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Establishing the basic science and technology for Iron-based superconducting electronics applications

Final Report Summary - IRON-SEA (Establishing the basic science and technology for Iron-based superconducting electronics applications)

Executive Summary:
In order to establish “the basic science and technology for Iron-based superconducting electronics applications”, EU consortium and the counterpart of Japanese consortium have worked together. For this objective, IRON-SEA has focused on 4 different area; 1) Fe-based superconducting thin films, 2) Technical basis research, 3) Scientific basis research, and 4) Educational basis activities. Executive summary for each category is described below.

1. Fe-based superconducting thin films
For device applications, high quality, epitaxial thin films are necessary. Therefore, we have made a lot of efforts to optimise the growth conditions in the first period. As a result, the growth condition of all Fe-based superconducting thin films studied in this project, namely “11”, “122” and “1111” have been almost established [“11”: Fe(Se,Te) mainly by pulsed laser deposition (PLD)1-2, “122”: BaFe2As2 by both PLD3-4 and molecular beam epitaxy (MBE)5, and “1111”: LnFeAs(O,F) (Ln: rare earth elements) by MBE6-7]. As mentioned in the 2nd periodic report as well as the detailed final report, PhD students were dispatched to the Japanese partner’s institute to prepare Ln-1111 films due to the technical difficulties for EU teams. The partner of CNR has changed a laser source from KrF to Nd:YAG for “11”. Optimum growth condition was newly achieved with a new Nd:YAG laser that showed better reproducibility. Now those films have been readily prepared and provided to all partners. It is worth mentioning that “1111” films have only been prepared exclusively by our consortium (Japanese partners, Nagoya university and Tokyo university of agriculture and technology) in the world. Additionally, the partners of CU Braislava and CNR have developed a synthesis technique for MgB2.

2. Technical basis research
A solid technology for high resolution patterning of Fe-based superconducting thin films is of fundamental importance for all electronic applications. Also for material properties investigations in order to obtain quantitative results from dc and, even more from ac analysis, it is important that the current flows in the sample through well-defined geometries. This implies the necessity of precise patterning of the thin films, a non-trivial process on Fe-based superconductors, due to their sensitivity to liquid and vapour water. Based on an acquired experience on cuprate superconductors, a working photolithography process has been developed.
We have developed new barrier materials, namely TiOx and AlOx for junctions using BaFe2As2. In particular, IcRn product has been increased significantly by using TiOx barrier8. Note that such barriers can be used for “11” and “1111” system. For realising SIS junctions (i.e. all Fe-based superconductors), “11”, “122” and “1111” films have been successfully prepared on [001]-tilted bicrystal substrates thanks to the optimised processing conditions. We have observed Shapiro steps for “11”9 and “122”10 bicrystal junctions, which indicates the realisation of “SIS” junctions. For “1111” structural analyses revealed that grain orientation was perfectly transferred from the substrates. Transport properties of the corresponding bicrystal films are under way. For this investigation, we concluded that P-doped Ba122 is the best candidate for grain boundary junctions. We have also developed “122”/Pt and “11”/Pt edge junctions by employing focused-ion-beam technique. Finally, we have also demonstrated hybrid junctions with MgB2 and P-doped BaFe2As2, which is one of the milestones in our project.
According to our investigations and literature survey, potential application for Fe-based superconductors may be nanowire detectors. Here the higher Tc of Fe-based superconductors with respect with currently employed materials (mostly NbN) could give a significant advantage in simplifying the cryogenic setup of the detectors, a key point for many applications, e.g. satellite. Fe-based superconductors could have also advantages with respect to traditional superconductors in terms of non-equilibrium properties.

3. Scientific basis research
By using high quality, epitaxial films, we have investigated physical properties (e.g. superconducting gap structures and its amplitude, local density of state) of this class of material by a point-contact Andreev reflection spectroscopy11-12 and femtosecond spectroscopy methods13-14. Intrinsic physical properties have been also investigated by various transport measurements such as noise spectroscopy15-16, resistivity and critical current density under an extremely high magnetic field of dc 45 T17.
Experimentally, gap symmetry is hard to identify for this multiband superconductor. Under these circumstances, we have designed phase-sensitive tests for Fe-based superconductors18. It is also note that the aforementioned results are interpreted with the aid of theoretical modelling: i) demonstration of the presence of electron / spin-fluctuation structures in Andreev reflection spectra in Co-doped Ba-122 films and relevant modeling, ii) analysis of pump-probe results in Co-doped Ba-122 by three-band s± Eliashberg model14, iii) four-band s± Eliashberg model applied to normal and superconducting state of LiFeAs19, and iv) adaption of the RCSJ model for asymmetric current voltage characteristics with excess current.
Currently it is of high interest to develop theoretical model of influence of disorder (impurity scattering) in this class of materials. We have formulated microscopic model for impurity scattering multiband superconductors with s± or s++ symmetry state20. We have shown that the suppression of transition temperature Tc can be described by a single parameter depending on the intraband and interband impurity scattering rates. Tc is shown to be more robust against nonmagnetic impurities than would be predicted in the trivial extension of Abrikosov-Gor’kov theory. We have found a disorder-induced transition from the s± state to a gapless and then to a fully gapped s++ state, controlled by a single parameter: the sign of the average coupling constant <λ>21.

4. Educational basis activities
Our project has also provided a lot of educational opportunities: 5 PhD students have worked for 3 years. 2 students will submit their PhD thesis using the project results. During the project, researcher’s exchanges have been frequently conducted. Additionally, one researcher from IFW Dresden has also submitted a habilitation thesis using the project results.

References:
1) E. Bellingeri et al, Appl. Phys. Lett. 100, 082601 (2012).
2) A. Kawale et al, IEEE Trans. Appl. Supercond. 23, 7500704 (2013).
3) F. Kurth et al, Appl. Phys. Lett. 102, 142601 (2013).
4) D. Daghero et al, Appl. Sur. Sci. 312, 23-29 (2014).
5) A. Sakagami et al, Physica C 494, 181-184 (2013).
6) H. Uemura et al, Solid Stat. Commun. 152, 735-739 (2012).
7) S. Ueda et al, Appl. Phys. Lett. 99, 232505 (2011).
8) S. Döring et al, J. Appl. Phys. 115, 083901 (2014).
9) E. Sarnelli et al, Appl. Phys. Lett. 104, 162601 (2014).
10) S. Schmidt et al, J. Phys.: Conf. Ser. 507, 012046 (2014).
11) P. Pecchio et al, Phys. Rev. B 88, 174506 (2013).
12) D. Daghero et al, Supercond. Sci. Technol. 27, 124014 (2014).
13) C. Bonavolonta et al, J. Phys.: Conf. Ser. 507, 012004 (2014).
14) C. Bonavolonta et al, Physica C 503, 132-135 (2014).
15) C. Barone et al, Supercond. Sci. Technol. 26, 075006 (2013).
16) C. Barone et al, Sci. Rep. 4, 6163 (2014).
17) K. Iida et al, Sci. Rep. 3, 02139 (2013).
18) A.A. Golubov and I.I. Mazin, Appl. Phys. Lett. 102 (2013) 032601.
19) G. A. Ummarino et al, J. Phys.: Condens. Matter 25 205701 (2013)
20) D. Efremov et al, New Journal of Physics 15, 013002 (2013).
21) D. Efremov et al, Phys. Rev. B 84, 180512(R) (2011).
Project Context and Objectives:
The discovery of high-temperature superconductivity in iron-based oxides and intermetallics triggered world-wide research activities to investigate the fundamental properties of these extraordinary materials. The investigation on this new class of materials has been still focused on their material processing and their physical properties using polycrystalline materials and/or single crystals, since a central issue in the iron-based superconductor (e.g. pairing symmetry, whose understanding paves the way to shedding light on the origin of superconductivity) has been controversial. Here it is worth mentioning that recent theoretical papers and some experimental results seem to indicate that pure s±-wave symmetry may not be a universal feature of all iron-based superconductors. Up to now, most of the research has been carried out on either bulk single crystals or polycrystalline materials due to the difficulties in synthesizing epitaxial thin films.

Whenever new superconducting materials are discovered, it becomes immediately interesting to explore their potential applications such as large scale power applications involving motors, transformers and superconducting magnets using wires, device applications based on the Josephson effect such as Superconducting Quantum Interface Devices (SQUIDs) and Single Flux Quantum (SFQ) devices. For realizing high integration Josephson junctions, a sandwitch type junction is suitable, however, this type of junction is not suitable for cuprates due to the redox problem of CuO2 plane. Additionally, crystal structure and physical parameters of cuprates are relatively high.

Fe-based materials may overcome those drawbacks. Recent success in epitaxially grown iron-based superconducting films achieved by the EU consortium members as well as the partner of the coordinated Japanese project members opens the way to electronics applications as well as a variety of experiments for understanding intrinsic properties. In this collaborative project, we will focus on establishing the fundamentals of the iron-based superconductors for electronics applications and address their feasibility.

We have several objectives in different workpackages. The first main objectives are to optimize the growth condition and provide films all partners for fundamental aspects as well as superconducting junctions. WP2 and WP3 are mainly involved in this work. One of the key objectives is to prepare various junctions (S-N, S-S, and hybrid S-S` with N and I barriers) and investigate Josephson effect (WP4). Additionally, fundamental aspects such as superconducting gap and order parameter symmetry have been investigated. Here S is superconductor, N is the normal conductor, S` is conventional superconductor and I is the insulator.

In order to evaluate the superconducting gap and its symmetry, point contact Andreev reflection spectroscopy, pump probe spectroscopy, and noise measurements have been carried out (WP5). Additionally, high field transport measurements have been also conducted to reveal intrinsic properties.

Theoretical aspects of the tunneling and Andreev reflection in unconventional superconductors will be studied (WP6). In particular, we shall concentrate on the Josephson effect and its dependence on the pairing symmetry. Furthermore, we will investigate the possibilities to use the Josephson effect to gain insight into deviations from the standard BCS-like behavior. In addition the results of the different spectroscopy measurements conducted in WP4 and WP5 will be analyzed in the framework of multiband Eliashberg theory with the main purpose of shedding light on the pairing mechanism and the nature of the mediating boson in Fe-based superconductors.as well as to make predictions on various other observables.

WP7 will propose new devices, such as nano-strip detectors and SQUIDs with the iron-based superconductors and assess their potential. In addition, assessment of technology for thin films patterning and multilayer fabrication will be carried out, which will provide a solid technological guideline for the realization of future devices. In particular a careful comparison with other existing superconductive devices and related fabrication technologies will be made to asses the relative points of strength and weakness of the iron-based superconductors.

In order to promote young scientists, IRON-SEA provides an opportunity for giving a scientific talk during the periodic meeting (WP9). The consortium offers unique opportunities to young researchers for staying at Japanese or European universities or institutes for a short period for the aim of training and transfer of knowledge. This kind of guest stay for short period clearly fulfills with the aim of more intensive exchange and training of researchers.

Project Results:
In terms of the films growth of Fe(Se,Te), doped BaFe2As2(Ba-122) and LnFeAs(O,F) (Ln=rare earth elements), our consortium is one of the leading groups in the world. Particularly, no groups in the world can prepare LnFeAs(O,F) (Ln=rare earth elements) except for our consortium. Additionally, high quality Fe(Se,Te) and doped Ba-122 have been readily fabricated.

Thanks to such progress, various physical properties (e.g superconducting gap amplitude) of Fe(Se,Te), Co-doped Ba-122 and P-doped Ba-122 thin films have been unveiled by using a point-contact Andreev reflection spectroscopy (PCARS) method and pump probe method, and noise spectroscopy. Here is the most significant result: 1) the puzzle of the gap amplitudes in FeTeSe has been solved: all the values reported in literature have found a systematic explanation, 2) the anisotropy of the order parameter on the electronlike FS in Fe(Te,Se) has been demonstrated by PCARS, 3) the first spectroscopic determination of the gap in a sample under pressure has been achieved, 4) the relationship between topological transition of the Fermi surface and the emerge of nodes in the gap has been demonstrated in CaFe2As2, 5) the effect of irradiation on the gaps and on the critical temperature of P-doped Ba-122 films has been assessed, 6) the effects of the surface degradation on the superconducting properties have been studied, 7) a peculiar noise generation and fast out-of-equilibrium behavior have been evidenced in Fe(Se,Te) films, 8) a pure 1/f noise has been found in Co-doped Ba-122 thin films, with a quadratic current current dependence; in bicrystal junctions the noise shows an anomalous frequency dependence.

We have investigated such physical observables as a function doping level. Using a high-filed dc up to 35 T, we have investigated transport properties of P-doped Ba-122 and NdFeAs(O,F) thin films. Unexpected high critical current is observed for P-doped Ba-122 at the quantum critical point. For NdFeAs(O,F), we also identified the intrinsic pinning along the crystallographic c-axis, which is similar to cuprates. It means that a possible Josephson effect along the c-axis like Bi-based cuprates.

For device fabrication, we have found TiOx may be a suitable material for insulating barrier for junctions. As a result, IcRn product is significantly improved compared to the previous junctions. Furthermore, Josephson junctions have been realized based on a wide range of different pnictides, substrates and mismatch angles and showed pure RSJ-like to flux-flow-like behavior. We have also prepared bicrystal junction using P-doped Ba-122, Co-doped Ba-122, Fe(Se,Te) and NdFeAs(O,F). Although the detailed characterization for NdFeAs(O,F) is still going, the most promising material would be P-doped Ba-122 based on both artificial and natural grain boundaries. Finally, a great variety of Josephson junctions has been optimized to enable the realization of hybrid phase-sensitive devices in the near future.

For theoretical works, we have achieved a lot of aspects: 1) Finding disorder-induced transition from the s± state to a gapless and then to a fully gapped s++ state, 2) Proposing manifestation of impurity induced s+- → s++ transition: multiband model for dynamical response functions, 3) Theory of effects of magnetic disorder in multiband superconductors, 4) Demonstration of the presence of electron / spin-fluctuation structures in Andreev reflection spectra in Co-doped 122 and in FeTe1-xSex films and relevant modeling, 5) Analysis of pump-probe results in Co-doped 122 by three-band s± Eliashberg model, 6) Four-band s± Eliashberg model applied to normal and superconducting state of LiFeAs, 7) Designing phase-sensitive tests for Fe-based superconductors, 8) Adaption of the RCSJ model for asymmetric current voltage characteristics with excess current, 9) Theory of Josephson effect in two-band superconductors with s+- symmetry, 10) Microscopic theory of tunneling spectroscopy of multiband superconductors, and 11) Explanation of the results of PCARS in Co-doped Ba-122 films within a three-band s± Eliashberg model based on a spin-fluctuation mediated pairing.

For aforementioned studies, Fe-based superconducting Josephson junction technology still needs further development to reach the high level of integration and good performances of low Tc superconductors (Nb, NbN). Therefore applications requiring large-scale integration or high quality, hysteretic Josephson junctions are not within reach for the moment. SQUIDs, by requiring few non-hysteretic junctions, are instead possible realizations. However a performance comparison with existing high- Tc SQUID technology suggests that specific advantages of Fe-based superconductors as SQUID have yet to be demonstrated.

Another application within reach for Fe-based superconductors is that of nanowire detectors. Here the higher Tc of Fe-based superconductors with respect with currently employed materials (mostly NbN) could give a significant advantage in simplifying the cryogenic setup of the detectors, a key point for many applications, e.g. satellite. Fe-based superconductors could have also advantages with respect to traditional superconductors in terms of non-equilibrium properties, as the pump-probe experiments performed within the IRON-SEA project have shown.

Future development of superconductive electronics aims at new devices with new physics involved, such as magnetic Josephson memories for RSFQ logic circuits, hybrid nanostrip high Tc detectors, intrinsic phase shift junctions (pi-junctions) for quantum computing, etc. Here again the wealth of different physical and material properties of the iron-based superconductors could become important prerequisites for the realization of innovative superconductive devices.

Potential Impact:
IRON-SEA created a high level of results, which would help to maintain the competitiveness of world research excellence in the field of superconductivity. As a proof, IRON-SEA members gave tremendous numbers of invited talks at the international conferences for 3 years (see the list of dissemination activities). Additionally, 45 peer review papers have been published. Note that majority of the papers are in high profile journals such as Nature Communications, Scientific Reports, Physical Review and Applied Physics Letters.

Prior to the project, we have already had existing collaborations. Owing to IRON-SEA activities, the existing networks have been further strong (e.g. a lot of scientific publications by FSU Jena and IFW Dresden). In a natural consequence, we have also developed a new network within EU as well as Japan. As a result, several joint publications by EU and Japanese teams have been appeared. Now IRON-SEA has been recognized as a new and strong research community in the world. Thanks to such a good recognition, a new project between Japanese partner (Nagoya University) and EU (Politecnico de Torino and Karlsruhe Institute of Technology) has been granted.

IRON-SEA has demonstrated a high level of integration between EU and Japanese partners. Not only senior researchers but also young scientists have precious experienced through the IRON-SEA activities in terms of research. As a training activity, young scientists have stayed at European or Japanese institute and universities for a short period. They have learnt new experimental techniques that cannot be performed in their home institutions. In addition, they have discussed the obtained data through experiments with the leading scientists. This kind of frequent communications and discussions always improve the quality of the research, which indeed leads to accelerating and strengthening research. It should be also noted that working in an international collaboration certainly has a strong, positive influence on young scientists. Additionally, IRON-SEA provided a good opportunity for young scientists as they gave a presentation, which would be a good practice for international conferences. It is worth mention that several scientific papers have been written by PhD student and published in high impact journals.

As can be seen in the dissemination lists, IRON-SEA members have actively disseminated the project results through the international conferences. This is mainly to the scientific community. The project homepage (www.ironsea.eu) also works as a central tool for dissemination, particularly to the public. As stated above, the scientific publications are also one of the dissemination.

List of Websites:

www.ironsea.eu

Associate Professor Kazumasa Iida (coordinator)
Nagoya University
Furo-cho, Chikusa-ku, Nagoya, 464-8603, Japan

Professor Paul Seidel
Friedlich Schiller University of Jena
Helmholtzweg 5, Jena, 07743, Germany

Prof. Andrej Plecenik
The Comenius University
Mlynska dolina F2, Bratislava 84248, Slovakia

Associate Prof. Renato S. Gonnelli and Dr. Dario Daghero
Politecnico di Torino
Corso Duca degli Abruzzi, 24, 10129 Torino, Italy

Associate Professor Sergio Pagano
SPIN Salerno
Via Ponte don Melillo, 84084 Fisciano (SA)-Italy

Associate Professor Alexander Golubov
University of Twente
7500 AE Enschede, The Netherland

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