The application of an overcritical current pulse I>Ic(T) to narrow YBa2Cu3O7 strips generates a phase-slip center after a certain delay time td . By analyzing the function td(I/Ic) through a time-dependent Ginzburg-Landau theory, one can extract the cooling time of the film on its substrate. This parameter was found to remain constant from 5 to 40 K, but also found to increase notably above 40 K and up to Tc . We interpret this behavior as a manifestation of heat transfer by a blackbody radiation of acoustic phonons, and the loss of emissivity at higher temperatures to the rise of phonon-phonon interaction processes.

K.H gratefully acknowledges the support of the King Fahd University of Petroleum and Minerals, Saudi Arabia, under the IN131034 DSR project.

It is known that spin-singlet superconducting (S) correlations, as well as triplet ones with zero spin projection, decay into an itinerant electron ferromagnetic (F) layer over a very short distance determined by the exchange energy. At the same time, a triplet component with a finite total spin generated at a magnetically inhomogeneous F/S interface may penetrate over a much longer distance limited only by temperature T. Although the long-range effect has been already observed for some F layers, up to now there is no consensus relating half-metallic manganites with 100% spin polarization where singlet superconducting correlations cannot exist.
We address this problem by studying transport characteristics of homogeneous composites formed by granules of a superconducting MgB₂ on μm scale and those of La_0.7Sr_0.3MnO₃ as small as 8-10 nm. The density of the composite with the relative volume fraction of superconducting grains p = 0.74 was as high as (96 ± 3) % of the theoretical value. Let us assume that the manganite grains form a sheath around MgB₂ granules, then its average thickness would be ~ 350 - 700 nm. If the ferromagnetic manganite grains are completely impenetrable for superconducting correlations except ultra-thin sheath parts, the transition interval to an S-state would be very narrow while in our samples with p = 0.9 and 0.74 it was about 10 and 25 - 30 K, respectively. Moreover, at high pressures of 12 kbar, we have observed a shift of the midpoint in the resistance-vs-T curves to higher temperatures which can be explained by the enhancement of the superconducting proximity effect in hybrid contacts formed by superconducting MgB₂ and ferromagnetic manganite components. Whereas in pure MgB₂ the critical current, as well as the critical temperature T_c, was reduced under the pressure of 12 kbar, in our hybrid samples it increased.

This work was performed within the German-Ukrainian project SE 664/18-1 supported by Deutsche Forschungsgemeinschaft (DFG).

A novel negative oxygen-isotope (16O/18O) effect (OIE) on the low-temperature tetragonal phase transition temperature T_LTT was observed in the stripe phase of La_(2-x)Ba_xCuO_4 (x = 1/8) using high-resolution neutron powder diffraction. The corresponding OIE exponent alpha_(T_LTT) = - 0.36(5) has the same sign as alpha_(T_so) = -0.57(6) found for the spin-stripe order temperature T_so [1]. The fact that the LTT transition is accompanied by charge ordering (CO) implies the presence of an OIE also for the CO temperature T_co. Furthermore, a temperature dependent shortening of the c-axis with the heavier isotope is observed. These results indicate that all transitions observed in the stripe phase of
La_(2-x)Ba_xCuO_4 (x = 1/8) are sensitive to oxygen lattice vibrations. The theoretical lattice dynamical calculations based on a dynamic anharmonic electron lattice interaction Hamiltonian consistently reproduce the experimental results. This structural instability is driven by phonon mode softening stemming from anharmonic electron-lattice interactions and these interactions are essential for the stripe formation in the cuprates. The present results may contribute to a better understanding of the complex microscopic mechanism of stripe formation and of high-temperature superconductivity in the cuprates in general.
[1] Z. Guguchia, R. Khasanov, M. Bendele, E. Pomjakushina, K. Conder, A. Shengelaya, and H. Keller,
Phys. Rev. Lett. 113, 057002 (2014).

Polycrystalline samples of RuSr₂(Eu_1.2Ce_0.8-xTb_x)Cu₂O_10-δ (0 ≤ x ≤ 0.3) (Ce/Tb samples) and RuSr₂(Eu_1.2-xTb_xCe_0.8)Cu₂O_10-δ (x = 0.3) (Eu/Tb sample) were synthesized by a solid-state reaction method. The samples were characterized by means of X-ray diffraction, Ru K-edge X-ray absorption spectroscopy (XAS), magnetization, and thermoelectric power measurements. It was found that Ce/Tb substitution resulted in a negligible change in the magnetic ordering temperature, T_m and a reduction of the weak-ferromagnetic component of the field-cooled (FC) magnetization as the Tb content increases, in contrast to the (Eu/Tb) sample which showed an increase in the magnetic ordering temperature with no prominent change in the FC magnetization at low temperatures. The XAS measurements indicated that substitution of Tb in both Ce/Tb and Eu/Tb samples caused an increase in Ru valence toward 5+. The magnetic properties are discussed in conjunction with the local structural changes in the Ru sublattice and the change in Ru valence induced by Tb substitution.

This work was supported by a National Research Foundation Grant (NRF-2012R1A1A2042519) and by a 2015 research grant from the Kangwon National University. We express our thanks to the Korea Basic Science Institute for help with the SQUID measurements and to the Pohang Accelerator Laboratory for help with the XAS measurements.

The complete penetration magnetic field BP is a very important parameter of superconducting bulks submitted to an applied magnetic field. It is useful to know it for applications as superconducting permanent magnet synchronous motors. In this study, cylindrical HTS pellets were submitted to an axial applied magnetic field. Theoretical and experimental studies are presented. For numerical simulations, the E-J characteristic of HTS bulks was described by a power law relationship, i.e. E(J) = EC(J/JC)n. The influence of the n-value and of the rising rate Vb of the applied field on the BP of HTS cylindrical pellets was investigated, and a linear relationship between BP, Vb and n-value could be established. The experiment was based on direct measurements of the local magnetic field in the gap between two bulk HTS pellets. The measurements of the field penetration has been carried out on HTS pellets at 77 K by applying increasing axial magnetic fields with a quasi-constant sweep rate. The influence of the gap size between the two HTS pellets was studied with a particular attention. Two values of the complete penetration field BP have been measured at two different rising rates Vb, allowing n-value determination. We conclude that using a combination of measurements and numerical simulations allow determining in a quite simple way the n-value and the critical current density JC of real HTS pellets.

Iridium is one of interesting 5d elements and a wide variety of physical properties at low temperature are observed in iridium related compounds. It is believed that electron correlations and spin-orbit copping play an important role in determining their electronic properties. Here we report the synthesis, experimental electronic characterization, and calculated electronic band structure of the cubic Laves phase superconductor CaIr2 (Tc = 6.1K) and SrIr2 (Tc = 5.8 K), which the Ir atoms have a pyrochlore lattice. The presence of a bulk superconducting transition was confirmed, and the properties of the superconductor were elucidated through magnetization and heat capacity measurements. The electronic band structure calculations indicate that the the Ir d states are dominant through the Fermi level. At last the effects of spin-orbit coupling in the Ir based compounds on the electronic structure, even the existence of superconductivity are discussed.

MgB2 superconductors have mainly been studied for the practical application for tapes and thin films owing to high Tc of about 39 K and high upper critical field among metallic superconductors. We have studied that the trapped field properties of MgB2 bulks fabricated by the capsule and hot isostatic pressing (HIP) methods and succeeded in producing the 2-3 T class bulk magnet. Recently, the trapped field of 5.4 T was reported for the hot-pressed MgB2 bulk using the mechanical alloyed precursor powder [1]. In this paper, to obtain the higher trapped field we studied the doping effect of Ti on the trapped field in MgB2 bulks fabricated by the HIP method. The (Mg1-xTix)B2 bulks were magnetized by the field cooling magnetization (FCM) in a magnetic field of 10 T using a 10-T cryogen-free superconducting magnet. The highest trapped field at the top surface of the single bulk (x=0.1) was 3.6 T and that at the center between the doubly stacked bulks (x=0.05 and 0.1) was 4.6 T. We discuss precisely the effect of Ti on the enhancement of the vortex pinning from the analysis of the microstructure observed by an electron backscatter diffraction method.
[1] G. Fuchs et al, Supercond. Sci. Technol. 26 (2013) 122002.

Pulse Field Magnetization is the most preferable method of field activation of HTS magnets. A serious drawback of this method is a substantial warming, and as a consequence, the occurrence of flux jumps. In this work the flux jump process were examined on melt-grown YBCO-annuli with outer diameter of 36 mm and inner diameter of 17 mm. A single pulse procedure was realized. The appearance of the jump depends on the pulse amplitude and its duration. At low amplitude the field trapped in the hole increases monotonically. At some critical amplitude the trapped field falls sharply (flux jump) and even changes sign. With further increase of the amplitude the trapped field remains negative and small. When pulse duration decreases the flux jump is observed at lower pulse amplitude. We believe that the critical parameter - H_a/t regulates the flux jump. Flux jump occurs due to the destruction of the circular shielding currents. The measurement of the radial distribution of the longitudinal magnetic field component B_z(r) after the flux jump shows that the annulus body is magnetized along the external field and the field in the hole has the opposite direction. The flux jump takes place when the circulating critical currents is insufficient to shield the rapid variation of the external magnetic field.

Authors thank Russian Fund for Assistance to Small Innovative Enterprises in Science and Technology for financial assistance.

This work is part of an Action de Recherches Concertées grant from the Ministry of Higher Education through the Research Council of the University of Liege (ARC 11/16-03).

The selective motions of the magnetic flux invasion into the Dy123-based HTS bulk magnets were experimentally evaluated during the pulsed field magnetization PFM processes. The flux motion and resultant trapped magnetic flux densities were measured at the surface center of the bulk sample at the temperature of 30 K. The invasion of the magnetic flux into the sample is expected to be affected by the microscopic structure and the geometrical distribution of the Jc values, we estimated the differences between the magnetic flux motions between those in the regions of growth sector boundary GSB and growth sector region GSR by measuring the magnetic flux density and temperature changes during the pulsed field application of 5-T in the superconducting state. The experimental results showed us that the magnetic flux invades into the sample through the path in the GSR earlier than that in the GRB region, which clearly reflects the different Jc values in each region. Furthermore, the data of flux-invading speed gradually changed according to the positions on the GSR, while the data irregularly changed on the GSB. This means that the direction of magnetic flux does not uniformly flow on this area. Since the temperature on the path of magnetic flux changes more rapidly than those in other regions, we can trace the invasion path of magnetic flux. As a result, the heat generation due to the invading flux motion occurs in the GSR region prior to the GSB region. Thus, the magnetic flux invasion behavior exhibits selective and non-uniform motion reflecting the difference of the macrostructure.

The authors give thanks to Dr. Teshima of Shinnittetsu-Sumikin Co. for his assistance.

Recent results obtained from iron-based high-temperature superconductors are promising in terms of performance, however, major improvements are still necessary for these materials to become competitive. Our experimental data and the model we developed to describe them suggest a strategy for achieving this performance boost. We studied the field dependence of the critical current density in Co- and K-doped Ba-122 polycrystals with different grain sizes. The use of SHPM (Scanning Hall Probe Microscopy) allowed us to directly visualize the magnetization profiles of the samples, from which the macroscopic critical current density as well as the currents flowing within the grains can be inferred. By performing SHPM measurements at various values of applied field, we were able to track the complex evolution of the magnetization profiles, which exhibit a pronounced dependence on the magnetization history. We present a model explaining this evolution and hence the history effect in the critical current density, which is based on the assumption that adjacent grains are coupled by Josephson currents. Within the framework of this model the macroscopic critical current density results from the interplay of intra-granular currents with these Josephson currents. Fitting the free parameters of this model to data obtained from SHPM measurements and from SQUID magnetometry results in very good agreement with these data and realistic values of the free parameters (the maximum inter-granular current density and the average thickness of the Josephson junctions). An important prediction of this model, which is also supported by our experimental data, is that the in-field macroscopic critical current density increases with decreasing grain size. This result may pave the way towards a significant improvement of the performance of granular high-temperature superconductors.

Magnetic field shielding is crucial for several applications requiring an ultralow magnetic field environment (e.g. biomedical application) or the mitigation of the magnetic field produced by an electronic device in order to guarantee electromagnetic compatibility with the surrounding environment or to prevent possible health hazards (e.g. workers safety).
In this framework, we investigated the efficiency of superconducting (SC) MgB₂ and hybrid MgB₂/Fe systems of different shapes (disk, tube, cup) for magnetic field mitigation. Shielding capability of these systems was measured as a function of temperature, external magnetic field, position and time by means of cryogenic Hall probes mounted on a custom-designed stage, moveable along the sample axis with micrometric resolution [1].
In all the investigated configurations, the presence of the ferromagnetic (FM) layer allows tuning the shielding efficiency of the superconductor. In particular, these hybrid systems result the most efficient solutions at applied fields higher than 0.5 T , where the use of SC/FM coaxial cups allow obtaining a shielding efficiency 3-4 times higher than that measured with a single MgB₂ cup, coupled with a reduction of the flux creep phenomena. On the contrary, at low field, the only superconducting solution is the most efficient.
Starting from these results, further shielding solutions, obtained by numerical simulations, are proposed in order to achieve additional improvements in the shielding efficiency of these hybrid systems, optimizing the relative sizing of the SC and FM layers and the air gap width between them.

[1] L. Gozzelino et al., Supercond. Sci. Technol. 25, 115013 (2012).

Authors wish to thank F. Gomory for fruitful discussions. This work was partially supported by the National Institute for Nuclear Physics under SR2S-RD experiment.

The capability of MgB2 bulks to trap high magnetic fields at temperatures around 20K could find applications in the domain of scientific equipment, for example for manufacturing small size high field permanent magnets. It was previously reported that a 2.6T field could be measured between two 25mm diameter MgB2 bulks at 20K [1]. In this contribution, we report measurements of the trapped field above 20mm diameter MgB2 bulks after the application and the removal of magnetic fields ranging between B=0T and B=4T and at temperatures between 20K and 32.5K.
The bulk MgB2 superconductors were fabricated by unconventional Spark Plasma Sintering (SPS) technique resulting in a very high relative density. For each temperature and field the trapped field was measured with 8 small Hall probes located along a diameter of the samples. The measurements were carried out i) as a function of time after the applied field zeroing and ii) as a function of the distance to the sample surface.The measurements as a function of time have shown a strong magnetic relaxation occurring immediately after the removal of the magnetic field. Stabilization occurs after a few thousands seconds. The field stabilized value was then measured as a function of the distance to the samples. While trapped field up to 1.6T was measured at a 1.5mm distance above samples after the application of a 3T magnetic field at 20K, the application of higher fields did not result in an increase of the trapped field. This suggests that at high field current lines with a density equal to the critical value flow in the whole bulk while they flow in a fraction of the samples at lower fields. Based on this result, the measurements recorded by various probes were reproduced with numerical techniques and those measured above the center of the bulks also by analytical considerations. Both methods have resulted in similar values for the critical current density at the different measurements temperatures.
[1] J.Durrel at al. Supercond. Sci. Technol. 25 (2012) 112002

The work of Louis Dupont was partially funded with a CIFRE grant of Agence Nationale de la Recherche et de la Technologie.

Magnetic fields are partially penetrated into a type II superconductor in the form of quantized vortices. Extrinsic pinning current flows together with intrinsic supercurrent. How the field is distorted by these currents has never been clearly discussed. Here, we report that mixed state shape effect is quite different from demagnetization effect. It was shown that peculiar features of irreversible magnetization, for example, a very steep magnetization slope, can be understood by shape dependent magnetization magnification. Reversible magnetization in the presence of vortices is a bit more complicated. Intrinsic current contribution at the edges of vortices near the surface is found to be very crucial, for example, for empirically observed magnetization orientation variations. More detailed free energy study, including the vortex-edge effect seems to be necessary.

This work was supported by Mid-career Researcher Program through NRF grant funded by the Ministry of Education, Science & Technology (MEST) (No. 2010-0029136).

As known, in superconductor Maglev and bearing system not only magnetic levitation but also magnetic stiffness are very critical parameter for industrial applications. In performed studies in literature, the vertical and lateral magnetic stiffness values of YBCOs could not be increased at the same time, because of different cooling process, even though some improvements of these stiffness have been achieved individually. In our study, we investigated vertical and lateral magnetic stiffness properties of multi–seeded YBCO bulk depending on the magnetic pole direction in PMG (Permanent magnet guideway) and different vertical position of onboard PM in different CHs to achieve larger vertical and lateral stiffness values together. Firstly, the vertical and lateral stiffness measurements were performed between multi–seeded YBCO and two type PMG with three PM vertical pole direction to understand physical mechanism of the magnetic stiffness and stability in different cooling height after the magnetic flux distributions modelling of PMG. Then, to enhance stiffness parameter and investigate applicability of multi–seeded YBCO for Maglev system the vertical and lateral magnetic stiffness measurements were carried out between multi–seeded YBCO and different PMGs (Conventional and Halbach PMG) depending on additional downward PMs in Conventional PMG and the onboard PM positions in onboard unit for different CHs. In our study, it has seen that the both vertical and lateral magnetic stiffness have different properties for different magnetic pole direction and relating stiffness values increases gradually with additional downward PMs and optimal onboard PM position.

This work was supported by the Scientific and Technological Research Council of Turkey (TÜBITAK), with project no. 112T090 and the Scientific Research Project Fund of Karadeniz Technical University in Turkey, with project no. 11060.

VKM, Shruti, and PS acknowledge the UGC (India) for providing UGC-BSR, UGC-SRF, and UGC-DS
Kothari fellowships, respectively. Technical support from AIRF (JNU) is gratefully acknowledged. SP would like to acknowledge financial support from DST-PURSE and DST-FIST programs of Department of Science and Technology (Government of India)

Proximity effect of a ferromagnetic insulator / Superconductor hybrid is studied by means of measuring differential conductance of NbN/GdN/TiN superconductor / ferromagnetic insulator / normal metal junctions. The data shows for high spin polarization of the GdN layer1 a large zero-field voltage offset interpreted as a spin-filtered Zeeman splitting of the NbN density of states by an effective exchange field (H0) from the GdN. The splitting increases linearly with applied field (Hext) enabling the relative sign of H0 and Hext to be determined. We show that the short NbN coherence length concentrates H0 at the NbN/GdN interface and eliminates any averaging over the GdN domain structure. Presence of large exchange field in the superconductor points to existence of unconventional superconductivity in an otherwise normal s-wave superconductor, and sheds light on the unconventional second harmonic current phase relation of corresponding NbN/GdN/NbN Josephson junctions 2,3.

1. Pal, A., Senapati, K., Barber, Z. H. & Blamire, M. G. Electric-Field-Dependent Spin Polarization in GdN Spin Filter Tunnel Junctions. Adv. Mater. 25, 5581–5 (2013).
2. Pal, A., Barber, Z. H., Robinson, J. W. A. & Blamire, M. G. Pure second harmonic current-phase relation in spin-filter Josephson junctions. Nat. Commun. 5, 3340 (2014).
3. Senapati, K., Blamire, M. G. & Barber, Z. H. Spin-filter Josephson junctions. Nat. Mater. 10, 849–52 (2011).

The low value of the intergranular J_c measured in polycrystals is the principal obstacle to applications of pnictide superconductor (RE1111; REFeAsO; RE = rare earth) and the concern that grain boundaries are weakly coupled with each other. To reduce this problem, we have synthesized a series of Sn added Sm1111 (SmFeAs(O,F) + xSn; x = 0-0.8) polycrystalline samples by low temperature synthesis method and studied the influence of the Sn addition on the intergranular J_c of Sm1111. These samples are well characterised by various measurements, such as XRD, Field Emission Scanning Electron Microscope (FESEM), resistivity under magnetic fields, magnetic susceptibility and remanent magnetization. Powder XRD measurements revealed the purity of the samples and showed that the lattice volume slightly increases with the Sn addition in the lightly F doped Sm1111 (SmFeAsO_0.88F_0.12), suggesting the reduced of fluorine concentrations inside the lattice, whereas there is almost no change in the lattice volume of the optimally doped sample (SmFeAsO_0.8F_0.2). The transition temperature (T_c) of lightly F doped Sm1111 (T_c = 51 K) shifted to lower temperature and reached to 40 K for 40% Sn addition. On the other hand, optimally F doped Sm1111 (T_c = 57 K) have the almost same value of T_c with Sn addition. Microstructural analysis depicts that the impurity phases in SmFeAsO_0.8F_0.2 reduced with Sn addition and, clean and well-connected grain boundaries increased. The Sn added SmFeAsO_0.8F_0.2 samples have the onset T_c up to 57 K and the intergranular J_c of over 10⁴ A cm^-2 at 5 K in the self-field, as assessed by remanent magnetization measurements. Sn addition improves the intergranular J_c of Sm1111 from 200 to 11000 A cm^-2 for 20% fluorine doped and 4000 to 9700 A cm^-2 for 12% fluorine doped samples respectively. We believe this superior intergrain J_c to be attributable to the strong intergrain coupling due to grain connectivity improved by the Sn addition. In addition, we will also discuss the optimization of the synthesis of Sn added Sm1111 below our conventional temperature (900°C) where the polycrystalline samples sintered at 750°C showed the intergrain J_c of over 3000 A cm^-2 at 5 K.
Reference:
S. J. Singh et al., Supercond. Sci. Tech. 27, 085010 (2014) and 26, 065006 (2013).

SJS thanks the Japan Society for the Promotion of Science (JSPS) and German Academic Exchange Service (DAAD) for JSPS and DAAD Postdoctoral Fellowships for Foreign Researchers, respectively.

Iron-based superconductors have enormous potential for high field magnet applications, since they have very high upper critical field, large critical current density (J_c), relatively small anisotropy and strong intrinsic pinning. Among the main families of iron-based superconductors, 1111 family attracted much interest for application as it has the highest transition temperature of 56 K. The wire fabrication study by powder-in-tube method of 1111 families had begun very early. However, the transport critical current density was severely limited by many difficulties including grain connectivity, fluorine control and impurity control. We are aimed to solve these problems. A Sn-presintering process of the SmFeAsO_1-xF_x precursor powder was applied. The intergranular Jc in SmFeAsO_1-xF_x tape was greatly improved, which was confirmed by transport current measurement and Magneto-optical imaging. Moreover, a series of low-temperature (300-500 °C) heat treatments under ambient pressure or uniaxial pressure were performed on SmFeAsO_1-xF_x tapes, which effectively avoided fast fluorine loss in tapes and prevented reaction layers between superconducting core and sheath metal. A highest transport J_c of 3.95 × 10⁴ A cm^-2 (at 4.2 K and self-field) is achieved in SmFeAsO_1-xF_x tapes.

This work was supported by the National “973” Program (Grant No. 2011CBA00105), National Natural Science Foundation of China (Grant Nos. 51025726, 51320105015 and 51202243), and China-Japan Bilateral Joint Research Project by NSFC and JSPS.

We present our study on the effect of hydrostatic pressure (applied during measurements) on the significant enhancement of flux pinning, Jc, and Tc in polycrystalline pnictide bulks, tapes and single crystals of Sr4V2O6Fe2As2, NaFe0.972Co0.028As, Ag-clad Sr0.6K0.4Fe2As2 tape with Sn addition, respectively. We demonstrate that the hydrostatic pressure up to 1.2 GPa can not only greatly increase Tc from 15 K (underdoped) to 22 K, but also significantly enhance the irreversibility field, Hirr, by a factor of 4 at 7 K, as well as the critical current density, Jc, by up to 30 times at both low and high fields for Sr4V2O6Fe2As2 polycrystalline bulks. Jc in NaFe0.972Co0.028As single crystals can be enhanced by at least tenfold at low fields and more than a hundredfold at high fields with Tc raising from 18 up 24 K. Remarkbly, Our team has found that hydrostatic pressure of 1.1GPa can significantly enhance Jc in Sr0.6K0.4Fe2As2 tapes by over an order of magnitude at both low and high fields with littile change in Tc. Record high Jc were obtained in the Ag-clad Sr0.6K0.4Fe2As2 tape under the hydrostatic pressure with valuves as high as 106 A/cm2 at low field and 1.8 × 105 A/cm2 at 13 T at 4K, which is roughly two times higher as compared to NbSn3. pinning is found to be dominant in all the samples under the hydrostatic pressure. Our flux pinning analysis indicates that the hydrostatic pressure can induce extra point defect pinning centers and increase the pinning force greatly, being mainly responsible for the significant Jc enhancement in all samples. In addition, a transformation from surface pinning to point pinning induced by pressure was accompanied by a reduction of anisotropy at high temperatures in granular samples. Our findings indicate that there are still plenty of room to further increase Jc in Fe-based superconductring tapes/wires, and also provide an effective method to significantly enhance Tc, Jc, Hirr, and Hc2, for other families of Fe-based superconductors in the forms of wires/tapes, films, and single crystals and polycrystalline bulks. The condensation energy enhanced by pressure which is in turn related to the pinning potential and Jc enhancement will be presented.


This work is supported by Australian Research Council throught ARC Discovery project on Fe-based superconductors.

The single crystals of Cu_xTaS₂ were grown via chemical vapor transport method in evacuated quartz tube with iodine. Different crystal growth regimes were used for producing of several Cu_xTaS₂ crystals. All copper intercalated TaS₂ single crystals were prepared from the same powder composition at the beginning of the crystal growth. The structural characterization results show, that the growth regime could have a significant influence on the stoichiometry of the sample and their superconducting properties.
In our work, we investigated properties of Cu_xTaS₂ single crystals by Scanning Electron Microscopy, X-ray analysis, SQUID magnetization measurements as well as thermal analyses.

This work was realized within the framework of the projects: VEGA 0090/13, APVV 0330/12, APVV Stefanik Project SK-FR-2013-0025, PHYSNET ITMS: 26110230097.

FeSe polycrystalline superconductors with dimensions over 10 × 10 × 3 mm³ were fabricated by diffusing liquid Se into an iron tablet at 650 °C. Scanning electron microscopy and X-ray diffraction analyses were performed on the FeSe samples, and they showed that the main phase throughout the bulk is tetragonal β-FeSe and the grains are well crystallized and connected. The critical transition temperature (onset) was determined to be 12 K, with a transition width of 3 K, by a resistivity measurement. The upper critical fields H_c2(0) was estimated to be 40 T, and the intragranular critical current density estimated from the Bean model is about 1.5 × 10⁵ A/cm² at 4 K in self-field. These results suggest that liquid-solid diffusion is a promising technique for fabricating large and high-quality FeSe bulk superconductors.

It is well known that iron pnictides and chalcogenides form a large variety of Fe based superconductors (FBS). Among FBS, iron chalcogenide superconductors has the simplest crystal structure which consists of only Fe₂Ch₂ (Ch = chalcogen) planes. Comparing with the other FBS, iron chalcogenide superconductors show lower superconducting critical temperature, T_c. However, the T_c of iron chalcogenide is easily modified by adding isotropic pressure or chemical intercalation. And also iron chalcogenides are known to indicate high upper critical field of superconductivity, H_c2.
Because iron chalcogenides can be a good candidate for application use, research on critical current density (J_c) of this materials is essential.
We have performed comprehensive and thorough research on substrate dependence of Fe(Se_1-xTe_x) thin films grown by pulsed laser deposition. Throughout the surveys, we have confirmed that CaF₂ substrate is the most appropriate for realizing fine superconducting properties of grown thin films. Critical current density (J_c) of FeSe_0.5Te_0.5 thin films on CaF₂ substrates were evaluated by result of magneto-optical (MO) observation and I-V characteristic measurements. The J_cs of the thin films obtained from MO observation (3.3×10⁶ A/cm² at 7.4 K) and I-V characteristic (2.1×10⁶ A/cm² at 7.0 K) show one of the highest values ever reported for superconducting thin films of iron chalcogenides. Besides, it implies homogenous critical current density that a typical rooftop pattern is observed by MO imaging. Since angular dependence of J_c under magnetic field can be understood as that for anisotorpic 3D superconductors by using effective magnetic field, H_eff = H·sqrt(sin²A+r^-2cos²A) , with mass anisotropy ratio r = 1.8, the thin film is considered not to have any extra pinning mechanism.

Among the recently discovered Fe-based superconducting compounds, the (K,Ba)Fe2As2 phase is raising large interest within the scientific community, because together with its pretty high TC, high and nearly isotropic upper critical field HC2 and its large critical current density weakly dependent on the magnetic field has shown to be not strongly affected by weak link at the grain boundaries. In fact, after some years of development, critical current densities Jc of about 105 A/cm2 at fields up to more than 10 T have been obtained in power in tube (PIT) processed wires and tapes. Here we explore the crucial points in the wire/tape fabrication by means of the ex-situ PIT method. We focus on a scaling up processes which are crucial for the industrial fabrication. In particular, we have addressed the issues concerning the phase purity, the internal porosity and crack formation in the superconducting core region. An extensive microstructural analysis correlated with the transport properties has been carried out as well. We performed the heat treatment without any additional external pressure, observing that, in these conditions, temperatures higher than 800 °C led to the formation of a secondary non-superconducting phases. We also pointed out how the brittleness of the superconducting compound is a crucial point for the cold working. Our best conductors with a filling factor of about 30% exhibited Tc= 38 K the highest value measured in such kind of superconducting tape. The microstructure analysis show clean and well connected grain boundaries but rather poor density: The measured Jc of about 3∙104 A/cm2in self-field is suppressed by less than a factor 7 at 7 T. Such not extremely high Jc values can be accounted for by the reduced density while the moderate in-field suppression and a rather high n-factor confirm the high homogeneity and uniformity of these tapes.

This work has been supported by FP7 European project SUPER-IRON (grant agreement No.283204) and “Compagnia di SanPaolo”. We thank Dr. Alice Scarpellini and Dr. Cristina Bernini for the help on scanning electron microscopy images acquisition.

The discovery of high-Tc Fe-based materials has catalyzed a revitalization in the field of superconductivity. However, it is remarkably little known about interfacial phenomena between non-perovskite superconductors and perovskite oxides. Among iron based superconductors, Co-doped BaFe2As2 can offer an ideal platform for studying the fundamental physics of superconductivity because these are much more chemically stable with well optimized growth technique [1,2]. It is now well established that the iron arsenides can have superconducting behavior which is induced by chemical substitution or pressure [3]. Here we report a kinetic and thermodynamic study of growth mechanisms in Co-doped BaFe2As2/SrTiO3 heterostructures. To get further insight into the initial stages of thin film growth, first principles calculations can predict the ground state of BaFe2As2 growth on a SrTiO3 substrate. They suggest a thermodynamically stable reaction layer between film and substrate. Additional structural and chemical analyses were performed by scanning transmission electron microscopy and electron energy loss spectroscopy. We believe that the interface structure can be tailored to optimize Fe-based superconducting thin films with various perovskite oxide substrates. Such studies can also suggest further research on the interface between non-perovskite and perovskite materials.
[1] S. Lee, J. Jiang, Y. Zhang, C.W. Bark, J. D.Weiss, C. Tarantini, C. T. Nelson, H.W. Jang, C. M. Folkman, S. H. Baek, A. Polyanskii, D. Abraimov, A. Yamamoto, J.W. Park, X. Q. Pan, E. E. Hellstrom, D. C. Larbalestier, and C. B. Eom, “Template engineering of Co-doped BaFe2As2 single-crystal thin films”, Nature Mater. 9, 397 (2010)
[2] S. Lee, C. Tarantini, P. Gao, J. Jiang, J. D.Weiss, F. Kametani, C. M. Folkman, Y. Zhang, X. Q. Pan, E. E. Hellstrom, D. C. Larbalestier and C. B. Eom, “Artificially engineered superlattices of pnictide superconductors”, Nature Mater. 12, 392 (2013)
[3] S. Kimber, A. Kreyssig, Y. Zhang, H. Jeschke, R. Valentí, F. Yokaichiya1, E. Colombier, J. Yan, T. C. Hansen, T. Chatterji, R. J. McQueeney, P. C. Canfield, A. Goldman, and D. N. Argyriou, “Similarities between structural distortions under pressure and chemical doping in superconducting BaFe2As2”, Nature Mater. 8, 471 (2009)

Work at the University of Wisconsin was supported by the DOE Office of Basic Energy Sciences under award number DE-FG02-06ER46327. The work at NHMFL was supported under NSF Cooperative Agreement DMR-1157490 and DMR-1006584.

We present the growth and characterization of SmFeAsO1-xFx and NdFeAsO1-xFx thin films which have been grown by a two-step method: metalorganic chemical vapor deposition and a postannealing diffusion processes. Precursor films containing a mixture of Sm and Fe or Nd and Fe oxides were deposited on LaAlO3 substrates by MOCVD, using as starting precursors: SamariumTris(2,2,6,6-tetramethyl-3,5-heptanedionato), NeodymiumTris(2,2,6,6-tetramethyl-3,5-heptanedionato) and Iron(III) 2,4-pentanedionate. Afterwards, the arsenic and fluorine are incorporated during an annealing of these precursor films in presence of a SmFeAsO0.75F0.25 or NdFeAsO0.75F0.25 pellets. The results of the composition, evaluated by an energy dispersive X-ray analysis probe (EDX), and results of crystallographic evolution, obtained by X-ray diffraction indicate that the method is suitable for the development of the SmFeAsO1-xFx and NdFeAsO1-xFx phases with the P4/nmm crystalline structure. The secondary ion mass spectroscopy indicates a successful uniform diffusion process of both fluorine and arsenic ions through all the film thickness. The resistance and magnetic measurements as a function of the temperature indicate that the SmFeAsO1-xFx films present a superconducting transition, with a Tc that depends on the preparation conditions; nevertheless, the NdFeAsO1-xFx films did not show a superconducting transition at all. We discuss such different behavior based on the incorporation of fluorine and the presence of spurious phases. These results show that our thin films have good quality and, the most important, that the two-step method reported in this work is an alternative technique to MBE or PLD to obtain pnictides- superconducting films.

This work was supported by the 163128 and 239895 projects of the CONACYT-México.

Fe(Te,Se) has the simplest crystal structure among all iron-based superconductors. In addition, it only contains less toxic chalcogens rather than arsenic, and has attracted interest both from basic science and application point of view. It is also known that as-grown crystals of Fe(Te,Se) do not show bulk superconductivity and post-annealing in O₂ [1], chalcogen [2,3], or pnictogen atmosphere [4] is necessary to induce bulk superconductivity. These annealing processes are believed to be effective in removing excess irons [5], which are harmful to superconductivity. We have characterized the value, distribution, and anisotropy of critical current density in the annealed Fe(Te,Se), and found that it is large, homogeneous, and isotropic [6,7]. We have also measured magnetic relaxation in the annealed Fe(Te,Se), and characterized vortex dynamics and vortex pinning mechanism [8]. We have also characterized some of normal state properties of well-annealed Fe(Te,Se).

[1] Y. Sun, T. Taen, Y. Tsuchiya, Z. X. Shi, and T. Tamegai, Supercond. Sci. Technol. 26, 015015 (2013).
[2] Y. Sun, Y. Tsuchiya, T. Yamada, T. Taen, S. Pyon, Z. X. Shi, and T. Tamegai, J. Phys. Soc. Jpn. 82, 093705 (2013).
[3] Y. Sun, Y. Tsuchiya, T. Yamada, T. Taen, S. Pyon, Z. X. Shi, and T. Tamegai, J. Phys. Soc. Jpn. 82, 115002 (2013).
[4] W. Zhou, Y. Sun, S. Zhang, J. C. Zhuang, F. Yuan, X. Li, Z. X. Shi, T. Yamada, Y. Tsuchiya, and T. Tamegai, J. Phys. Soc. Jpn. 83, 065001 (2014).
[5] Y. Sun, Y. Tsuchiya, T. Taen, T. Yamada, S. Pyon, A. Sugimoto, T. Ekino, Z. X. Shi, and T. Tamegai, Sci. Rep. 4, 4585 (2014).
[6] Y. Sun, T. Taen, Y. Tsuchiya, Q. P. Ding, S. Pyon, Z. X. Shi, and T. Tamegai, Appl. Phys. Express 6, 043101 (2013).
[7] Y. Sun, T. Taen, T. Yamada, Y. Tsuchiya, S. Pyon, and T. Tamegai, Supercond. Sci. Technol. 28, 044002 (2015).
[8] Y. Sun, T. Taen, Y. Tsuchiya, S. Pyon, Z. X. Shi, and T. Tamegai, Europhys. Lett. 103, 57013 (2013).

Support of this work by the European Union's Seventh Framework Programme (FP7/2007-2013) under grant agreement number 283141 (IRON-SEA) and Strategic International Collaborative Research Program (SICORP), Japan Science and Technology Agency. We think I. Mönch, J. Scheiter, A. Ichinose, I. Tsukada, A. Sala, M.Putti, M. Kühnel and U. Besold for help

The antiferromagnetic iron pnictide BaFe₂As₂ exhibits superconductivity when the long-range spin density wave (SDW) order is sufficiently suppressed either by carrier doping, isovalent substitution (e.g. Ru) or external pressure. The phase diagram of Ba(Fe_1-xRu_x)₂As₂ shows a superconducting dome up to an unusually high value of x ~ 40% in contrast to other substitution elements on the Fe-site [1]. In the last years, several explanations for the superconductivity in this system have been proposed, involving magnetic dilution, weakening of electronic correlations and the introduction of charge carriers in spite the fact that Ru is isovalent to Fe. Wang et al. tried to settle this debate by focusing their theoretical study on the effect of disorder introduced by Ru-substitution on the electronic structure and found a surprising insensitivity to the impurity scattering. Additionally, they found a reduction in the density of states at the Fermi level upon Ru-substitution [2]. Recently, we discovered that a tensile in-plain strain can induce superconductivity in undoped BaFe₂As₂ epitaxial thin films [3]. We also observed similarities between structural changes caused by Ru substitution and in-plain strain. Therefore, it is important to investigate the effect of in-plain strain on the phase diagram of Ru-substituted BaFe₂As₂ thin films. It is possible to introduce different amount of in-plane strain to the thin films using various single crystal substrates. It was found that the strain strongly affects both superconducting and antiferromagnetic SDW phases, revealing a strong structural dependence of the electronic properties. The results are discussed in comparison to data obtained on single crystals.

[1] F. Rullier-Albenque et al., Phys. Rev. B 81, 224503 (2010)
[2] L. Wang et al., Phys. Rev. Lett. 110, 037001 (2013)
[3] J. Engelmann et al., Nat. Commun. 4, 2877 (2013)

Support of this work by the German Research Foundation (DFG) within the framework of the Research Training Group 1621 and from European Union`s Seventh Framework Programme under grant number 283204 (Super-Iron) is gratefully acknowledged. M. Langer thanks J. Scheiter for technical assistance and J. Engelmann for fruitful discussions.

Superconducting multilayers are of great interest for fundamental research and electronic applications. One of the most interesting architectures, involves the superconductor/ferromagnet/superconductor systems (SFS) for the study of characteristics like the order parameter symmetry or the electrical properties of this type of Josephson juctions. Only a few works have been reported on the fabrication of pnictide superconducting multilayers.

Given this, in this work we report an approach to the fabrication and characterization of a trilayer system SFS of Fe(Se,Te)/Fe/Fe(Se,Te). As a first step we deposited trilayers of Fe(Se,Te)/Fe/Fe(Se,Te) on Fe-buffered MgO and MgAl2O4 by Pulsed Laser Deposition (PLD). The trilayers grow c-axis oriented and biaxially textured, as θ-2θ scans and texture measurements revealed. This find, was confirmed by in-situ RHEED measurements where both superconducting layers present good crystalline properties. They show a superconducting transition of 11.1 K on MgO and 13.1 K on MgAl2O4. This is slightly lower than a single layer of Fe(Se,Te) on the respective templates.

Oscar Rodríguez acknowledges to Colciencias for the scholarship support provided through its National Doctoral Program. We thank Michael Kühnel for technical support as well as Sabine Wurmehl and Michael Schulze for the provision of the FeSe_0.5Te_0.5 target. This work ws supported in part by the European Commision under project SUPERIRON grant 283204

This work was supported by the National Basic Research Program of China (973 Program) Grants 2011CBA00107, 2011CBA00202 and 2014CB339804, the National Natural Science Foundation of China (NSFC) Grants 11227904 and 61101010.

In this work, we study theoretically possible mechanisms of carrier localization, metal–insulator transitions (MITs) and stripe formation in hole-doped cuprates. Three distinctly different scenarios are proposed for the carrier localization in three-dimensional (3D) lightly doped cuprates. The binding energies and radii of the extrinsic and intrinsic large (bi)polarons in cuprates are calculated variationally using the continuum model and adiabatic approximation. We have shown that the extrinsic and intrinsic 3D large bipolarons exist in lightly doped cuprates at η = ε_∞/ε₀ < 0.127 and η < 0.138, respectively, where ε_∞(ε₀) is the optic (static) dielectric constant. While the optical bipolarons can exist if η < 0.134 and the Fröhlich coupling constants α are greater than 5.8. The dopant- and carrier-driven inhomogeneities favor the specific charge ordering in the form of a 3D network of carrier-rich and carrier-poor stripes and the formation of different superlattices and in-gap bands of dopants and large polarons. The localized in-gap states develop into metallic states at some critical doping levels. We use the uncertainty relation to obtain the specific conditions for the Anderson and new MITs in cuprates. The applicability limits of these MITs in cuprates are clarified. We argue that the new MITs in the cuprates caused by the strong carrier–defect–phonon and carrier–phonon interactions are accompanied by the formation of a 3D self-organized network of carrier-poor (insulating) and carrier-rich (metallic) stripes, which coexist in a wide range of doping x ≈ 0.02-0.20, and the suppression of superconductivity observed in underdoped region near the x = 1/8 is caused by the formation of insulating stripes on a global scale and by the preponderance of insulating phase compared with the metallic one. Our results are in good agreement with the existing experiments on La-based and other cuprates.

The HgBa₂Ca₂Cu₃O_8+δ high temperature superconductor shows superconducting transition temperature T_c above 135 K and T_c≈165K under the external pressure. These characteristics make the Hg-1223 phase a very attractive material for producing superconducting wires, tapes, and thin-films. Therefore, creation simpler and reliable technologies for synthesized high stability Hg-1223 phase are very important subjects in order to achieve the large-scale technological applications of mercury-based superconductors.
We report the wet chemistry methods for preparation Hg-based polycrystalline superconductors. For synthesized Hg-1223 samples, we used two-step method. In the first stage non-mercury containing precursor was prepared before proceeding to the second stage where HgO was added to the precursor before final sintering. The synthesis of Ba₂Ca₂Cu₃O_y is carried out by dissolving separately CaCO₃ in acetic acid, CuO in ammonium chloride and BaCO₃ in distilled water. The solutions are mixed afterward in the stoichiometric ratio. To this solution were added poly(vinyl alcohol) (PVA). The solution dried at 180^oC temperature for 12 h and after obtained gel was dreaded at 300^oC. The xerogels are then homogenised in an agate mortar and calcined at 750^oC, during 4 h in a muffled furnace. The obtained precursor is then mixed with HgO in nominal stoichiometric quantity. The resulting powder is pelletized and inserted in a quartz tube that is closed under vacuum. These pellets were sintered in furnace at 800^oC for 6 hours. The structure of the obtained materials was determined by XRD. Transition temperature was measured by ac susceptibility and critical currents density Jc by high harmonic methods. Finally, including poly(vinyl alcohol) in preparation precursor Ba₂Ca₂Cu₃O_y we observed single-phase and purity high temperature Hg-1223 phase.

This work has been fulfilled by financial support of the Shota Rustaveli National Science Foundation, Grant No. FR/423/6-260/12.

Nb₃Sn is one of the widely used practical superconductors for dc high field magnet applications. There is also much interest in using Nb₃Sn for radio-frequency (RF) applications, particularly in superconducting resonator cavities for particle accelerators [1]. However, the small lower critical field H_c1 limits the ability of Nb₃Sn to withstand high RF fields > 100 mT as penetration of vortices causes strong dissipation. To address this problem it was suggested to coat the Nb resonator cavities with superconductor-Insulator-Superconductor (SIS) multilayers in which the thickness of Nb₃Sn films is smaller than the London penetration depth [2]. In this work we have grown and tested thin films and multilayers of Nb₃Sn and dielectrics on oxide substrates. These structures have been fabricated by two different approaches: first, by the alternating deposition of Nb and Sn layers followed by annealing; and second, by co-deposition of Nb and Sn at high temperature. Temperature dependencies of the low-field quality factor were measured at 7.4 GHz for 50 and 100 nm thick Nb₃Sn bilayers. The Nb₃Sn films have shown good screening properties and the onset of superconductivity close to the bulk T_c. SIS multilayers were also fabricated and tested for their RF properties. The results were compared to previous studies conducted on SIS multilayers of other materials such as NbN [3].
[1]. S.M. Deambrosis, G. Keppel, V. Ramazzo, C. Roncaloto, R.G. Sharma, and V. Palmieri, Physica C 441 (2006) 108.
[2]. A. Gurevich, Appl. Phys. Lett. 88 (2006) 012511.
[3]. C. Z. Antoine, J. C. Villegier, and G. Martinet, Appl. Phys. Lett. 102 (2013) 102603.

Work supported by the US Department of Energy under grant No. DE-SC0010081.

In this study, the high temperature superconductor (Bi,Pb)₂Sr₂Ca₂Cu₃O_x (Bi-2223) thick films were fabricated by dip-coating method on silver substrates. Two deposited precursor films were stacked “Face-to-face” (FTF) to prevent the volatilization of specific elements during sintering. Two-step heat treatment was studied. The sintering temperature was fixed on 835 ^oC and different sintering time was investigated in an 8.5% O₂ (N₂ balanced) for the first period heat treatment (HT1). After pressed by 1.0 GPa, the samples were sintered at 825 ^oC in an 8.5% O₂ (N₂ balanced) for the second heat treatment (HT2). The thick film which was sintered for 10 h in HT1 showed a zero-resistance transition temperature (T_C0) of 107.9 K and a critical current density (J_C) of 1100 A/cm² (77 K, 0 T). And its J_C value was 5500 A/cm² (77 K, 0 T) after HT2.

The authors own great thankfulness for the support from the National Natural Science Foundation of China (51475257), the Development Foundation of Shenzhen (JCYJ20130402145002389), and the Tribology Science Fund of State Key Laboratory of Tribology, China.

In this study, textured (Bi,Pb)₂Sr₂Ca₂Cu₃O_x ((Bi,Pb)-2223) thin films were successfully fabricated on LaAlO₃ (100) substrates by using chemical solution deposition (CSD) method. The precursor solution was produced from acetates and nitrites dissolved in acetic acid and ammonia water. The as prepared solution was spin-coated on LAO substrates. The heat treatment includes a pyrolysis process at 500 ^oC and a sintering process at above 800 ^oC in reduced oxygen atmosphere. The growth of pure (Bi,Pb)-2223 phase through CSD is quite challenging due to the volatilization of specific elements during high temperature heat treatments. This element loss can be reduced by sealing the pyrolyzed films inside a silver foil with (Bi,Pb)-2223 precursor powder surrounded. Films with the thickness of about 150 nm were obtained after sintering. The microstructures and superconducting properties were investigated. It is shown from the X-ray diffraction patterns that the content of (Bi,Pb)-2223 phase varied with increasing sintering temperature, with the maximum value achieved at 845 ^oC. Strong bi-axial texture was also observed. The critical temperature for all samples is around 104 K. The critical current density for the optimized sample reached more than 0.05 MA/cm² at 77K.

The authors own great thankfulness for the support from the National Natural Science Foundation of China (51475257), the Development Foundation of Shenzhen (JCYJ20130402145002389), and the Tribology Science Fund of State Key Laboratory of Tribology, China

The superconducting, structural and mechanical properties of BSCCO bulk samples have been studied as a function of pressure by means of AC susceptibility, XRD and microhardness measurements, respectively. In this study, 3 samples have been prepared. All samples are kept for 120 h at 845 οC and then cooled by 1 οC/min rates from annealing temperature to 200 οC in air atmosphere. During the preparation of the samples one and two intermediate pressed are applied at 75MPa. X-ray diffraction analysis shows that both Bi-2223 and Bi-2212 phases detect for all samples. However, according to intermediate pressing, Bi-2223 phases getting higher. The critical transition temperature, phase purity and crystallinity of the prepared bulk samples are compared with each other. Elasticity (E), brittleness (Bi), fracture toughness (KIC) and yield strength (Y) values are also determined according to annealing time, applied load and production parameters of materials.

This work has been supported by Turkish Research and Scientific Council (TUBITAK) under grant contract No: 113F150. We also acknowledge The Republic of Turkey, Ministry of Development under the Project Number 2010K120520.

The proximity effect is theoretically studied for thin and bulk pure HTS/F contacts, where F is a ferromagnet and HTS is a high-temperature superconductor. It is shown that there is no continuous matching between the s-wave and d-wave pair amplitudes at the HTS/F boundary. In thin HTS/F nanostructures the exchange field strongly suppresses the averaged s- and d-pairing and leads to the competition between Bardeen-Cooper-Schrieffer (BCS) and Fulde-Ferrell-Larkin-Ovchinnikov scenarios of superconductivity. On the contrary, in bulk HTS/F contact the exchange field has no effect on the critical temperature T_cd of d-wave pairing. This is due to the lack of d-wave pairing in F layers, resulting in full internal reflection of d-wave pairs even from perfectly transparent HTS/F boundary. The s-wave superconductivity localized at the HTS/F interface is predicted. Due to a strong damping of the surface critical temperature T_cd for the d-wave component, one should expect spontaneous symmetry change of the order parameter at the HTS/F interfaces from initially d-wave type to s-type, because T_cs of the s-wave component is proved to be higher than T_cd. It qualitatively agrees with experimental observation of the mixture of s- and d- wave components at the Ag/BiSrCaCuO and Fe/Ag/BiSrCaCuO interfaces [1,2].
Moreover, many of HTS experts suppose, that the spin-fluctuation mechanism in HTS is realized, leading to the d-wave symmetry of superconducting order parameter. On the other hand, K.A. Müller, discoverer of HTS systems, considers that the d-wave symmetry of the order parameter is realized on the surface of HTS while the BCS mechanism with s-wave pairing is implemented in the bulk [3]. We believe, that the problem of the symmetry of the HTS order parameter could be studied based on the proximity effect in HTS/F and HTS/S nanostructures, if conventional BCS superconductor (S) or ferromagnet is used as a peculiar kind of probe (see [4] for F/S systems).
1. M. Freamat, K.-W. Ng, Phys. Rev. B 68, 060507 (2003)
2. K.-W. Ng, M. Freamat, Int. Journ. Mod. Phys. B 19, 495 (2005)
3. K.A. Müller, Physica C 387, 57 (2003)
4. M.G. Khusainov, et al., JETP Lett. 89, 626 (2009); ibid. 90, 124 (2009)

The work is partially supported by Russian Government Program of Competitive Growth of Kazan Federal University. Marat Khusainov is also thankful to the RFBR (13-02-01202) for partial support.

Growths and interfacial properties of NbN films deposited on MgO substrates by magnetron sputtering were investigated. The epitaxial NbN Films can be deposited on the MgO substrates. However, the interface effect between film and substrate is often reported to be strongly defective due to lattice mismatch, so that the conductivity of NbN films gets worse. In this paper, we investigate the transport properties and crystal structures of NbN films on different crystal oritentation MgO sunstrates. Meanwhile, we also investigate the relationships between the growth parameters including growrh temperature, growth atmosphere, substrate pretreatment and superconductivity of NbN films. Moreover, the surface structures of films and substrates are also investigated by Low energy diffraction (LEED) and high energy electron diffraction (RHEED).

This work was supported by 863 project of china (No.2014AA032703) and The National Natural Science Fund of china (No.61171003) and CAST-BISEE innovation fund of Beijing Institute of Spacecraft Environment Engineering.

Granular superconductivity occurs when microscopic superconducting grains are connected with neighbors either by non-superconducting materials, or by less robust superconductors, or even by a physical constriction, i.e., an equally strong superconductor of smaller cross section. The intragranular component is related to the essence of the grains, governed by the inherent properties of the material while, on the other hand, the intergranular part is associated with grain boundaries coupling neighboring grains - the so-called weak-link. One characteristic feature of granular superconductors is the occurrence of a double resistive superconducting transition and the typical two-peaks and two-plateaus in the ac-susceptibility components. Using ac-susceptibility and magneto-transport measurements, we have investigated the superconducting properties of weak-links created artificially in Nb films. The 200 nm thick Nb thin films with rectangular shape (3 x 1 mm2), were deposited by magnetron sputtering in a UHV system. A Focused Ion Beam apparatus was employed to carve a groove in the central region of the film, along its width, creating thus two identical grains. The groove, which acts as a single weak-link, had its depth and width controlled by the FIB dose. The topographical characterization was performed by use of atomic force microscopy. Magnetic and magneto-transport measurements, as well as magneto-optical imaging, were used to determine the superconducting parameters associated with the grains and with the weak-link.

This paper reflects on the investigation of high-quality 5nm thin NbN films deposited by means of reactive DC magnetron sputtering at room temperature. The deposition without intentional substrate heating offers major advantages from a processing point of view and motivates the extensive composition- and surface characterization and comparison of present films with high quality films grown at elevated temperatures. Monocrystalline NbN films have been epitaxially grown onto hexagonal GaN buffer-layers (0002) and show a distinct, low defect interface as confirmed from High-Resolution TEM. The critical temperature (Tc) of those films reached 10.4K. Furthermore, a poly-crystalline structure was observed on films grown onto Si (100) substrates, exhibiting a Tc of 8.1K albeit a narrow transition from the normal to the superconducting state. X-ray photoelectron spectroscopy and reflected electron energy loss spectroscopy verified that the composition of NbN did not differ irrespectively of applied substrate heating. Moreover, the native oxide layer at the surface of NbN has been identified as NbO₂ and thus, is in contrast to the Nb₂O₅, usually being formed at the surface of Nb when exposed to air. These findings are of significance since it was proven the possibility of growing epitaxial NbN onto GaN buffer layer in the absence of high temperatures hence paving the way to employ NbN in more advanced fabrication processes involving a higher degree of complexity. The eased integration and employment of lift-off techniques could, in particular, lead to improved performance of cryogenic ultra-sensitive terahertz electronics.

In the deposition of MgO films by radio frequency magnetron sputtering method, the growing film is usually bombarded by energetic particles mainly composed of oxygen atoms and negative oxygen ions with energy of several hundred eV. The bombardment could cause reduction in the film thickness , which is usually called re-sputtering effect. In this study, the density distribution of energetic particles was obtained from the thickness profile of as-deposited MgO film. It was found that the energetic particle flux was well collimated. Thus, a new method was designed to fabricate bi-axially textured MgO buffer layer using the energetic particle flux. In the deposition area simultaneously bombarded by energetic particles from 55° tilted target, MgO layer could develop bi-axial texture within 10 nm thickness on the substrate of amorphous Y2O3 buffered Hastelloy C276. The XRD measurement indicated that the MgO layer exhibited a (111) out-of-plane orientation and a 3-fold symmetric in-plane orientation. This method performed similar to the ion beam assisted deposition (IBAD) method but didn’t need any ion sources. The influence of sputtering parameters, such as duration of deposition and so on, on the bi-axial texture of MgO was investigated.

This study was supported by National Natural Science Foundation of China (51402165), Guangdong Natural Science Foundation (S2013040014199), and Fundamental Research Program of Shenzhen (JCYJ20140827160129762).

To enhance the J_c of YBCO films both in self-field and under magnetic field, an effective strategy is to introduce artificial pining centers and keep a good YBCO matrix at the same time. Here we propose a new dopant ---LaAlO₃ (LAO), based on its chemical stability and small mismatch towards YBCO. A series YBCO films with different LAO doping contents were fabricated on LAO single crystal substrates by metal organic deposition. We observed that though a lot of LAO is added to YBCO, by XRD measurement and SEM observations YBCO still keep a good epitaxial growth relationship. Superconductivity is investigated by VSM, and an unusual curve of moment loop is found at low temperature. There is a moment peak appoximately at -0.01T and +0.01T respectively and a result is that the highest J_c is obtained at 0.01T. Besides, the improvement observed under magnetic field, the J_c of the doped sample also is significantly enhanced in self-field compared to a pure YBCO film. The J_c of the 5% LAO doped sample is enhanced more than 3times in self-field 77K, whereas at 77K and 1.5T, the J_c value is 7 times larger than in the pure YBCO film. These results indicate that LAO doping can effectively enhance the Jc of YBCO films both in self-field and in applied magnetic fields.

This work is financially supported by the Chnia Scholarship, National Natural Science Foundation of China（51171002), the Project of Construction of Innovative Teams and Teacher Career Development for Universities and Colleges Under Beijing Municipality（IDHT20130510), and by 211 Program of Beijing City

The proposed study investigates the aging of two precursor solutions used for chemical solution deposition (CSD) of YBa₂Cu₃O_7-x (YBCO) and composite YBCO films with nanostructured inclusions of BaZrO₃ (YBCO-BZO). The stability of precursor solutions plays a crucial role to determine the crystalline quality and the superconducting properties of YBCO films. Even though low or free fluorine solutions are promising in order to improve the synthesis route and to reduce the environmental impact, the poor stability of the solution is the major drawback of the low-fluorine precursors, since they degrade faster (4-6 weeks) than other precursor solutions for superconducting layers (e.g. trifluoroacetates solutions are stable up to 6 months) or buffer layers (stable for several months). Therefore, a deeper comprehension of the degradation mechanisms is fundamental to properly optimize the precursor solution composition. The current work studies two precursor solutions (for YBCO and YBCO-BZO) that are complex systems in which different interactions need to be considered: most of all, the nature of ligands (acetate, propionate, ammonia) that complex each metal and influence not only the solution stability but also the quality of the deposited film. Thus, information obtained from different analyses [Fourier transformed infrared spectroscopy (FT-IR), thermal analyses (DTA/TG) coupled with mass spectrometry (MS), X-ray photoelectron spectroscopy (XPS), Raman Spectroscopy, Nuclear Magnetic Resonance (NMR)] is correlated with morphological, structural and superconducting properties of the deposited film in order to understand the underlying chemical mechanisms of aging (salts precipitation, oxidation, polymerization,..) and its effects.

This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission.

YBCO/BYNO nano-composite was successfully fabricated by metal organic deposition method based on trifluoroeacetate salts (TFA-MOD). The paper systematically studied the influence on the microstructure, surface quality and on the superconducting properties of the YBCO films with different content of Nb dopant. It is demonstrated that the microstructure of YBCO films had not an obvious impact for a Nb doping amount of less than 10mol.%, while the surface quality of YBCO is still improved. Although the critical transition temperature (Tc) of YBCO decreased slightly after Nb doping, it was kept as high as 90K for the present experiments. In addition, a significant enhancement of the critical current density (Jc) was observed for YBCO films with BYNO nanoparticles compared to those of pure YBCO films both in self-field and applied field. The epitaxial and random oriented BYNO nanoparticles coexisted in YBCO nanocomposites, which can be explained by the nucleation model. The size of the BYNO is between 20nm and 30nm, while the random fraction of BYNO was above 90%. The stacking fault parallel to the film and a serious lattice distortion of YBCO can be found around the BYNO nanoparticles, which could strengthen the nanostrain in the nanocomposite. As increasing the random fraction of BYNO, the nanostrain within nanocomposites was strengthened. According to that, the enhancement of field properties of BYNO resulted from the enhanced nanostrain. The present results can provide the basis for the enhancement of the in-field properties of YBCO nanocomposite.

The low fluorine solution was useful for the epitaxial growth of YBa₂Cu₃O_y (YBCO) film. In this paper, we developed a series of low fluorine solutions containing different content of fluorine to fabricate YBCO film on buffered NiW substrates and single crystal LaAlO₃ substrates. In comparison with the traditional all TFA solution, these new low fluorine solutions had 0% - 50% of fluorine content. Fast pyrolysis process and smooth surface of YBCO precursor films could be acquired by using the low fluorine solution. The phase, texture, microstructure and superconducting properties of YBCO films were characterized by X-ray diffraction, scanning electron microscopy and four-probe method. When the optimal fluorine content was 23%, the solution could significantly reduce the formation of BaCO₃ and increase the formation of BaF₂ in the pyrolyzed film. The YBCO films with a critical current density (J_c) of ~ 2.6 MA/cm² (77K, 0T) could be obtained on buffered NiW substrates (CeO₂/YSZ/CeO₂/NiW) from the low fluorine solution.

This work was financially supported by the International Science & Technology Cooperation Program of China (No.2012DFA50780), the National Science Fund Program of China (No. 51302225 and 51202201), the Innovative Research Team of Shaanxi province (No.2013KCT-07), the Science & Technology Nova Program of Shaanxi Province (No.2014KJXX-23).

​Strontium titanate (STO) is an ideal candidate to use as a single buffer layer for growing high quality YBa₂Cu₃O_7-X (YBCO) film.
Biaxially textured STO buffer layer has been grown by pulsed laser deposition (PLD) on rolling assisted biaxially textured substrates (RABiTS) made of low cost Cu-clad stainless steel substrate with textured electroplated Ni provided by Tanaka Kikinzoku, which exhibits a very strong cube texture and a roughness below 10 nm.
This study presents the development and optimization of the maximum achievable thickness of highly textured (00l) STO films on metallic substrates by means of PLD process using reducing atmosphere (Ar/5%H₂) to prevent the oxidation of the substrate.
STO was deposited by PLD using excimer laser KrF at different temperatures from 300 to 850 ºC. Optimization of the parameters of PLD process shows that STO film deposited at 500 ºC exhibits the best quality texture with high repeatability.
On the other hand, STO films were deposited with different thickness, i.e. 0.3, 0.5, 0.8 and 1 µm, to achieve the maximum thickness with the best texture and quality surface.
XRD analyses reveal an excellent quality of biaxial texture and SEM images show an adequate two dimensional growth of the layer and a smooth surface, in all the cases.
EBSD analyses are in line with XRD measurements from 0.3 to 0.8 µm and show that STO films grow epitaxially, with a fraction of oriented area exceeding 97% with 12º tolerance angle, whereas at 1 µm of thickness the surface texture is strongly deteriorated.
The optimum thickness with the best texture quality is 0.8 µm of STO, with a rms roughness of about 5 nm and the full width at half maximum (FWHM) of omega-scans RD and TD of (002) STO and phi-scan (111) STO are 4.5º, 6.2º and 5.3º, respectively.
STO/Ni/Cu-clad stainless steel architecture prepared by PLD under reducing atmosphere and low temperature may provide useful templates for the growth of YBCO coated conductors.

The research leading to these results has received funding from the European Union’s Seventh Framework Programme (FP7/2007-2013) under grant agreement nº 280432.
 

A series of Dy1-xGdxBa2Cu3O7- d (DGBCO) thin films, with partial substitution of Dy with Gd (with 0 ≤ x ≤ 0.2), was fabricated on LaAlO3 single crystal substrates by means of TFA-MOD, using trifluoroacetates. Thin film properties were investigated using Vibrating Sample Magnetometry, X-ray diffraction and Scanning Electron Microscopy. Analysis of X-ray diffraction patterns for all samples reveals that the (123)-phase is formed and minor reflections representing other phases are observed. The critical temperature was about 89 K and found to be independent of the substitution ratio between Dy and Gd. Analysis of critical current densities and pinning forces are also presented in this work.

This work was supported by Capes agency - Proc. n° BEX 13491/13-0. Authors thanks the Department of Energy Conversion and Storage of Technical University of Denmark.

Chemical solution deposition (CSD) offers a versatile and low cost methodology for the production of coated conductors; however, reaching thick films by CSD becomes one of the main challenges for this technique due to the high stress produced during pyrolysis, which may lead to buckling and crack formation, so degraded superconducting properties.
The pyrolysis study of low fluor metalorganic Y, Ba and Cu precursor solutions deposited by a single-nozzle inkjet printer (IJP) was carried out for gel deposited layers of several microns. The main investigated parameters were the effect of the heating rate and the solution formulation.
Thermo gravimetric analysis (TGA), evolved gas analysis (EGA) and infrared spectroscopy (IR) were performed to study the evaporation/decomposition of organic products along pyrolysis. Thickness evolution was studied in different stages by profilometry and the mechanical properties of the gel by nanoindentation. In addition a new analytical tool was implemented (Pyrolyzer) based on a heating stage device coupled to an optical microscope that allows the recording of the whole process.
First of all, we indentified the zone of maximum shrinkage before the decomposition of organic matter (T<280ºC), where also buckling may be reversible in certain cases. During this shrinkage zone, buckling and cracking may appear as stress relief mechanism which shows up clearly separated. Buckling formation was observed at rather low temperatures (~200ºC) associated to an elastic regime. Cracking, on the other hand, was identified as an irreversible phenomenon at higher temperatures (~270ºC), once the layer becomes stiffer. All this kinetic study has enabled us to optimize the overall pyrolysis process to obtain single deposited thick pyrolyzed films (~2 µm) with the homogeneous morphology required to obtain high performance, ~1 µm, YBCO epitaxial films Finally, we demonstrate that multideposition of these YBCO thick layers by inkjet printing is a successful and very promising approach to achieve film thickness beyond 1µm. Following the optimized pyrolysis process high quality thick films (> 1 µm) may be achieved.

Acknowledgements. The research leading to these results has received funding from EU-FP7 NMP-LA-2012-280432 EUROTAPES project, MP1201 Cost action and MAT2014-51778-C2-1-R national project.

Abnormal increase of deposition speed by pulsed laser deposition process (PLD) is investigated. Deposition speeds from 0.1 to 10 nm per elementary laser pulse were analysed regarding critical current in HTS coated conductors.
Experiments were performed in PLD installation employed for deposition of both ceria cap buffer and YBCO layers. Cr-Ni stainless steel tape preliminary coated with bi-textured yttria-stabilized-zirconia buffer layer via alternating beam assisted deposition (ABAD) is used as a substrate.
Perturbations of crystallinity in YBCO films were observed at highest deposition speeds. In spite of reduction of crystalline and morphology perfection only a moderate reduction of critical current in HTS films was found.
Maxima in dependences of critical current versus deposition speed at different temperatures are demonstrated and discussed.
Influence of tape tension during PLD process as well as initial substrate/buffer defects are shown to be a reason for dropouts in transport current across the tape length. Alternative possibilities for suppression of these defects are discussed.
Effects of smoothening of local peculiarities in YBCO films with increased thickness are identified and reported.
Feasibility of ultra-high speed PLD is analysed taking into account magnetic field strength under which operation of tape is expected.

In order to reveal the origin of T_c increase of Sr-substituted YBa₂Cu₄O₈ (Y124), we have fabricated Sr-doped Y124 crystals by KOH flux method, which can grow high quality single crystals. In the previous work done for powder crystals, we found that the T_c determined by magnetization measurement enhances as fabrication temperature decreases. In this study, in order to confirm the evidence of T_c increase by means of electrical resistivity measurement and investigate the correlation between T_c and lattice constant, we deposited Sr-doped Y124 films on NdGaO₃ (001) substrates by the KOH flux method.
Y₂O₃, BaCO₃, SrCO₃ and CuO were used for raw materials, and NdGaO₃ (001) substrate was used for a seed crystal. Nominal composition was Y:Ba:Sr:Cu = 1:1.8:0.2:4 in molar ratio. 100 wt% KOH to the raw materials were used as a flux. The raw material powders and flux were hated up to 600-700ºC in an alumina crucible and kept for 12 hours.
The fabrication of Y124 films were identified by XRD measurement. The film fabricated at 700ºC showed zero-resistance at 80 K (T_c^zero) and the c-axis length of 27.19 Å. In contrast, the film fabricated at 600ºC showed T_c^zero = 84.5 K and c = 27.24 Å, showing T_c enhancement. As is the case of powder crystals, Sr-doped Y124 film fabricated at a low temperature showed a higher T_c value than at a high temperature. Additionally, compared to film fabricated at 600ºC, the c-axis length of film fabricated at 700ºC contracted.
Some reports dealing with Sr-substituted Y124 concluded that the T_c increases by lattice construction and chemical pressure effect by the incorporation of Sr to the Ba sites. This study indicates the opposite relationship between T_c enhancement and c-lattice contraction. Consequently, the origin of the improvement of T_c observed on the Sr-doped Y124 fabricated at low temperature by the KOH flux method is not chemical pressure effect but hole-dope effect by the incorporation of Sr²⁺ to the Y³⁺ sites.

Rare-earth based oxide superconductors REBa₂Cu₃O_7-d (RE123: RE=rare-earth elements) and REBa₂Cu₄O₈ (RE124) can be grown from the solution made of molten potassium hydroxide (KOH) at low temperatures around 500ºC. The low temperature growth using KOH flux is quite suitable for the production of RE123 films on various substrate materials. Since KOH flux growth is a near equilibrium process, RE123 and RE124 can be obtained depending on temperature and oxygen partial pressure (PO₂). To find out the proper growth conditions of RE123 from KOH flux, temperature and PO₂ dependence in phase formation was investigated. Superconducting property was also measured by SQUID magnetometer and transport measurement to find out whether there are some effects of KOH flux on RE123 superconductivity depending on temperature and PO₂.
As same as the phase relations reported for solid-state reaction process, Y123 phase was formed from KOH flux at a high temperature and low PO₂ region. The obtained Y123 powder crystals showed superconducting critical temperature (Tc) more than 90 K after oxygenation annealing regardless of formation temperature and PO₂. On the other hand, Eu123 and Eu124 phase were co-obtained at low temperature region in air growth and no Eu124 but Eu123 phase was formed at PO₂=10^-3 atm at temperatures from 500ºC to 700ºC. Therefore, phase relation of Eu123 and Eu124 was slightly different form that of the Y-based system. Tc of Eu123 phase depended on growth temperature and PO₂. Tc decreased as growth temperature decreased and PO₂ increased. The Tc degradation of Eu123 phase and the phase relations of Eu-system slightly different from Y-system are possibly occurred by the partial substitution of Eu for Ba site in E123 crystal structure by KOH flux.
RE123 films on single crystalline substrates were obtained from KOH flux. This indicates that KOH flux method has a process potential for the application to the preparation of RE123/substrate composite devices.

The authors thank F. Nakayama for experimentation discussion.

The growth of YBa2Cu3O6+x (YBCO) films deposited by pulsed laser deposition on a high-quality biaxially textured NiW substrates with chemical-solution-deposited buffer layers, Gd2Zr2O7 barrier and Ce0.9La0.1O2 cap, is systematically investigated. The optimization process is executed by varying the number of laser pulses i.e. the thickness of the film, to get better understanding about the quality of YBCO films near the interface of the buffer layer. The structural characterizations as well as the magnetic and resistivity properties are determined by x-ray diffractometry, atomic force microscopy, and with conventional magnetometry and transport measurements.
The structural analysis shows that the YBCO films are fully textured, c-axis out-of-plane, but in-plane 45º oriented grains occur in all the thicknesses. In addition, the full width at half maximum of the YBCO (005) peak increases in 2theta and the c-axis lengthens with increasing film thickness, indicating modified unit cell structure in thicker films. When comparing the superconducting properties with the properties of YBCO films grown on single crystal SrTiO3 substrates, the critical temperature in films on buffered NiW is slightly lower but the transitions are clearly broader. However, remarkable thickness dependent variation in Tc or in Delta Tc cannot be observed. In any case, the obtained results are in the significant role when developing the optimal buffer layer structure for NiW metal substrate based future generation technology solutions.

Jenny and Antti Wihuri Foundation and Finnish Cultural Foundation are acknowledged for financial support.

YBa₂Cu₃O_7-x (YBCO) films with co-doping BaTiO3 (BTO) and Y₂O₃ nanostructures were prepared by metal organic deposition using trifluoroacetates (TFA-MOD) method. The properties of the BTO/Y₂O₃ co-doped YBCO films with different excess yttrium have been systematically studied by x-ray diffraction (XRD), Raman spectra and scanning electron microscope (SEM). The optimized content of yttrium excess in the BTO/Y₂O₃ co-doped YBCO films is 10 mol %, and the critical current density (JC) is as high as ~ 17 MA/cm2 (self-field, 65K) by the measurement of magnetic signal. In addition, the Y2Cu2O5 was formed when the content of yttrium excess rising to 24 mol %, which may be result in the deterioration of the superconducting properties and the microstructure. The unique combination of the different types of nanostructures of BTO and Y₂O₃ in the doped YBCO films, compared with the pure YBCO films and BTO doped YBCO films, enhance the critical current density (JC) not only at self-magnetic field, but also in applied magnetic field.

The surface roughness of substrate has a significant effect on the initial texture of IBAD_MgO thin films. Biaxially textured MgO thin film layer was deposited on chemical solution derived planarization Y₂O₃ buffer substrate by IBAD(Ion Beam Assisted Deposition) method for HTS(High temperature superconductor) coated conductor. Planarization Y₂O₃ layer was prepared on unpolished Hastelloy metal tape using a continuous reel-to-reel solution multi-nano layer coating process. Unpolished Hastelloy substrate has a typical RMS roughness around 21 nm over 5ｘ5 um² area. Planarization Y₂O₃ buffer substrates were prepared with various degrees of RMS roughness values between 0.5 and 15 nm. We studied the influence of the surface roughness of the planarization Y₂O₃ buffer substrate for IBAD_MgO films.

Magnesium diboride thin films show promise for use as electronic devices, in RF cavities, and as high field conductors. The most popular method for producing MgB2 thin films is through hybrid physical chemical vapor deposition. This technique utilizes magnesium pellets and diborane gas to yield high quality, epitaxial thin films that exhibit good high field properties and higher-than-bulk Tc. Other techniques have been studied including chemical vapor deposition, pulsed laser deposition, and sputtering. These techniques involve expensive equipment and make scaling up to industrial quantities difficult. Using solution-based approaches to create MgB2 thin films, scalability is increased substantially while cost is kept low. One such process involves synthesizing a magnesium borohydride solution, depositing this solution on various substrates through spin coating and dip coating, and annealing the film to evolve hydrogen and sinter the film. The other approach is to use a magnesium diboride suspension, coat the sample through an electric field, and anneal the sample to sinter the particles. The electronic and magnetic properties of these thin films are shown and compared to epitaxially grown films.

This work was performed within Partnership program - PN II, funded by MEN-UEFISCDI, project No. 214/2014. Authors thank Dr. V. Mihalache for help with XRD measurements.

Superconducting disks of MgB₂ with addition of Sb₂O₅ with different stoichiometric compositions ((MgB₂) + (Sb₂O₅)_x, x = 0.0025, 0.005, 0.015 were obtained by the ex-situ Spark Plasma Sintering (SPS) technique. All added samples have high density (>95%). The critical temperature T_c shows a small variation within 38-38.8K. This result and XRD data suggest that Sb does not enter the lattice of MgB₂. X-ray diffraction investigations indicate that impurity phases are Mg₃Sb₂, MgO, and MgB₄. We found that there is an enhancement of the irreversibility magnetic field for the optimum doped sample with x = 0.005 or 0.01 of the Sb₂O₅ additive. The complex correlation between starting compositions, phases, microstructure and vortex pinning aspects are discussed.

This work was performed within Partnership program - PN II, funded by MEN-UEFISCDI, project No. 214/2014. M.B. acknowledges Sectoral Operational Programme Human Resources Development 2007-2013 of the Romanian Ministry of European Funds through the Financial Agreement POSDRU/159/1.5/S/132395.

This research has been financially supported by Scientific and Technological Research Council of Turkey, TUBITAK (Grant No. 113F205).

Since the discovery of superconductivity in MgB₂, various preparation methods have been developed for films, filaments, or wires of this material. Physical vapor deposition methods require a well-adjusted atmosphere and the prevention of oxygen contamination. Another possibility is the Mg diffusion method where different Mg and B precursors undergo a reaction. Of all the deposition methods for MgB₂ films, hybrid physical-chemical vapor deposition has been the most effective one for epitaxial, yet only thin films. A new method for preparing films is using pre-reacted MgB₂ powder by the technique called Aerosol Deposition technique which is known for preparation of ceramic films. Multifilament conductors are prepared with the powder-in-tube technique using in-situ and/or ex-situ reacted MgB₂ powders. This shows that most promising preparation methods need MgB2 powder. However, the processing of MgB₂ as a material itself is difficult.
One important aspect is the formation of the hazardous gas B₂H₆ during processing of MgB₂ powder. MgB₂ can react with water from the atmosphere and forms B₂H₆ or higher borides. This gas is harmful by inhalation and flammable. However, this gas is colorless and the odor threshold is higher than the highest permissible concentration of B₂H₆ in the air at the workplace (AGW value 0.1 ppm). We will demonstrate how B₂H₆ is formed. In order to do so, we investigated the influence of temperature and humidity on as-received and milled MgB₂ powder in a closed system. The B₂H₆ content is measured by a commercially available Diboran sensor. Finally, we will make a few recommendations for working with MgB₂.

Work was funded by Federal Ministry of Education and Research (BMBF) through 03SF0480C

Both the authors would like to thank Prof. Dr. Ekrem Yanmaz (at Karadeniz Technical University, Trabzon, Turkey) for his support and encouragement. This work was supported by the Scientific Research Coordination Unit of Bulent Ecevit University of Turkey, with project No. 2013-76962555-03.

Disk-shaped, bulk MgB₂ superconductors (sample diameter up to 4 cm) were prepared in order to improve their performance for use as superconducting super-magnets. Several samples were fabricated using two different processes, one set using a solid state reaction in pure Ar atmosphere at temperatures ranging from 750 to 950 ^oC, and another type prepared via high-pressure sintering. On both types of samples, magnetization and transport measurements revealed that at the low reaction temperatures flux pinning at grain boundaries is dominant, which is decreasing on increasing temperature. At the highest reaction temperature, j_c was found to increase again indicating a change of the pinning mechanism [1]. In order to clarify this behavior, the samples were characterized in detail by means of transmission electron microscopy (TEM) and transmission electron backscatter diffraction (t-EBSD).
[1] M.R.Koblischka et al., IEEE Trans. Magn. 50, 9000504 (2014).

Although the various compound morphologies play a decisive role to control its properties, the synthesis method and doping define additional, crucial parameters that markedly impact the nature and concentration of intrinsic or extrinsic defects and the materials’ electronic properties. To this end, it has been reported that carbon doping significantly improves the electromagnetic properties and the critical current density of magnesium diboride (MgB₂). Nonetheless, the effect of carbon doping in MgB₂ has also been discussed controversially with respect to its substitution site and solubility limit. Moreover, previous studies on C doping into MgB₂ were mainly focused on its effect on superconductivity. From the defect structure point of view, the effect of C doping on the electronic properties is equally important and has to be investigated. In this work, the defect structure of C-doped MgB₂, and its impact on the material properties will be investigated by electron paramagnetic resonance (EPR) and Raman spectroscopy. EPR is well suited for this task because it provides a direct method to monitor different paramagnetic states of defects and, thus, complements other experimental techniques, such as X-ray powder diffraction (XRPD) and Raman spectroscopy. In this sense, EPR does not only allow for an identification of paramagnetically active intrinsic and extrinsic defects, but also yields insight into the correlation of such defects to the structural and superconductive properties of MgB₂.
Here, undoped and C-doped MgB₂ samples were synthesized using two sets of mixtures prepared from the precursors, amorphous nanoboron and as-received amorphous carbon-doped nanoboron. Mg vacancies and C-related dangling-bond active centers could be distinguished and sp³-hybridized carbon radicals were detected. A strong reduction in the critical temperature T_c was observed due to defects and crystal distortion. The symmetry effect of the latter is also reflected on the vibrational modes in the Raman spectra.

This work is financially supported by the Deutsche Forschungsgemeinschaft (DFG) with the grant number Er662/1-2 and Koc University Graduate Program. Authors associated with Koc University would like to thank Koc University Surface Science and Technology Center (KUYTAM).

The paper presents the results of flux-pinning mechanism investigations by magnetic measurements in the oxide-containing magnesium diboride thin films (MgB₂), which are deposited on the dielectric wafers by a magnetron sputtering. The investigations of crystal structure, phase content, relief of the MgB₂ thin films have been done by XR-diffraction, SEM microcopy and Auger spectroscopy. It’s demonstrated that depending on the parameters values of the deposition and annealing processes the thin films with various degree of the structure perfection and various phase content could be fabricated.. The density of superconducting critical currents of these films reaches values up to 1,8·10¹¹ – 8,2 · 10¹⁰ А/m² at 10 К and 8·10¹⁰ – 2,8 · 10¹⁰А/м² at 20 К in the fields 0-1 Т ( if an external magnetic field is oriented in parallel for the wafer surface) .
The magnetic field dependence of the critical current density, j_c(B), was analyzed within the frames of the collective pinning models. The various combinations of the Abrikosov vortices pinning types could be realized in the films ( such as a pinning in the places of fluctuation of superconducting critical temperature (δТс -type) or of fluctuation of the free path length (δl-type) ) , on the appearance of which the synthesis conditions can do influence. The MgB₂ films that have the combined type of mechanism of the Abrikosov vortices pinning (mainly, in the disordered MgB_xO_y boundaries in them) are demonstrated the highest values of the superconducting critical current density.

The authors thank King Fahd University of Petroleum and Minerals for its support.