. The superconducting properties of these samples still have to be investigated.


The group at NIMS proposed a unique technique for fabricating FeSe films on RABiTS provided by evico GmbH.<cit.> They performed electrochemical deposition using three electrodes, Pt (counter electrode), Ag/AgCl (reference electrode) and RABiTS (working electrode) at 70^∘C. The electrolyte was prepared from a solution of FeCl_2·4H_2O, SeO_2, and Na_2SO_4 in distilled water. A pH value of 2.1 for the electrode was maintained during the deposition. X-ray diffraction confirmed FeSe phase formation without heat treatment. Although the resulting film had a low T_ c of 2.5 K, this technique is promising for further development and optimization.<cit.>




 §.§ Co- and P-doped Ba-122

The first report on Co-doped Ba-122 on technical substrates was published by Iida et al.<cit.> They fabricated Co-doped Ba-122 films on Fe-buffered IBAD-MgO templates by PLD. Due to the relatively large in-plane mosaicity of the MgO (Δϕ_ MgO∼6^∘), the crystalline quality of the intermediate Fe as well as Co-doped Ba-122 layers was poor (i.e., Δϕ_ Fe∼5^∘ and Δϕ_ Ba-122∼5^∘). Although biaxially textured Co-doped Ba-122 showed an onset T_ c of 21 K, the in-field J_ c at 8 K was almost one order of magnitude lower compared to the film on single crystalline MgO due to the poor crystalline quality. Later, employing a high-quality MgO template (Δϕ_ MgO∼2.4^∘) improved the in-field J_ c of Co-doped Ba-122 to a level almost comparable to films on single crystalline substrates.<cit.>


Almost at the same time, Katase et al. also reported on the fabrication of Co-doped Ba-122 directly on IBAD-MgO.<cit.> A sharp texture of the Co-doped Ba-122
with Δϕ_ Ba122∼3.2^∘-3.5^∘ was obtained irrespective of the in-plane mosaicity of MgO templates due to the self-epitaxy effect. The onset T_ c of Co-doped Ba-122 on IBAD-MgO was 22 K. In-field J_ c reached almost 0.1 MA/cm^2 under 9 T at 4 K.

Very recently, Co-doped Ba-122 was also fabricated on IBAD-MgO with LMO by Xu et al.<cit.> They used SrTiO_3 as buffer layer between LMO and Co-doped Ba-122. The resultant film showed almost isotropic J_ c. The respective J_ c for H∥ c and ∥ ab were 0.86 and 0.96 MA/cm^2 at 9 T and 4.2 K.
 


P-doped Ba-122 shows the second highest T_ c among the 122 compounds and, therefore, J_ c`-H properties are usually superior to Co-doped Ba-122. Additionally, growth-related and artificial pinning centers can be introduced into the P-doped Ba-122 matrix as discussed in the previous section.
Sato et al. fabricated P-doped Ba-122 thin films with PLD on IBAD-MgO with two different in-plain texture qualities, Δϕ_ MgO=4^∘ and 8^∘.<cit.> The structural characterization by X-ray diffraction revealed Δϕ_ Ba-122=2.7^∘ and 8^∘ for P-doped Ba-122 deposited on the well and poorly textured MgO template, respectively. The film on poor crystalline MgO template had an onset T_ c of 23 K, which is comparable to the Co-doped Ba-122 on IBAD-MgO
 (fig. <ref>(a)).
On the other hand, the P-doped Ba-122 on the well textured MgO template had a higher T_ c of 26 K (fig. <ref>(a)). 
The low-T_ c sample showed nonetheless better in-field J_ c properties: almost isotropic J_ c exceeding 0.1 MA/cm^2 at 9 T and 4 K (fig. <ref>(b)). This can be explained by grain boundary pinning. Figure <ref> shows the J_ c`-H properties of a further optimized P-doped Ba-122 thin film on IBAD-MgO (T_ c,90%Rn of 28.3 K) and the corresponding normalized pinning force density (f_ p) as a function of h=H/H_ max, where H_ max is the field at which the pinning force density (F_ p) shows the maximum value.<cit.> As can be seen, f_ p curves for H∥ c at different temperatures almost fall onto a master curve described by


    f_ p = 25/16h^0.5(1-h/5)^2

This formula is analogous to

    (H/H_ irr)^p(1-H/H_ irr)^q

(p=0.5, q=2, and H_ irr is the irreversibility field) found by Dew-Hughes for pinning by 2D defects such as grain boundaries.<cit.> Recently, Paturi et al. found that the exponent p=0.5 for YBCO regardless of q for a defect size in the order of coherence length.<cit.> On the other hand, p increases to 1 with increasing defect size. This confirms that the pinning in P-doped Ba-122 on IBAD-MgO is dominated by the dislocations with nano-size.





Hosono's group even fabricated a 10 cm long P-doped Ba-122 tape using a reel-to-reel system by PLD,<cit.> although the growth conditions were not fully optimized. The resultant film showed a slightly lower T_ c,10%Rn of 18.7 K and a low I_ c of 0.47 mA per cm-width at 4.2 K, corresponding to a J_ c of 4.7×10^4 A/cm^2. They also found that even short samples had a low T_ c. To improve the superconducting properties, they fabricated Fe_3P/P-doped Ba-122 bilayers on IBAD-MgO. As a result, the superconducting properties were improved to T_ c,10%Rn=24 K and I_ c=975 mA per cm-width (J_ c=1.75×10^5 Acm^2) at 4.2 K.




 §.§ NdFeAs(O,F)



Biaxially textured NdFeAs(O,F) films on IBAD-MgO have been fabricated by MBE at Nagoya University.<cit.> The 102 peak ϕ-scan revealed a small amount of
45^∘ rotated grains. Shown in fig. <ref> are J_ c`-H properties of NdFeAs(O,F) for H∥ c in comparison to data on PIT-processed SmFeAs(O,F) wires and a NdFeAs(O,F) film on single crystal MgO. Here, J_ c of NdFeAs(O,F) on IBAD-MgO was calculated from the hysteresis loops using the Bean model. The NdFeAs(O,F) thin film on IBAD template had a self-field
J_ c of 7×10^4 A/cm^2 at 5 K, whereas the corresponding value for PIT-processed SmFeAs(O,F) was around 4×10^4 A/cm^2 at 4.2 K.<cit.> Now the performance gap between the tape and wire is very small. However, given that J_ c of NdFeAs(O,F) on single crystalline MgO substrates is above 10^4 A/cm^2 at 4.2 K even at fields as high as at 35 T and self-field values reach 3 MA/cm^2,<cit.> there is certainly room for further improvement of superconducting properties of NdFeAs(O,F) on IBAD-MgO. The reason for the low J_ c values on IBAD templates is most likely the strong F diffusion in the GB regions, as revealed by bicrystal experiments, see above.




 §.§ Summary of FBS thin films on technical substrates




Figure <ref> compares J_ c`-H characteristics at 4.2 K of various FBS thin films on technical substrates and of high-quality FBS thin films on single-crystalline substrates. For Co-doped Ba-122, although low-field J_ c for the film on IBAD <cit.> is lower than that for the film on single crystal,<cit.> a crossover is visible at 7.5 T.
As can be seen in fig. <ref>(b), in-field J_ c performance for the P-doped Ba-122 on IBAD-MgO<cit.> is slightly inferior to the film on MgO single-crystalline substrate<cit.>(fig. <ref>(b)). Such a difference in J_ c`-H for the P-doped Ba-122 is mainly explained by the fact that the films on single-crystalline substrates are pinning-improved. It is expected that similar microstructures transferred to tapes would lead to similar J_ c levels. Additionally, the phase formation has to be optimized so that the full potential of the films on single crystals can be achieved on tape. On the other hand, J_ c`-H of Fe(Se,Te) on RABiTS <cit.> is almost comparable to that of the pinning-enhanced Fe(Se,Te) thin film <cit.> (fig. <ref>(c)). Finally, the largest discrepancy is seen for NdFeAs(O,F) on IBAD-MgO and single crystalline MgO(fig. <ref>(d)), which can be explained by the strong F diffusion along GBs and a concomitant broadening of their regions. This might be improved by employing flourine-free oxygen-deficient LnFeAsO compounds.


To evaluate the best possible J_ c performance, the ultimate critical current density, i.e., depairing current density at 4.2 K (J_ d(4.2  K)) for various FBS estimated by eqs. (4) and (5) is tabulated in table <ref>. 


    J_ d(0) = ϕ_0/3√(3)πμ_0λ_ ab^2(0)ξ_ ab(0)



    J_ d(T) =J_ d(0){1-(T/T_ c)^2}^1.5{1+(T/T_ c)^2}
 


Clearly, Ba-122 compounds possess the highest J_ d(4.2  K) among the FBS. The maximum achievable J_ c by core pinning is around 30% of the depairing current density <cit.>. However, the pinning efficiency, here defined as J_ c^ s.f./J_ d at 4.2 K for FBS even on single crystalline substrates reaches only 3.5∼5.8%. 





§ SUMMARY AND DISCUSSION

In order to understand the performance potential of FBS, we discussed the main application-related properties like pinning improvement by natural and artificial defects, the transparency of grain boundaries and the properties of GB networks. Naturally grown defects (BaFeO_2 in Co-doped Ba-122 and threading dislocations in Fe(Se,Te)) mainly contribute to the c-axis correlated pinning, and point pinning introduced by strain is beneficial as well. Artificial pinning centers can be introduced in the superconducting matrix by multilayer deposition, irradiation, and the addition of nanoparticles such as BaZrO_3 to the PLD targets. Although all methods described here are effective for improving in-field properties, the J_ c
level at 4.2 K achieved so far is still only up to ∼6% of the depairing current density in films on single crystals, suggesting room for further improvement of superconducting properties of FBS.


Bicrystal experiments on Ba-122 and Fe(Se,Te) revealed the metallic nature of grain boundaries. The critical angle above which inter-grain J_ c falls off is around 9^∘. However, the initial slope for the logarithmic J_ c decrease with GB angle for FBS is similar to that of YBCO.
A distinct difference between FBS and YBCO is recognised above θ_ GB=24^∘: for YBCO J_ c^ GB continuously decreases with θ_ GB, whilst J_ c^ GB remains constant for FBS. Overall, those properties are highly beneficial for making cheap FBS conductors.
 
   
The three main systems with largest application potential (11, 122 and 1111) have already been realized on technical substrates.
Most of the FBS coated conductor prototypes showed a comparable T_ c to their counterparts on single-crystalline substrates. 
Fe(Se,Te) and P-doped Ba-122 reached the practical level (0.1 MA/cm^2 at 15 T). It is noted that PIT-processed FBS wires (K-doped Ba-122 and Sr-122) also have reached this level,<cit.> despite the presence of many GBs. Therefore, further GB experiments on FBS, especially for K-doped 122, are desired. To further improve the in-field J_ c properties of FBS on technical substrates, the introduction of artificial pinning centers without compromising T_ c is necessary .


Even though FBS tapes will never compete with YBCO tapes regarding their superconducting properties, they could compete with YBCO in terms of cost. For YBCO tapes, expensive highly-textured metallic substrates and Ag layers to allow oxygenation are necessary. However, FBS can be realized on poorly textured templates without expensive cover layers. For realizing FBS conductors, there are other properties that need to be evaluated: i) thick FBS thin films (∼1 μm) on technical substrates should be fabricated and characterized for realizing FBS coated conductors. ii) Strain tolerance dependence of J_ c should be investigated, which is important for making a magnet.



We thank for financial support from the EU (Iron-Sea under project number FP7-283141, SUPERIRON, Grant number 283204). This work was partially supported by the JSPS Grant-in-Aid for Scientific Research (B) Grant Number 16H04646. A portion of this work was performed at the National High Magnetic Field Laboratory, which was supported by National Science Foundation Cooperative Agreement No. DMR-1644779, and the State of Florida.


105
fxundefined [1]
 ifx#1

fnum [1]
 #1firstoftwo
 secondoftwo
 
fx [1]
 #1firstoftwo
 secondoftwo
 





noop [0]secondoftwo

ref[1]@startlink#1@href
href[1]#1@endlink
anitize@url [0]`
12`$12`&12`#12`1̂2`_12`%12
startlink[1]
endlink[0]

rl [1]href #1












@bib@innerbibempty

[Kamihara et al.(2008)Kamihara, Watanabe, Hirano, and  Hosono]Kamihara
  author author Y. Kamihara, author T. Watanabe,
  author M. Hirano,  and author H. Hosono, title
  title Iron-Based Layered Superconductor
  La[O_1-xF_x]FeAs (x=0.05-0.12) with T_ c=26 K,  @noop  journal journal J. Am. Chem.
  Soc. volume 130, pages 3296–3297
  (year 2008)NoStop
[Li, Si, and Dimitrov(2011)]Li
  author author Q. Li, author W. Si,  and  author I. K. Dimitrov,  title title Films of iron chalcogenide
  superconductors, @noop  journal journal Rep. Prog. Phys. volume 74, pages 124510 (year 2011)NoStop
[Hiramatsu et al.(2012a)Hiramatsu, Katase, Kamiya, and Hosono]Hiramatsu-1
  author author H. Hiramatsu, author T. Katase,
  author T. Kamiya,  and author H. Hosono, title
  title Thin Film Growth and Device Fabrication of
  Iron-Based Superconductors, @noop  journal
  journal J. Phys. Soc. Jpn. volume
  81, pages 011011 (year
  2012a)NoStop
[Mele(2012)]Mele
  author author P. Mele, title title Superconducting
  properties of iron chalcogenide thin films, @noop  journal journal Sci. Technol. Adv. Mater. volume 13, pages 054301 (year
  2012)NoStop
[Haindl et al.(2014)Haindl,
  Kidszun, Oswald, Hess,
  Büchner, Kölling, Wilde, Thersleff, Yurchenko, Jourdan, Hiramatsu, and Hosono]Haindl
  author author S. Haindl, author M. Kidszun,
  author S. Oswald, author C. Hess, author
  B. Büchner, author
  S. Kölling, author
  L. Wilde, author T. Thersleff, author V. V.  Yurchenko, author M. Jourdan, author H. Hiramatsu,
   and author H. Hosono,  title title Thin film growth of
  Fe-based superconductors: from fundamental properties to functional devices.
  A comparative review, @noop  journal journal Rep. Prog. Phys. volume 77, pages 046502 (year 2014)NoStop
[Imai, Nabeshima, and Maeda(2017)]Imai
  author author Y. Imai, author F. Nabeshima,   and author A. Maeda, title title Comparative Review on Thin Film Growth
  of Iron-Based Superconductors, @noop  journal
  journal Condens. Matter volume 2,  pages 25 (year 2017)NoStop
[Hosono et al.(2015)Hosono,
  Tanabe, Takayama-Muromachi, Kageyama, Yamanaka, Kumakura, Nohara, Hiramatsu, and Fujitsu]Hosono-1
  author author H. Hosono, author K. Tanabe,
  author E. Takayama-Muromachi,
  author H. Kageyama, author S. Yamanaka, author
  H. Kumakura, author
  M. Nohara, author H. Hiramatsu,  and author S. Fujitsu, title title Exploration of new superconductors and functional materials, and
  fabrication of superconducting tapes and wires of iron pnictides,  @noop  journal journal Sci. Technol. Adv.
  Mater. volume 16, pages 033503
  (year 2015)NoStop
[Hosono et al.(2017)Hosono,
  Yamamoto, Hiramatsu, and Ma]Hosono-2
  author author H. Hosono, author A. Yamamoto,
  author H. Hiramatsu,  and  author Y. Ma, title title Recent advances in iron-based
  superconductors towards applications, @noop  journal journal Mater. Today volume
  21, pages 278–302 (year 2017)NoStop
[Sakoda, Iida, and Naito(2018)]Sakoda
  author author M. Sakoda, author K. Iida,  and  author M. Naito, title title Recent progress in thin-film growth of
  Fe-based superconductors: superior superconductivity achieved by thin
  films, @noop  journal journal
  Supercond. Sci. Technol.  (year 2018), note
  DOI:10.1088/1361-6668/aabddbNoStop
[Yang et al.(2017)Yang,
  Chen, Zhu, Xing, and  Wen]Yang
  author author H. Yang, author G. Chen, author X. Zhu, author
  J. Xing,  and author
  H.-H. Wen, title title BCS-like critical fluctuations with limited overlap of
  Cooper pairs in FeSe, @noop  journal journal Phys. Rev. B volume 96, pages 064501 (year 2017)NoStop
[Klein et al.(2010)Klein,
  Braithwaite, Demuer, Knafo,
  Lapertot, Marcenat, Rodière, Sheikin, Strobel, Sulpice, and Toulemonde]Klein
  author author T. Klein, author D. Braithwaite,
  author A. Demuer, author A. Knafo, author
  G. Lapertot, author
  C. Marcenat, author
  P. Rodière, author
  I. Sheikin, author P. Strobel, author P. Sulpice,  and author P. Toulemonde, title title Thermodynamic phase diagram of Fe(Se_0.5Te_0.5) single
  crystals in fields up to 28 tesla, @noop  journal
  journal Phys. Rev. B volume 82,  pages 184506 (year 2010)NoStop
[Yamamoto et al.(2009)Yamamoto, Jaroszynski, Tarantini,
  Balicas, Jiang, Gurevich,
  Larbalestier, Jin, Sefat,
  McGuire, Sales, Christen,  and Mandrus]Yamamoto
  author author A. Yamamoto, author J. Jaroszynski, author C. Tarantini, author L. Balicas,
  author J. Jiang, author A. Gurevich, author
  D. C. Larbalestier, author
  R. Jin, author A. S. Sefat, author M. A.  McGuire, author B. C.  Sales, author D. K. Christen,  and author D. Mandrus, title title Small
  anisotropy, weak thermal fluctuations, and high field superconductivity in
  Co-doped iron pnictide Ba(Fe_1-xCo_x)_2As_2, @noop 
  journal journal Appl. Phys. Lett. volume 94, pages 062511 (year
  2009)NoStop
[Chaparro et al.(2012)Chaparro, Fang, Claus, Rydh, Crabtree, Stanev, Kwok, and Welp]Chaparro
  author author C. Chaparro, author L. Fang,
  author H. Claus, author A. Rydh, author
  G. W. Crabtree, author
  V. Stanev, author W. K. Kwok,  and author U. Welp, title title
  Doping dependence of the specific heat of single-crystal
  BaFe_2(As_1-xP_x)_2, @noop  journal
  journal Phys. Rev. B volume 85,  pages 184525 (year 2012)NoStop
[Kacmarcik et al.(2009)Kacmarcik, Marcenat, Klein, Pribulova, van der Beek, Konczykowski,
  Bud'ko, Tillman, Ni, and  Canfield]Kacmarcik
  author author J. Kacmarcik, author C. Marcenat,
  author T. Klein, author Z. Pribulova, author
  C. J. van der Beek, author
  M. Konczykowski, author
  S. K. Bud'ko, author
  M. Tillman, author N. Ni,  and author P. C.  Canfield, title title
  Strongly dissimilar vortex-liquid regimes in single-crystalline NdFeAs(O,F)
  and (Ba,K)Fe_2As_2: A comparative study, @noop  journal journal Phys. Rev. B volume
  80, pages 014515 (year 2009)NoStop
[Jaroszynski et al.(2008)Jaroszynski, Riggs, Hunte, A. Gurevich, Boebinger, Balakirev,
  Migliori, Ren, Lu,
  Yang, Shen, Dong,
  Zhao, Jin, Sefat,
  McGuire, Sales, Christen,  and Mandrus]Jaroszynski
  author author J. Jaroszynski, author S. C. Riggs, author F. Hunte,
  author D. C. L. A. Gurevich,
  author G. S. Boebinger, author F. F. Balakirev, author
  A. Migliori, author
  Z. A. Ren, author W. Lu, author J. Yang, author X. L. Shen,
  author X. L. Dong, author Z. X. Zhao, author
  R. Jin, author A. S. Sefat, author M. A.  McGuire, author B. C.  Sales, author D. K. Christen,  and author D. Mandrus, title title Comparative
  high-field magnetotransport of the oxypnictide superconductors
  RFeAsO_1-xF_x (R=La, Nd) and SmFeAsO_1-δ, @noop
   journal journal Phys. Rev. B volume 78, pages 064511 (year
  2008)NoStop
[Zehetmayer et al.(2002)Zehetmayer, Eisterer, Jun, Kazakov, Karpinski, Wisniewski, and  Weber]Zehetmayer
  author author M. Zehetmayer, author M. Eisterer, author J. Jun,
  author S. M. Kazakov, author J. Karpinski, author
  A. Wisniewski,  and author
  H. W. Weber, title title Mixed-state properties of superconducting MgB_2 single
  crystal, @noop  journal journal Phys.
  Rev. B volume 66, pages 052505
  (year 2002)NoStop
[Putti et al.(2010)Putti,
  Pallecchi, Bellingeri, Cimberle, Tropeano, Ferdeghini,
  Palenzona, Tarantini, Yamamoto, Jiang, Jaroszynski, Kametani, Abraimov, Polyanskii,
  Weiss, Hellstrom, Gurevich,
  Larbalestier, Jin, Sales,
  Sefat, McGuire, Mandrus,
  Cheng, Jia, Wen,
  Lee, and Eom]Putti
  author author M. Putti, author I. Pallecchi,
  author E. Bellingeri, author M. R. Cimberle, author
  M. Tropeano, author
  C. Ferdeghini, author
  A. Palenzona, author
  C. Tarantini, author
  A. Yamamoto, author
  J. Jiang, author J. Jaroszynski, author F. Kametani, author D. Abraimov, author A. Polyanskii, author J. D.  Weiss, author E. E. Hellstrom, author A. Gurevich,
  author D. C. Larbalestier,
  author R. Jin, author
  B. C. Sales, author
  A. S. Sefat, author
  M. A. McGuire, author
  D. Mandrus, author P. Cheng, author Y. Jia, author H. H.  Wen, author S. Lee,  and author C. B. Eom, title title New Fe-based
  superconductors: properties relevant for applications, @noop 
  journal journal Supercond. Sci. Technol.  volume 23, pages 034003 (year 2010)NoStop
[Blatter et al.(1994)Blatter, Feigel'man, Geshkenbein,
  Larkin, and Vinokur]Blatter
  author author G. Blatter, author M. V. Feigel'man, author V. B. Geshkenbein, author A. I. Larkin,  and author V. M. Vinokur, title title Vortices in
  high-temperature superconductors, @noop  journal
  journal Rev. Mod. Phys. volume 66,  pages 1125–1388 (year 1994)NoStop
[Eley et al.(2017)Eley,
  Miura, Maiorov, and Civale]Eley
  author author S. Eley, author M. Miura,
  author B. Maiorov,  and  author L. Civale, title title Universal lower limit on vortex creep
  in superconductors, @noop  journal journal Na. Mater. volume 16, pages
  409–413 (year 2017)NoStop
[Lee et al.(2010)Lee,
  Jiang, Zhang, Bark,
  Weiss, Tarantini, Nelson,
  Jang, Folkman, Baek,
  Polyanskii, Abraimov, Yamamoto, Park, Pan, Hellstrom, Larbalestier, and Eom]Lee-1
  author author S. Lee, author J. Jiang, author Y. Zhang, author
  C. W. Bark, author
  J. D. Weiss, author
  C. Tarantini, author
  C. T. Nelson, author
  H. W. Jang, author
  C. M. Folkman, author
  S. H. Baek, author
  A. Polyanskii, author
  D. Abraimov, author
  A. Yamamoto, author
  J. W. Park, author
  X. Q. Pan, author E. E. Hellstrom, author D. C.  Larbalestier,  and author C. B. Eom, title title Template engineering of Co-doped BaFe_2As_2 single-crystal thin
  films, @noop  journal journal Nat.
  Mater. volume 9, pages 397–402
  (year 2010)NoStop
[Ishida et al.(2017)Ishida,
  Song, Ogino, Iyo,
  Eisaki, Nakajima, Shimoyama,  and Eisterer]Ishida
  author author S. Ishida, author D. Song,
  author H. Ogino, author A. Iyo, author
  H. Eisaki, author M. Nakajima, author J. Shimoyama,  and author M. Eisterer, title title Doping-dependent critical current properties in K, Co, and P-doped
  BaFe_2As_2 single crystals, @noop  journal
  journal Phys. Rev. B volume 95,  pages 014517 (year 2017)NoStop
[Kurth et al.(2015)Kurth,
  Tarantini, Grinenko, Hänisch, Jaroszynski, Reich,
  Mori, Sakagami, Kawaguchi,
  Engelmann, Schultz, Holzapfel, Ikuta, Hühne, and  Iida]Fritz
  author author F. Kurth, author C. Tarantini,
  author V. Grinenko, author J. Hänisch, author
  J. Jaroszynski, author
  E. Reich, author Y. Mori, author A. Sakagami, author T. Kawaguchi,
  author J. Engelmann, author L. Schultz, author
  B. Holzapfel, author
  H. Ikuta, author R. Hühne,  and author K. Iida, title title
  Unusually high critical current of clean P-doped BaFe_2As_2 single
  crystalline thin film, @noop  journal journal Appl. Phys. Lett. volume 106, pages 072602 (year 2015)NoStop
[Iida et al.(2011a)Iida, Hänisch, Schulze, Aswartham,
  Wurmehl, Büchner, Schultz, and Holzapfel]Iida-0
  author author K. Iida, author J. Hänisch,
  author M. Schulze, author S. Aswartham, author
  S. Wurmehl, author B. Büchner, author L. Schultz,  and author B. Holzapfel, title title Generic Fe buffer layers for Fe-based superconductors: Epitaxial
  FeSe_1-xTe_x thin films, @noop  journal
  journal Appl. Phys. Lett. volume 99,  pages 202503 (year 2011a)NoStop
[Sun et al.(2013)Sun,
  Taen, Tsuchiya, Ding,
  Pyon, Shi, and Tamegai]Sun
  author author Y. Sun, author T. Taen, author Y. Tsuchiya, author
  Q. Ding, author S. Pyon, author Z. Shi,  and author T. Tamegai, title title Large,
  Homogeneous, and Isotropic Critical Current Density in Oxygen-Annealed
  Fe_1+yTe_0.6Se_0.4 Single Crystal, @noop  journal journal Appl. Phys. Express volume 6, pages 043101 (year
  2013)NoStop
[Tarantini et al.(2014a)Tarantini, Iida,
  Hänisch, Kurth, Jaroszynski, Sumiya, Chihara, Hatano, Ikuta, Schmidt, Seidel, Holzapfel, and Larbalestier]Tarantini-6
  author author C. Tarantini, author K. Iida,
  author J. Hänisch, author F. Kurth, author
  J. Jaroszynski, author
  N. Sumiya, author M. Chihara, author T. Hatano, author H. Ikuta, author S. Schmidt, author P. Seidel, author B. Holzapfel,  and author D. C. Larbalestier, title title Intrinsic and extrinsic pinning in NdFeAs(O,F): vortex
  trapping and lock-in by the layered structure, @noop  journal journal Sci. Rep. volume
  6, pages 36047 (year
  2014a)NoStop
[Eisterer et al.(2014)Eisterer, Mishev, Zehetmayer, Zhigadlo, Katrych, and Karpinski]Eisterer-0
  author author M. Eisterer, author V. Mishev,
  author M. Zehetmayer, author N. D. Zhigadlo, author
  S. Katrych,  and author
  J. Karpinski, title title Critical current anisotropy in Nd-1111 single crystals
  and the influence of neutron irradiation, @noop  journal journal Supercond. Sci. Technol. volume 27, pages 044009 (year
  2014)NoStop
[Matsumoto and Mele(2010)]Matsumoto
  author author K. Matsumoto and author P. Mele, title title Artificial
  pinning center technology to enhance vortex pinning in YBCO coated
  conductors, @noop  journal journal
  Supercond. Sci. Technol. volume 23, pages 014001 (year 2010)NoStop
[Tarantini et al.(2010a)Tarantini, Lee,
  Zhang, Jiang, Bark,
  Weiss, Polyanskii, Nelson,
  Jang, Folkman, Baek,
  Pan, Gurevich, Hellstrom,
  Eom, and Larbalestier]Tarantini-0
  author author C. Tarantini, author S. Lee,
  author Y. Zhang, author J. Jiang, author
  C. W. Bark, author
  J. D. Weiss, author
  A. Polyanskii, author
  C. T. Nelson, author
  H. W. Jang, author
  C. M. Folkman, author
  S. H. Baek, author
  X. Q. Pan, author A. Gurevich, author E. E.  Hellstrom, author C. B.  Eom,  and author D. C.  Larbalestier, title title Strong vortex pinning in Co-doped BaFe_2As_2 single crystal
  thin films, @noop  journal journal
  Appl. Phys. Lett. volume 96, pages
  142510 (year 2010a)NoStop
[Tarantini et al.(2012)Tarantini, Lee, Kametani, andJ. D. Weiss, Jaroszynski, Folkman,
  Hellstrom, Eom, and Larbalestier]Tarantini-1
  author author C. Tarantini, author S. Lee,
  author F. Kametani, author J. J. andJ. D. Weiss, author J. Jaroszynski, author
  C. M. Folkman, author
  E. E. Hellstrom, author
  C. B. Eom,  and author
  D. C. Larbalestier, title
  title Artificial and self-assembled vortex-pinning
  centers in superconducting Ba(Fe_1-xCo_x)_2As_2 thin films as a
  route to obtaining very high critical-current densities, @noop 
  journal journal Phys. Rev. B volume 86, pages 214504 (year
  2012)NoStop
[Zhang et al.(2011)Zhang,
  Nelson, Lee, Jiang,
  Bark, Weiss, Tarantini,
  Folkman, Baek, Hellstrom,
  Larbalestier, Eom, and X]Zhang-Yi
  author author Y. Zhang, author C. T. Nelson,
  author S. Lee, author
  J. Jiang, author C. W. Bark, author J. D.  Weiss, author C. Tarantini, author C. M. Folkman, author S. H. Baek,
  author E. E. Hellstrom, author D. C. Larbalestier, author C. B. Eom,  and author P. X, title
  title Self-assembled oxide nanopillars in epitaxial
  BaFe_2As_2 thin films for vortex pinning, @noop  journal journal Appl. Phys. Lett. volume 98, pages 042509 (year
  2011)NoStop
[Bellingeri et al.(2012)Bellingeri, Kawale, Pallecchi,
  Gerbi, Buzio, Braccini,
  Palenzona, Putti, Adamo,
  Sarnelli, and Ferdeghini]Bellingeri
  author author E. Bellingeri, author S. Kawale,
  author I. Pallecchi, author A. Gerbi, author
  R. Buzio, author V. Braccini, author A. Palenzona, author M. Putti, author M. Adamo, author E. Sarnelli,  and author C. Ferdeghini, title title Strong vortex pinning in FeSe_0.5Te_0.5 epitaxial thin
  film, @noop  journal journal Appl.
  Phys. Lett. volume 100, pages 082601
  (year 2012)NoStop
[Sato et al.(2014)Sato,
  Hiramatsu, Kamiya, and Hosono]Sato-1
  author author H. Sato, author H. Hiramatsu,
  author T. Kamiya,  and author H. Hosono, title
  title High critical-current density with less
  anisotropy in BaFe_2(As,P)_2 epitaxial thin films: Effect of
  intentionally grown c-axis vortex-pinning centers, @noop 
  journal journal Appl. Phys. Lett. volume 104, pages 182603 (year
  2014)NoStop
[Hänisch et al.(2014)Hänisch, Iida, Kurth, Thersleff, Trommler, Reich, Hühne, Schultz, and Holzapfel]Jens
  author author J. Hänisch, author K. Iida,
  author F. Kurth, author T. Thersleff, author
  S. Trommler, author
  E. Reich, author R. Hühne, author L. Schultz,  and author B. Holzapfel, title title The effect of 45^∘ grain boundaries and associated Fe
  particles on J_ c and resistivity in
  Ba(Fe_0.9Co_0.1)_2As_2 thin films, @noop  journal journal AIP Conference Proceedings volume 1574, pages 260 (year
  2014)NoStop
[Sakagami et al.(2013)Sakagami, Kawaguchi, Tabuchi, Ujihara, Takeda, and Ikuta]Sakagami
  author author A. Sakagami, author T. Kawaguchi,
  author M. Tabuchi, author T. Ujihara, author
  Y. Takeda,  and author
  H. Ikuta, title title Critical current density and grain boundary property of
  BaFe_2(As,P)_2 thin films, @noop  journal
  journal Physica C volume 494,  pages 181–184 (year 2013)NoStop
[MacManus-Driscoll et al.(2004)MacManus-Driscoll, Foltyn, Jia,
  Wang, Serquis, Civale,
  Maiorov, Hawley, Maley, and  Peterson]Driscoll
  author author J. L. MacManus-Driscoll, author S. R. Foltyn, author Q. X. Jia,
  author H. Wang, author
  A. Serquis, author L. Civale, author B. Maiorov, author M. E.  Hawley, author M. P.  Maley,  and author D. E.  Peterson, title title
  Strongly enhanced current densities in superconducting coated conductors of
  YBa_2Cu_3O_7-x + BaZrO_3, @noop  journal
  journal Nat. Mater. volume 3,  pages 439–443 (year 2004)NoStop
[Miura et al.(2013)Miura,
  Maiorov, Kato, Shimode,
  Wada, Adachi, and Tanabe]Miura
  author author M. Miura, author B. Maiorov,
  author T. Kato, author
  T. Shimode, author K. Wada, author S. Adachi,  and author K. Tanabe, title title Strongly
  enhanced flux pinning in one-step deposition of
  BaFe_2(As_0.66P_0.33)_2 superconductor films with uniformly
  dispersed BaZrO_3 nanoparticles, @noop  journal
  journal Nat. Commun. volume 4,  pages 2499 (year 2013)NoStop
[Lee et al.(2017)Lee,
  Jiang, Kametani, Oh,
  Weiss, Collantes, Seo,
  Yoon, Tarantini, Jo,
  Hellstrom, and Lee]Lee-J
  author author J. Lee, author J. Jiang, author F. Kametani, author
  M. J. Oh, author J. D. Weiss, author Y. Collantes, author S. Seo, author S. Yoon, author C. Tarantini,
  author Y. J. Jo, author E. E. Hellstrom,  and author S. Lee, title
  title High critical current density over 1
  MAcm^-2 at 13 T in BaZrO_3 incorporated Ba(Fe,Co)_2As_2 thin
  film, @noop  journal journal
  Supercond. Sci. Technol. volume 30, pages 085006 (year 2017)NoStop
[Chepikov et al.(2017)Chepikov, Mineev, Degtyarenko,
  Lee, Petrykin, Ovcharov,
  Vasiliev, Kaul, Amelichev,
  Kamenev, Molodyk, and Samoilenkov]Chepikov
  author author V. Chepikov, author N. Mineev,
  author P. Degtyarenko, author S. Lee, author
  V. Petrykin, author
  A. Ovcharov, author
  A. Vasiliev, author
  A. Kaul, author V. Amelichev, author A. Kamenev, author A. Molodyk,  and author S. Samoilenkov, title title Introduction of BaSnO_3 and BaZrO_3 artificial pinning centres
  into 2G HTS wires based on PLD-GdBCO films. Phase I of the industrial R&D
  programme at SuperOx, @noop  journal journal Supercond. Sci. Technol. volume 30,  pages 124001 (year 2017)NoStop
[Birlik et al.(2010)Birlik,
  Erbe, Freudenberg, Celik,
  Schultz, and Holzapfel]Birlik
  author author I. Birlik, author M. Erbe,
  author T. Freudenberg, author E. Celik, author
  L. Schultz,  and author
  B. Holzapfel, title title Flux pinning improvement of ybco superconducting films
  with BaZrO_3 nanoparticles prepared by chemical solution deposition
  method, @noop  journal journal J.
  Phys. : Conf. Ser. volume 234, pages
  012004 (year 2010)NoStop
[Horide et al.(2017)Horide,
  Kametani, Yoshioka, Kitamura, and Matsumoto]Horide
  author author T. Horide, author F. Kametani,
  author S. Yoshioka, author T. Kitamura,  and author K. Matsumoto, title
  title Structural Evolution Induced by Interfacial
  Lattice Mismatch in Self-Organized YBa_2Cu_3O_7-δ Nanocomposite
  Film, @noop  journal journal ACS
  Nano volume 11, pages 1780–1788
  (year 2017)NoStop
[Goyal et al.(2005)Goyal,
  Kang, Leonard, Martin,
  Gapud, Varela, Paranthaman,
  Ijaduola, Specht, Thompson,
  Christen, Pennycook, and List]Goyal
  author author A. Goyal, author S. Kang,
  author K. J. Leonard, author P. M. Martin, author
  A. A. Gapud, author
  M. Varela, author M. Paranthaman, author A. O.  Ijaduola, author E. D.  Specht, author J. R.  Thompson, author D. K.  Christen, author S. J.  Pennycook,  and author F. A.  List, title title
  Irradiation-free, columnar defects comprised of self-assembled nanodots and
  nanorods resulting in strongly enhanced flux-pinning in
  YBa_2Cu_3O_7-δ films, @noop  journal
  journal Supercond. Sci. Technol. volume
  18, pages 1533 (year 2005)NoStop
[Lee et al.(2013)Lee,
  Tarantini, Gao, Jiang,
  Weiss, Kametani, Folkman,
  Zhang, Pan, Hellstrom,
  Larbalestier, and Eom]Lee-2
  author author S. Lee, author C. Tarantini,
  author P. Gao, author
  J. Jiang, author J. D. Weiss, author F. Kametani, author C. M.  Folkman, author Y. Zhang, author X. Q. Pan,
  author E. E. Hellstrom, author D. C. Larbalestier,  and  author C. B. Eom, title title Artificially engineered superlattices
  of pnictide superconductors, @noop  journal
  journal Nat. Mater. volume 12,  pages 392–396 (year 2013)NoStop
[Nabeshima et al.(2017)Nabeshima, Imai, Ichinose, Tsukada, and Maeda]Nabeshima
  author author F. Nabeshima, author Y. Imai,
  author A. Ichinose, author I. Tsukada,  and author A. Maeda, title
  title Growth and transport properties of FeSe/FeTe
  superlattice thin films, @noop  journal journal Jpn. J. Appl. Phys. volume 56, pages 020308 (year 2017)NoStop
[Tarantini et al.(2014b)Tarantini, Kametani, Lee, Jiang, Weiss, Jaroszynski, Hellstrom,
  Eom, and Larbalestier]Tarantini-2
  author author C. Tarantini, author F. Kametani,
  author S. Lee, author
  J. Jiang, author J. D. Weiss, author J. Jaroszynski, author E. E.  Hellstrom, author C. B.  Eom,  and author D. C.  Larbalestier, title title Development of very high J_ c in
  Ba(Fe_1-xCo_x)_2As_2 thin films grown on CaF_2, @noop
   journal journal Sci. Rep. volume 4, pages 7305 (year
  2014b)NoStop
[Engelmann et al.(2013)Engelmann, Iida, Kurth, Behler, Oswald, Hühne, Holzapfel, Schultz, and Haindl]Engelmann-1
  author author J. Engelmann, author K. Iida,
  author F. Kurth, author C. Behler, author
  S. Oswald, author R. Hühne, author B. Holzapfel, author L. Schultz,  and author D. Haindl, title title
  Fe/Ba(Fe_1-xCo_x)_2As_2 multilayers and quasi-multilayers with
  T_ c=29 K, @noop  journal journal Physica C volume 494, pages
  185–188 (year 2013)NoStop
[Chen et al.(2012)Chen,
  Tsai, Zhu, Chen,
  Bi, Lee, and Wang]Chen-Li
  author author L. Chen, author C. F. Tsai,
  author Y. Zhu, author
  A. Chen, author Z. Bi, author J. Lee,  and author H. Wang,  title title Enhanced flux pinning
  properties in superconducting FeSe_0.5Te_0.5 thin films with
  secondary phases, @noop  journal journal Supercond. Sci. Technol. volume 25,  pages 025020 (year 2012)NoStop
[Eisterer et al.(2011)Eisterer, Raunicher, Weber, Bellingeri, Cimberle, Pallecchi,
  Putti, and Ferdeghini]Eisterer-2
  author author M. Eisterer, author R. Raunicher,
  author H. W. Weber, author E. Bellingeri, author
  M. R. Cimberle, author
  I. Pallecchi, author
  M. Putti,  and author
  C. Ferdeghini, title
  title Anisotropic critical currents in
  FeSe_0.5Te_0.5 films and the influence of neutron irradiation,  @noop  journal journal Supercond. Sci.
  Technol. volume 24, pages 065016
  (year 2011)NoStop
[Ozaki et al.(2016)Ozaki,
  Wu, Zhang, Jaroszynski,
  Si, Zhou, Zhu, and  Li]Ozaki
  author author T. Ozaki, author L. Wu, author C. Zhang, author
  J. Jaroszynski, author
  W. Si, author J. Zhou, author Y. Zhu,  and author Q. Li,  title title A route for a strong
  increase of critical current in nanostrained iron-based superconductors,  @noop  journal journal Nat. Commun.  volume 7, pages 13036 (year
  2016)NoStop
[Ozaki et al.(2018)Ozaki,
  Wu, Zhang, Si, Jie, and Li]Ozaki-2
  author author T. Ozaki, author L. Wu, author C. Zhang, author
  W. Si, author Q. Jie,  and author Q. Li, title title
  Enhanced critical current in superconducting FeSe_0.5Te_0.5 films
  at all magnetic field orientations by scalable gold ion irradiation,  @noop  journal journal Supercond. Sci.
  Technol. volume 31, pages 024002
  (year 2018)NoStop
[Ahmad et al.(2017)Ahmad,
  Choi, Seo, Seo, Lee, Park, Mosqueira, Gu,  and Kwon]Ahmad
  author author D. Ahmad, author W. J. Choi,
  author Y. I. Seo, author S. Seo, author
  S. Lee, author T. Park, author J. Mosqueira, author G. Gu,   and author Y. S. Kwon,  title title Effect or proton
  irradiation on the fluctuation-induced magnetoconductivity of
  FeSe_1-xTe_x thin films, @noop  journal
  journal New J. Phys. volume 19,  pages 093004 (year 2017)NoStop
[Sylva et al.(2018)Sylva,
  Bellingeri, Ferdeghini, Martinelli, Pallecchi, Pellegrino,
  Putti, Ghigo, Gozzelino,
  Torsello, Grimaldi, Leo,
  Nigro, and Braccini]Sylva
  author author G. Sylva, author E. Bellingeri,
  author C. Ferdeghini, author A. Martinelli, author
  I. Pallecchi, author
  L. Pellegrino, author
  M. Putti, author G. Ghigo, author L. Gozzelino, author D. Torsello, author G. Grimaldi, author A. Leo, author A. Nigro,  and author V. Braccini, title title Effects of
  high energy proton irradiation on the superconducting properties of Fe(Se,Te)
  thin films, @noop  journal journal
  Supercond. Sci. Technol.  (year 2018), note
  https://doi.org/10.1088/1361-6668/aab3bdNoStop
[Janaki et al.(2014)Janaki,
  Kumar, Mani, Amaladass,
  Raveendran, Magudapathy, Kalavathi, Sairam, and Ravindran]Janaki
  author author J. Janaki, author T. Kumar,
  author A. Mani, author
  E. P. Amaladass, author
  N. Raveendran, author
  P. Magudapathy, author
  S. Kalavathi, author
  T. N. Sairam,  and author
  T. R. Ravindran, title
  title Characterization and Low Temperature Study of
  Iron Telluride Thin Films Upon Ageing and Oxygen Ion Irradiation,  @noop  journal journal J. Supercond. Nov.
  Magn. volume 27, pages 2639–2643
  (year 2014)NoStop
[Maiorov et al.(2012)Maiorov, Katase, Usov, Weigand, Civale, Hiramatsu, and  Hosono]Boris
  author author B. Maiorov, author T. Katase,
  author O. Usov, author
  M. Weigand, author L. Civale, author H. Hiramatsu,  and author H. Hosono, title title
  Competition and cooperation of pinning by extrinsic point-like defects and
  intrinsic strong columnar defects in BaFe_2As_2 thin films,  @noop  journal journal Phys. Rev. B  volume 86, pages 094513 (year 2012)NoStop
[Schilling et al.(2016)Schilling, Baumgartner, Gorshunov,
  Zhukova, Dravin, Misten,
  Efremov, Dolgov, Iida,
  Dressel, and Zapf]Schilling
  author author M. B. Schilling, author A. Baumgartner, author B. Gorshunov, author E. S. Zhukova, author V. A. Dravin,
  author K. V. Misten, author D. V. Efremov, author
  O. V. Dolgov, author
  K. Iida, author M. Dressel,  and author S. Zapf, title title
  Tracing the s± symmetry in iron pnictides by controlled disorder,  @noop  journal journal Phys. Rev. B  volume 93, pages 174515 (year 2016)NoStop
[Daghero et al.(2017)Daghero, Tortello, Gozzelino, Gonnelli, Hatano, Kawaguchi, and  Ikuta]Daghero-1
  author author D. Daghero, author M. Tortello,
  author L. Gozzelino, author R. S. Gonnelli, author
  T. Hatano, author T. Kawaguchi,  and author H. Ikuta, title title
  Effect of ion irradiation on surface morphology and superconductivity of
  BaFe_2(As_1-xP_x)_2 films, @noop  journal
  journal Appl. Surf. Sci. volume 395,  pages 9–15 (year 2017)NoStop
[Tarantini et al.(2018)Tarantini, Iida, Sumiya, Chihara, Hatano, Ikuta, Singh, Newman, and Larbalestier]Tarantini-4
  author author C. Tarantini, author K. Iida,
  author N. Sumiya, author C. Chihara, author
  T. Hatano, author H. Ikuta, author R. K.  Singh, author N. Newman,  and author D. C. Larbalestier, title title Effect of
  α-particle irradiation on a NdFeAs(O,F) thin film, @noop 
  journal journal Supercond. Sci. Technol.  volume 31, pages 034002 (year 2018)NoStop
[Eisterer(2018)]Eisterer-1
  author author M. Eisterer, title title Radiation
  effects on iron-based superconductors, @noop  journal journal Supercond. Sci. Technol. volume 31, pages 013001 (year
  2018)NoStop
[Efremov et al.(2011)Efremov, Korshunov, Dolgov, Golubov, and Hirschfeld]Efremov
  author author D. V. Efremov, author M. M. Korshunov, author O. V. Dolgov, author A. A. Golubov,
   and author P. J. Hirschfeld,  title title Disorder-induced transition
  between s± and s++ states in two-band superconductors, @noop
   journal journal Phys. Rev. B volume 84, pages 180512 (year
  2011)NoStop
[Tarantini et al.(2010b)Tarantini, Putti,
  Gurevich, Shen, Singh,
  Rowell, Newman, Larbalestier,
  Cheng, Jia, and Wen]Tarantini-3
  author author C. Tarantini, author M. Putti,
  author A. Gurevich, author Z. Shen, author
  R. K. Singh, author
  J. M. Rowell, author
  N. Newman, author D. C. Larbalestier, author P. Cheng, author Z. Jia,  and author H.-H.  Wen, title title
  Suppression of the Critical Temperature of Superconducting NdFeAs(O,F)
  Single Crystals by Kondo-Like Defects Sites Induced by α-Particle
  Irradiation, @noop  journal journal
  Phys. Rev. Lett. volume 104, pages
  087002 (year 2010b)NoStop
[Held et al.(2009)Held,
  Schneider, Mannhart, Allard,
  More, and Goyal]Held
  author author R. Held, author C. W. Schneider,
  author J. Mannhart, author L. F. Allard, author
  K. L. More,  and author
  A. Goyal, title title Low-angle grain boundaries in YBa_2Cu_3O_7-δ
  with high critical current densities, @noop  journal journal Phys. Rev. B volume
  79, pages 014515 (year 2009)NoStop
[Omura et al.(2017)Omura,
  Matsumoto, Hatano, Iida,  and Ikuta]Omura
  author author T. Omura, author T. Matsumoto,
  author T. Hatano, author K. Iida,  and author
  H. Ikuta, title title Fabrication of grain boundary junctions using NdFeAs(O,F)
  superconducting thin films, @noop    (year
  2017), http://arxiv.org/abs/arXiv:1712.05966 arXiv:1712.05966
  NoStop
[Deutscher(2010)]Deutscher
  author author G. Deutscher, title title Origin of
  weak-link behavior of grain boundaries in superconducting cuprates and
  pnictides, @noop  journal journal
  Appl. Phys. Lett. volume 96, pages
  122502 (year 2010)NoStop
[Durrell et al.(2011)Durrell, Eom, Gurevich, Hellstrom, Tarantini, Yamamoto, and  Larbalestier]John
  author author J. H. Durrell, author C.-B. Eom,
  author A. Gurevich, author E. E. Hellstrom, author
  C. Tarantini, author
  A. Yamamoto,  and author
  D. C. Larbalestier, title
  title The behavior of grain boundaries in the
  Fe-based superconductors, @noop  journal journal Rep. Prog. Phys. volume 74, pages 124511 (year 2011)NoStop
[Sarnelli et al.(2014)Sarnelli, Adamo, Nappi, Braccini, Kawale, Bellingeri, and  Ferdeghini]Sarnelli-1
  author author E. Sarnelli, author M. Adamo,
  author C. Nappi, author V. Braccini, author
  S. Kawale, author E. Bellingeri,  and author C. Ferdeghini, title title Properties of high-angle Fe(Se,Te) bicrystal grain boundary
  junctions, @noop  journal journal
  Appl. Phys. Lett. volume 104, pages
  162601 (year 2014)NoStop
[Sarnelli et al.(2017a)Sarnelli, Nappi,
  Camerlingo, Enrico, Bellingeri, Kawale, Braccini, Leveratto, and Ferdeghini]Sarnelli-2
  author author E. Sarnelli, author C. Nappi,
  author C. Camerlingo, author E. Enrico, author
  E. Bellingeri, author
  S. Kawale, author V. Braccini, author A. Leveratto,  and author C. Ferdeghini, title title Properties of Fe(Se,Te) Bicrystal Grain Boundary Junctions, SQUIDs,
  and Nanostrips, @noop  journal journal
  IEEE Trans. Appl. Supercond. volume 27, pages 7400104 (year 2017a)NoStop
[Sarnelli et al.(2017b)Sarnelli, Nappi,
  Leveratto, Bellingeri, Braccini, and Ferdeghini]Sarnelli-3
  author author E. Sarnelli, author C. Nappi,
  author A. Leveratto, author E. Bellingeri, author
  V. Braccini,  and author
  C. Ferdeghini, title
  title Fe(Se,Te) superconducting quantum interference
  devices, @noop  journal journal
  Supercond. Sci. Technol. volume 30, pages 065003 (year 2017b)NoStop
[Si et al.(2015)Si,
  Zhang, Shi, Ozaki,
  Jaroszynski, and Li]Si-1
  author author W. Si, author C. Zhang, author X. Shi, author
  T. Ozaki, author J. Jaroszynski,  and author Q. Li, title title
  Grain boundary junctions of FeSe_0.5Te_0.5 thin films on SrTiO_3
  bi-crystal substrates, @noop  journal journal Appl. Phys. Lett. volume 106, pages 032602 (year 2015)NoStop
[Lee et al.(2009)Lee,
  Jiang, Weiss, Folkman,
  Bark, Tarantini, Xu,
  Abraimov, Polyanskii, Nelson,
  Zhang, Baek, Jang,
  Yamamoto, Kametani, Pan,
  Hellstrom, Gurevich, Eom,  and Larbalestier]Lee-3
  author author S. Lee, author J. Jiang, author J. D. Weiss, author
  C. M. Folkman, author
  C. W. Bark, author
  C. Tarantini, author
  A. Xu, author D. Abraimov, author A. Polyanskii, author C. T.  Nelson, author Y. Zhang, author S. H. Baek,
  author H. W. Jang, author A. Yamamoto, author
  F. Kametani, author
  X. Q. Pan, author E. E. Hellstrom, author A. Gurevich, author C. B.  Eom,  and author D. C.  Larbalestier, title title Weak-link behavior of grain boundaries in superconducting
  Ba(Fe_0.5Co_x)_2As_2 bicrystals, @noop  journal journal Appl. Phys. Lett. volume 95, pages 212505 (year
  2009)NoStop
[Schmidt et al.(2013)Schmidt, Döring, Schmidl, Tympel, Haindl, Iida, Kurth, Holzapfel, and Seidel]Schmidt-1
  author author S. Schmidt, author S. Döring,
  author F. Schmidl, author V. Tympel, author
  S. Haindl, author K. Iida, author F. Kurth, author B. Holzapfel,   and author P. Seidel,  title title Bicrystalline Grain
  Boundary and Hybrid SNS Junctions Based on Ba-122 Thin Films, @noop
   journal journal IEEE Trans. Appl. Supercond.  volume 23, pages 7300104 (year 2013)NoStop
[Katase et al.(2013)Katase,
  Ishimaru, Tsukamoto, Hiramatsu, Kamiya, Tanabe, and  Hosono]Katase-1
  author author T. Katase, author Y. Ishimaru,
  author A. Tsukamoto, author H. Hiramatsu, author
  T. Kamiya, author K. Tanabe,  and author H. Hosono, title title
  Advantageous grain boundaries in iron pnictide superconductors,  @noop  journal journal Nat. Commun.  volume 2, pages 409 (year
  2013)NoStop
[Hiramatsu et al.(2012b)Hiramatsu, Katase, Ishimaru, Tsukamoto, Kamiya, Tanabe, and Hoson]Hiramatsu-2
  author author H. Hiramatsu, author T. Katase,
  author Y. Ishimaru, author A. Tsukamoto, author
  T. Kamiya, author K. Tanabe,  and author H. Hoson, title title
  Microstructure and transport properties of [001]-tilt bicrystal grain
  boundaries in iron pnictide superconductor, cobalt-doped BaFe_2As_2,  @noop  journal journal Mater. Sci. Eng.,
  B volume 177, pages 515–519
  (year 2012b)NoStop
[Iida et al.(2012)Iida,
  Haindl, Kurth, Hänisch,
  Schultz, and Holzapfel]Iida-1
  author author K. Iida, author S. Haindl,
  author F. Kurth, author J. Hänisch, author
  L. Schultz,  and author
  B. Holzapfel, title title BaFe_2As_2/Fe bilayers with [001]-tilt grain boundary
  on MgO and SrTiO_3 bicrystal substrates, @noop  journal journal Physics Procedia volume 45, pages 189–192 (year
  2012)NoStop
[Katase et al.(2010a)Katase, Ishimaru,
  Tsukamoto, Hiramatsu, Kamiya,
  Tanabe, and Hosono]Katase-junction-1
  author author T. Katase, author Y. Ishimaru,
  author A. Tsukamoto, author H. Hiramatsu, author
  T. Kamiya, author K. Tanabe,  and author H. Hosono, title title
  Josephson junction in cobalt-doped BaFe_2As_2 epitaxial thin films on
  (La,Sr)(Al,Ta)O_3 bicrystal substrates, @noop  journal journal Appl. Phys. Lett. volume 96, pages 142507 (year
  2010a)NoStop
[Katase et al.(2010b)Katase, Ishimaru,
  Tsukamoto, Hiramatsu, Kamiya,
  Tanabe, and Hosono]Katase-junction-2
  author author T. Katase, author Y. Ishimaru,
  author A. Tsukamoto, author H. Hiramatsu, author
  T. Kamiya, author K. Tanabe,  and author H. Hosono, title title
  DC superconducting quantum interference devices fabricated using bicrystal
  grain boundary junctions in Co-doped BaFe_2As_2 epitaxial films,  @noop  journal journal Supercond. Sci.
  Technol. volume 23, pages 082001
  (year 2010b)NoStop
[Schmidt et al.(2014)Schmidt, Döring, Schmidl, Kurth, Iida, Holzapfel, Kawaguchi, Mori, Ikuta, and Seidel]Schmidt-2
  author author S. Schmidt, author S. Döring,
  author F. Schmidl, author F. Kurth, author
  K. Iida, author B. Holzapfel, author T. Kawaguchi, author Y. Mori, author H. Ikuta,  and author P. Seidel, title title Bicrystalline
  Grain Boundary Junctions of Co-doped and P-doped Ba-122 Thin Films,  @noop  journal journal J. Phys.: Conf.
  Ser. volume 507, pages 012046
  (year 2014)NoStop
[Seidel(2011)]Seidel-1
  author author P. Seidel, title title Josephson
  effects in iron based superconductors, @noop  journal journal Supercond. Sci. Technol. volume 24, pages 043001 (year
  2011)NoStop
[Kim et al.(2014)Kim,
  Weiss, Hellstrom, Larbalestier, and Seidman]Kim
  author author Y.-J. Kim, author J. D. Weiss,
  author E. E. Hellstrom, author D. C. Larbalestier,  and  author D. N. Seidman,  title title Evidence for composition
  variations and impurity segregation at grain boundaries in high
  current-density polycrystalline K- and Co-doped BaFe_2As_2
  superconductors, @noop  journal journal Appl. Phys. Lett. volume 105, pages 162604 (year 2014)NoStop
[Weiss et al.(2012)Weiss,
  Tarantini, Jiang, Kametani,
  Polyanskii, Larbalestier, and Hellstrom]Weiss
  author author J. D. Weiss, author C. Tarantini,
  author J. Jiang, author F. Kametani, author
  A. A. Polyanskii, author
  D. C. Larbalestier,  and author E. E. Hellstrom, title title High intergrain critical current
  density in fine-grain (Ba_0.6K_0.4)Fe_2As_2 wires and bulks,  @noop  journal journal Nat. Mater.  volume 11, pages 682–685 (year 2012)NoStop
[Weiss et al.(2015)Weiss,
  Yamamoto, Polyanskii, Richardson, Larbalestier, and Hellstrom]Weiss-2
  author author J. D. Weiss, author A. Yamamoto,
  author A. A. Polyanskii,
  author R. B. Richardson,
  author D. C. Larbalestier,   and author E. E. Hellstrom,  title title Demonstration of an
  iron-pnictide bulk superconducting magnet capable of trapping over 1 T,  @noop  journal journal Supercond. Sci.
  Technol. volume 28, pages 112001
  (year 2015)NoStop
[Zhang et al.(2015)Zhang,
  Lin, Yuan, Zhang,
  Yao, Wang, Dong,
  Ma, Awaji, and Watanabe]Zhang
  author author Q. Zhang, author H. Lin, author P. Yuan, author
  X. Zhang, author C. Yao, author D. Wang, author C. Dong, author Y. Ma, author
  S. Awaji,  and author
  K. Watanabe, title title Low-temperature synthesis to achieve high critical
  current density and avoid a reaction layer in SmFeAsO_1-xF_x
  superconducting tapes, @noop  journal journal Supercond. Sci. Technol. volume 28,  pages 6105005 (year 2015)NoStop
[Hilgenkamp and Mannhart(2002)]Hilgenkamp
  author author H. Hilgenkamp and author J. Mannhart, title title Grain
  boundaries in high–T_ c superconductors, @noop 
  journal journal Rev. Mod. Phys. volume 74, pages 485–549 (year
  2002)NoStop
[Graser et al.(2010)Graser,
  Hirschfeld, Kopp, Gutser,
  Andersen, and Mannhart]Graser
  author author S. Graser, author P. J. Hirschfeld, author T. Kopp,
  author R. Gutser, author B. M. Andersen,  and author J. Mannhart, title
  title How grain boundaries limit supercurrents in
  high-temperature superconductors, @noop  journal
  journal Nat. Phys. volume 6,  pages 609–614 (year 2010)NoStop
[Sato et al.(2016)Sato,
  Hiramatsu, Kamiya, and Hosono]Sato-2
  author author H. Sato, author H. Hiramatsu,
  author T. Kamiya,  and author H. Hosono, title
  title Enhanced critical-current in P-doped
  BaFe_2As_2 thin films on metal substrates arising from poorly aligned
  grain boundaries, @noop  journal journal Sci. Rep. volume 6, pages
  36828 (year 2016)NoStop
[Iida et al.(2017)Iida,
  Sato, Tarantini, Hänisch,
  Jaroszynski, Hiramatsu, Holzapfel, and Hosono]Iida-3
  author author K. Iida, author H. Sato, author C. Tarantini, author
  J. Hänisch, author
  J. Jaroszynski, author
  H. Hiramatsu, author
  B. Holzapfel,  and author
  H. Hosono, title title High-field transport properties of a P-doped
  BaFe_2As_2 film on technical substrate, @noop  journal journal Sci. Rep. volume
  7, pages 39951 (year 2017)NoStop
[Chan(1994)]Chan
  author author S.-W. Chan, title title Nature of grain
  boundaries as related to critical currents in superconducting
  YBa_2Cu_3O_7-x, @noop  journal journal J. Phys. Chem. Solids volume 55,  pages 1415–1432 (year 1994)NoStop
[Hammerl et al.(2000)Hammerl, Schmehl, Schultz, Götz, Bielefeldt, Schneider,
  Hilgenkamp, and Mannhart]Hammerl
  author author G. Hammerl, author A. Schmehl,
  author R. R. Schultz, author B. Götz, author
  H. Bielefeldt, author
  C. W. Schneider, author
  H. Hilgenkamp,  and author
  J. Mannhart, title title Enhanced supercurrent density in polycrystalline
  YBa_2Cu_3O_7-δ at 77 K from calcium doping grain
  boundaries, @noop  journal journal
  Nature volume 407, pages 162–164
  (year 2000)NoStop
[Song et al.(2005)Song,
  Daniels, Feldmann, Gurevich,  and Larbalestier]Song
  author author X. Song, author G. Daniels,
  author D. M. Feldmann, author A. Gurevich,  and author D. C. Larbalestier, title title Electromagnetic, atomic structure and
  chemistry changes induced by Ca-doping of low-angle
  YBa_2Cu_3O_7-δ grain boundaries, @noop  journal journal Nat. Mater. volume
  4, pages 470–475 (year 2005)NoStop
[Si et al.(2011)Si,
  Zhou, Jie, Dimitrov,
  Solovyov, Johnston, Jaroszynski, Matias, Sheehan, and  Li]Si-2
  author author W. Si, author J. Zhou, author Q. Jie, author
  I. Dimitrov, author
  V. Solovyov, author
  P. D. Johnston, author
  J. Jaroszynski, author
  V. Matias, author C. Sheehan,  and author Q. Li, title title
  Iron-chalcogenide FeSe_0.5Te_0.5 coated superconducting tapes for
  high field applications, @noop  journal journal Appl. Phys. Lett. volume 98, pages 262509 (year 2011)NoStop
[Si et al.(2013)Si,
  Han, Shi, Ehrlich,
  Jaroszynski, Goyal, and Li]Si-3
  author author W. Si, author S. J. Han,
  author X. Shi, author
  S. N. Ehrlich, author
  J. Jaroszynski, author
  A. Goyal,  and author
  Q. Li, title title High current superconductivity in
  FeSe_0.5Te_0.5-coated conductors at 30 tesla, @noop 
  journal journal Nat. Commun. volume 4, pages 1347 (year 2013)NoStop
[Xu et al.(2017)Xu,
  Yuan, Ma, and Cai]Xu
  author author Z. Xu, author P. Yuan, author Y. Ma,  and author
  C. Cai, title title High-performance FeSe_0.5Te_0.5 thin films
  fabricated on less-well-textured flexible coated conductor templates,  @noop  journal journal Supercond. Sci.
  Technol. volume 30, pages 035003
  (year 2017)NoStop
[Braccini(2017)]Braccini
  author author V. Braccini, @noop  title Fe(Se,Te)
  epitaxial thin films deposited on flexible metallic substrates, 
  (year 2017), note 13 th biennial European
  Conference on Applied Superconductivity, EUCAS 2017, Geneva,
  SwitzerlandNoStop
[Demura et al.(2016)Demura,
  Tanaka, Yamashita, Denholme,
  Okazaki, Fujioka, Yamaguchi,
  Takeya, Iida, Holzapfel,
  Sakata, and Takano]Demura
  author author S. Demura, author M. Tanaka,
  author A. Yamashita, author S. J. Denholme, author
  H. Okazaki, author M. Fujioka, author T. Yamaguchi, author H. Takeya, author K. Iida, author B. Holzapfel, author H. Sakata,   and author Y. Takano,  title title Electrochemical Deposition
  of FeSe on RABiTS Tapes, @noop  journal journal J. Phys. Soc. Jpn. volume 85, pages 015001 (year 2016)NoStop
[Yamashita et al.(2018)Yamashita, Matsumoto, Tanaka, Hara, Iida, Holzapfel, Takeya, and Takano]Yamashita
  author author A. Yamashita, author R. Matsumoto, author M. Tanaka,
  author H. Hara, author
  K. Iida, author B. Holzapfel, author H. Takeya,  and author Y. Takano, title title
  Observation of zero resistance in as-electrodeposited FeSe, @noop
   journal journal Solid State Commun. volume 270, pages 72–75 (year
  2018)NoStop
[Iida et al.(2011b)Iida, Hänisch, Trommler, Matias,
  Haindl, Kurth, del Pozo,
  Hühne, Kidszun, Engelmann, Schultz, and Holzapfel]Iida-2
  author author K. Iida, author J. Hänisch,
  author S. Trommler, author V. Matias, author
  S. Haindl, author F. Kurth, author I. L.  del Pozo, author R. Hühne, author M. Kidszun,
  author J. Engelmann, author L. Schultz,  and author B. Holzapfel, title
  title Epitaxial Growth of Superconducting
  Ba(Fe_1-xCo_x)_2As_2 Thin Films on Technical Ion Beam Assisted
  Deposition MgO Substrates, @noop  journal journal Appl. Phys. Express volume 4, pages 013103 (year 2011b)NoStop
[Trommler et al.(2012)Trommler, Hänisch, Matias,
  Hühne, Reich, Iida,
  Haindl, Shultz, and Holzapfel]Trommler
  author author S. Trommler, author J. Hänisch, author V. Matias,
  author R. Hühne, author E. Reich, author
  K. Iida, author S. Haindl, author L. Shultz,  and author B. Holzapfel, title title Architecture, microstructure and J_ c anisotropy of highly
  oriented biaxially textured Co-doped BaFe_2As_2 on Fe/IBAD-MgO-buffered
  metal tapes, @noop  journal journal
  Supercond. Sci. Technol. volume 25, pages 084019 (year 2012)NoStop
[Katase et al.(2011)Katase,
  Hiramatsu, Matias, Sheehan,
  Ishimaru, Kamiya, Tanabe,  and Hosono]Katase-2
  author author T. Katase, author H. Hiramatsu,
  author V. Matias, author C. Sheehan, author
  Y. Ishimaru, author
  T. Kamiya, author K. Tanabe,  and author H. Hosono, title title
  Biaxially textured cobal-doped BaFe_2As_2 films with high critical
  current density over 1 MA/cm^2 on MgO-buffered metal-tape flexible
  substrates, @noop  journal journal
  Appl. Phys. Lett. volume 98, pages
  242510 (year 2011)NoStop
[Xu et al.(2018)Xu,
  Yuan, Fan, Chen, and  Ma]Zhongtang-Xu
  author author Z. Xu, author P. Yuan, author F. Fan, author
  Y. Chen,  and author
  Y. Ma, title title Transport properties and pinning analysis for Co-doped
  BaFe_2As_2 thin films on metal tapes, @noop  journal journal Supercond. Sci. Technol. volume 31, pages 055001 (year
  2018)NoStop
[Iida et al.(2014)Iida,
  Kurth, Chihara, Sumiya,
  Grinenko, Ichinose, Tsukada,
  Hänisch, Matias, Hatano,
  Holzapfel, and Ikuta]Iida-4
  author author K. Iida, author F. Kurth,
  author M. Chihara, author N. Sumiya, author
  V. Grinenko, author
  A. Ichinose, author
  I. Tsukada, author J. Hänisch, author V. Matias, author T. Hatano, author B. Holzapfel,  and author H. Ikuta, title title
  Highly textured oxypnictide superconducting films on metal substrates,  @noop  journal journal Appl. Phys.
  Lett. volume 105, pages 172602
  (year 2014)NoStop
[Huang et al.(2018)Huang,
  Yao, Dong, Zhang,
  Wang, Cheng, Li,
  Awaji, Wen, and Ma]Huang-He
  author author H. Huang, author C. Yao, author C. Dong, author
  X. Zhang, author D. Wang, author Z. Cheng, author J. Li, author S. Awaji, author
  H. Wen,  and author
  Y. Ma, title title High transport current superconductivity in
  powder-in-tube Ba_0.6K_0.4Fe_2As_2 tapes at 27 T,  @noop  journal journal Supercond. Sci.
  Technol. volume 31, pages 015017
  (year 2018)NoStop
[Zhang et al.(2017a)Zhang, Oguro,
  Yao, Dong, Xu, Wang, Awaji, Watanabe, and Ma]Zhang-x
  author author X. Zhang, author H. Oguro,
  author C. Yao, author
  C. Dong, author Z. Xu, author D. Wang, author S. Awaji,
  author K. Watanabe,  and  author Y. Ma, title title Superconducting Properties of 100-m
  Class Sr_0.6K_0.4Fe_2As_2 Tape and Pancake Coils superconducting
  tapes, @noop  journal journal IEEE
  Trans. Appl. Supercond. volume 27, pages 7300705 (year 2017a)NoStop
[Dew-Hughes(1974)]Hughes
  author author D. Dew-Hughes, title title Flux
  pinning mechanism in type II superconductors, @noop  journal journal Philos. Mag. volume
  30, pages 293–305 (year 1974)NoStop
[Paturi et al.(2016)Paturi,
  Malmivirta, Palonen, and Huhtinen]Paturi
  author author P. Paturi, author M. Malmivirta,
  author H. Palonen,  and  author H. Huhtinen, title title Dopant Diameter Dependence of J_
  c(B) in Doped YBCO Films, @noop  journal
  journal IEEE Trans. Appl. Supercond. volume 26, pages 8000705 (year
  2016)NoStop
[Zhang et al.(2017b)Zhang, Zhang,
  Yao, Huang, Wang,
  Dong, Ma, Ogino, and  Awaji]Q-Zhang
  author author Q. Zhang, author X. Zhang,
  author C. Yao, author
  H. Huang, author D. Wang, author C. Dong, author Y. Ma, author H. Ogino,  and author
  S. Awaji, title title Enhanced transport current density in Sn-doped
  SmFeAsO_1-xF_x tapes prepared by the PIT method, @noop 
  journal journal Supercond. Sci. Technol.  volume 30, pages 065004 (year 2017b)NoStop
[Wimbush(2015)]Paul-Seidel
  author author S. C. Wimbush, title Flux pinning in Applied
  Superconductivity Handbook on Devices and Applications,  (publisher WILEY, year 2015) Chap. chapter 1,
  pp. pages 85–87NoStop
[Prozorov et al.(2009)Prozorov, Tanatar, Gordon, Martin, Kim, Kogan, Ni,
  Tillman, Bud'ko, and Canfield]Porozorov
  author author R. Prozorov, author M. A. Tanatar, author R. T. Gordon,
  author C. Martin, author H. Kim, author
  V. G. Kogan, author
  N. Ni, author M. E. Tillman, author S. K.  Bud'ko,  and author P. C.  Canfield, title title
  Anisotropic London penetration depth and superfluid density in single
  crystals of iron-based pnictide superconductors, @noop 
  journal journal Physica C volume 469, pages 582–589 (year
  2009)NoStop


