[1] Y. Mizuguchi et al., Appl. Phys. Lett. 93, 152505 (2008).
[2] S. Margadonna et al., Phys. Rev. B 80, 064506 (2009).
[3] S. Masaki et al., J. Phys. Soc. Jpn. 78, 063704 (2009).
[4] T. Ozaki et al., Euro. Phys. Lett., 98, 27002 (2012).

Superconductivity in Fe-based superconductors emerges in the vicinity of magnetic and nematic phases. Usually, the transition temperatures of those two latter states are very close to each other. This situation obscures the leading degree of freedom which drives the phase diagram. Here we show that the splitting between the nematic and the magnetic transition temperatures is enlarged considerably in the case of strained Ba(Fe1-xCox)2As2 epitaxial thin films that makes it possible to investigate those phenomena independently. We suggest the presence of two quantum critical points in strained films, corresponding to the magnetic and the nematic states, based on the temperature dependence of the resistivity and the Hall effect. The band structure calculations indicate that in-plain strain modifies the electronic structure and hence affects the whole phase diagram. The transition temperatures of the magnetic, nematic, and superconducting states for the strained films as well as single crystals (i.e., fully relaxed) scale with the Hall number. The scaling indicates that the phase diagram of the Ba(Fe1-xCox)2As2 system is controlled by its crystal structure and electron count whereas the disorder produced by Co doping does not play a major role.

The authors thank Helge Rosner and Andrey Chubukov for fruitful discussions, Juliane Scheiter for preparing FIB cuts, and Michael Kuehnel and Ulrike Besold for their technical support.

It is believed that spin fluctuations at the nesting vector (π; π) connecting the hole and electron Fermi surfaces (FS) are responsible for the superconducting pairing in iron pnictides. In this case the superconducting gap has an opposite sign on the hole and electron FS sheets. For this unconventional s_± superconductivity theory predicts that interband impurity scattering does not enhance the slope of the upper critical field H_c2 at T_c [1], in contrast to the s₊₊ case [2]. Indeed, the scaling dH_c2/dT~ T_c holds for the 1111 family for the large range of T_c. However, for the 122 family the scaling was observed with significant scattering, only [1]. Here we investigate the effect of disorder on the dH_c2/dTat T_c for BaFe₂(As,P)₂ thin films with various doping levels prepared by molecular beam epitaxy (MBE) and pulsed laser deposition (PLD). The MBE films are single crystalline layers with a very low amount of defects [3]. It was found that the dH_c2^||c/dT of these films is comparable to that of single crystals. In contrast, our PLD films contain a large amount of defects such as dislocations, low angle grain boundaries (<2˚), and impurity phases. This leads to a significantly lower residual resistivity ratio (RRR = R₃₀₀/R_Tc) as compared to MBE films. The T_c are 31K at optimum doping for the MBE films and about 26K for the PLD films. However, the PLD films exhibit a higher -dH_c2/dT as compared to the MBE in spite of the lower T_c. The enhancement of the slope with disorder is typical for conventional s₊₊ superconductivity. Taking into account a random nature of the disorder we suppose that various types of scattering processes (intra- and inter- band) happen in our PLD films. Therefore, our data are inconsistent with a simple d or s_± superconductivity in BaFe₂(As,P)₂ system and indicate that intraband interactions such as electron- phonon or/and electron- orbital fluctuations with short scattering momenta involved are also relevant for the superconductivity in this system. These findings are also relevant for practical applications using BaFe₂(As,P)₂ since it opens a possible avenue to enhance H_c2 by introducing disorder to the material without a sizable penalty for T_c.
1. V. G. Kogan PRB 80, 214532 (2009).
2. A. Gurevich PRB 67, 184515 (2003).
3. F. Kurth et al., APL 106, 072602 (2015).

We acknowledge Dr. H. Fujii, Dr. H. Takeya, Dr. S. J. Ye and Miss Y. C. Zhang of the National Institute for Materials Science for their assistance in the experiments.

Very recently a new type of iron based superconductors (IBS) have been discovered, namely the 112 type Ca_1-xRE_xFeAs₂ with RE = La, Ce, Pr, Nd, Sm, Eu and Gd [[1],[2],[3]], with Tc up to 34 K for x(La)=0.16 and up to 43 - 47 K in the case of La-Sb co-doping [[4],[5]]. In contrast with the other IBS crystal structures, these compounds present two intriguing features: (i) they crystallize in a monoclinic structure P21 (No. 4) or P21/m (No. 11); (ii) they have a layered structure with superconducting planes of anti-fluorite Fe₂As₂ stacked with a Ca,Re and As₂ zigzag chains layers. DC-squid magnetization measurements on Ca_1-xLa_xFeAs₂ single crystals let evaluate a critical current density J_C up to 10⁵ A/cm2 [[6]], but no direct transport measurements have been reported up to now to the best of our knowledge.

In this work we present the main transport properties and the first measurement of the magnetic field dependence of the in-plane transport critical current density in Ca_1-xLa_xFeAs₂ single crystals realized at the University of Tokyo (Jp) and at the AIST Institute of Tsukuba (Jp), patterned through Focus-Ion-Beam (FIB) techniques at the NIMS Institute of Tsukuba (Jp). The transport characterization has been performed at the University of Genova (It) and CNR-SPIN institute of Genova (It).

The zero-field J_C value measured at 3 K is as high as 0.9 10⁵ A/cm2 and it is reduced of about 30% under an applied field of 7 T.

[1] N. Katayama et al, J. Phys. Soc. Jpn. 82, 123702 (2013).
[2] H. Yakita et al, J. Am. Chem. Soc., 136, 846 (2014).
[3] A. Sala et al,, Appl. Phys. Express 7, 073102 (2014)
[4] K. Kudo et al, J. Phys. Soc. Jpn. 83, 025001 (2014)
[5] K. Kudo et al, J. Phys. Soc. Jpn. 83 (2014) 093705
[6] W. Zhou et al, Appl. Phys. Express 7, 063102 (2014)


FP7 European project SUPER-IRON (Grant Agreement No. 283204)
Progetti Seed (PGESE004) CNR-SPIN

122 type pnictide superconductor is of particular interest for applications as it has high upper critical fields H_c2 (> 100 T) and low anisotropy γ (< 2). In the last few years a lot of efforts has been put into the development of high performance 122 iron-based tape conductors, since the large J_c is necessary to appear in applied fields for practical applications. Here we report on recent experiments to enhance in-field transport critical current densities of 122 tapes based on different techniques. Up to now, the superconducting properties have been significantly improved, thus critical current densities up to ~10⁵ A/cm² (at 14 T, 4.2 K) can be achieved. Present actions are aimed to fabricate long length wires and to decrease production costs by using less expensive sheathed materials. Recently developed Cu-cladded 122 tapes reached critical current densities of more than 2.7×10⁴ A/cm² in 14 T at 4.2 K, by carefully controlling the microstructure. Also discussed is recently fabricated 11 meter-long 122 tape conductor using a scalable rolling process.

Iron-based superconductors have great potential for high magnetic field applications such as superconducting wires and tapes due to their good grain boundary properties [1], large upper critical magnetic fields over 50 T, and small anisotropy factors. Recently, we reported that BaFe₂(As_1–xP_x)₂ (Ba122:P) epitaxial films exhibit high critical current density (J_c) [2]. c-axis oriented vortex pinning centers were successfully introduced by optimizing the growth rate and their isotropy of J_c and self-field J_c were improved. As a result, these films exhibit the highest J_c over 1 MA/cm² at 9 T among iron-based superconductor thin films reported so far.
In this study, we fabricated Ba122:P on IBAD-MgO buffered metal-tape substrates (IBAD, made by iBeam Materials, Inc.) by pulsed laser deposition and characterized their properties.
We employed IBAD with a twisting angle of 4 degrees and 35 % P-doped polycrystal disks as tape substrates and targets, respectively. Phosphorus concentration in the films was 28 %, which is lower than that of targets. Zero resistivity temperature (T_{c zero}) of the films was 23 K, which is slightly lower than Ba122:P epitaxial films on MgO single-crystals (25 K).
Self-field J_c of the films at 4 K was 1 MA/cm² and the J_c decreased to 0.1 MA/cm² at 9 T. These values were comparable to that of Ba122:Co films on IBAD [3] but lower than those of Ba122:P epitaxial films on MgO single-crystals [2]. Angular dependence of J_c shows broad peaks around H // ab-plane and H // c-axis. These results indicates c-axis oriented pinning centers were successfully introduced also in these IBAD samples. The ratio of J_c ^max and J_c ^min was 0.82 at 12 K and 3 T, which is comparable to that of Ba122:P films on the MgO single-crystals. The present films on IBAD have the highest isotropy among coated conductors of iron-based superconductors.
At the conference, we present the results of Ba122:P films with further optimized P concentrations and discuss the relationship between twisting angles of IBAD and J_c properties of the films.

References
[1] T. Katase et al., Nat. Commun. 2, 409 (2011). [2] H. Sato et al., Appl. Phys. Lett. 104, 182603 (2014). [3] Katase et al., Appl. Phys. Lett. 98, 202510 (2011).

This work was supported by Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, through the “Element Strategy Initiative to Form Core Research Center.”