In this paper, we will describe a practical method of flux pumping in superconductors such as YBCO bulks. This technique only requires a small magnetic field which can be generated by permanent magnets and will lead to a much higher field trapped within the superconductor after multiple pumps. A thermal material whose permeability drops strongly as its temperature exceeds a given point has been used as a magnetic flux regulator to create a travelling magnetic field. Ferrites with different dopants were synthesized by ceramic method as TMs. Various testing methods have been used to investigate thecharacteristics of ferrite samples, including non-destructive test, SEM, and SQUID. The TC sweep of TMs was obtained at temperatures ranging from 77 K to 300 K. Gadolinium (Gd)-epoxy composites with different volume fractions were also made as TMs due to their better thermal properties compared to ferrites. A 1D thermal modelling of Gd-epoxy composite has been built up to optimize the thermal diffusivity and the relative permeability of the composite.
This work aims to investigate the variation of the thermal andmagnetic properties of two TM candidates and gives a smart insight of finding better TM which is more efficient for the thermally actuated magnetization flux pumping method.

The authors would like to thank all contributes from the EPEC Superconductivity Group in the University of Cambridge, UK.

REBa2Cu3O7-x conductors show promise for use in a multitude of next generation high-field magnet, energy storage, and power transmission systems due to its high Jc. During fabrication and operation the superconducting tape is subjected to many stresses and strains that can potentially degrade the performance of the conductor. In addition, many of these systems operate in AC mode and as such the fatigue properties must be well understood. Current studies have determined the performance of YBCO tapes under single cycle tensile stresses, but limited data is available regarding the fatigue life. In this study we present fatigue data from Superpower 4 mm HTS tapes; up to 0.5% strain and 100,000 cycles. Further investigation under SEM reveals important information showing how microcracks propagate from defects during deposition and cutting of tape. Reducing these defects could lead to greatly improved fatigue life.

Superconducting wires based on Nb₃Sn are perspective pretenders for using these materials in large magnetic systems of Large Hadron Collider and future fusion reactors. Therefore it is an important task to investigate the changes of critical properties of this material after intensive irradiation by fast particles. Here we present the results on magnetization measurements of Nb₃Sn multifilamentary wires after fast proton irradiation. We have investigated 4 types of wires (samples with 0.8 – 1.2 mm in diameter and ~3 mm length) supplied by various vendors. The irradiation of all samples at room temperature was performed on cyclotron in “Kurchatov Institute” by protons with energy of 32 MeV with 4 steps of fluencies up to 10¹⁸ cm^-2. The measurements of magnetization curves on unirradiated and irradiated Nb₃Sn samples were carried out in VSM (Quantum Design PPMS system) in field range up to 6 Tesla at temperatures of 7 and 12 K. Temperature dependence of remanent magnetic momentum of all samples was measured too for estimating of T_c. Very small changes of T_c were observed for all samples with fluence up to 10¹⁷ cm^-2 and dramatically decreasing of T_c was measured in all samples irradiated with higher fluencies. The critical current density for all 4 types of irradiated wires decreasing with fluence more than 10¹⁷ cm^-2. The obtained experimental results are discussed here according calculated radiation damages profiles in irradiated samples.

We would like to thank CERN and NRC “Kurchatov institute” for financial support of this work.

Characterisation of the critical current density of high temperature superconducting bulks is typically undertaken using magnetometery. The conversion between sample magnetisation and critical current using Bean's model (or modifications thereof) involves numerous assumptions including, importantly, that J_c within the sample is uniform. Where it is desired to obtain a local measurement of J_c across a feature such as a grain boundary, magnetometry is challenging to apply. Transport critical current measurements of superconducting tapes and films are relatively commonplace, but very little work has been carried out on bulks. It is challenging to reduce the cross sectional area of bulks significantly enough to achieve reasonable current densities, despite their critical current densities being somewhat lower than tapes. Unlike in tapes, the field dependency of J_c across grain boundaries in bulk superconductors has been difficult to quantify; this method promises significant improvements in this regard.
In the work described here, metallographic techniques and resin reinforcement were used to create an I-shaped sample of bulk superconductor from a section cut from a GdBCO-Ag bulk. The resultant superconducting track had a cross sectional area of 0.4 mm². Current contacts were created by annealing silver paint onto the sample. The sample was suspended in liquid nitrogen and was found to support a critical current of 110 A using a field criterion in the narrowed track of 1 μV/cm. We find, therefore, that it is possible to measure critical current densities in excess of 2.5 x 10⁸ Am^-2 in sections of bulk superconductor.

This work was supported by the Engineering and Physical Sciences Research Council, via a Doctoral Training Award and funding from the grant EP/K02910X/1.

In the report, we propose a method for calculation of current paths in the HTS tape with defects such as cracks of arbitrary shape, non-superconducting inclusions and inclusions with low local critical current density. The calculation method is based on the model of critical state which takes into account a precision two-exponential approximation of field dependence of critical current [Bulletin of the Lebedev Physics Institute 41, 351 (2014)]. The method allows to calculate the currents flowing through the HTS tape with defects both for currents induced by the external magnetic field and for transport currents from an external source. The calculations of the current distribution in HTS tapes with different types of defects in the magnetic field and transport current were performed. It was shown that the combined using of the simultaneous action of a magnetic field and transport current can greatly simplify the identification of the boundaries and shape of the defects. Thus, the proposed method may be more useful as compared with the conventional magnetometric diagnostic studies, when the tape is affected by the magnetic field or transport current only.

Research was supported by Russian Science Foundation (project №14-22-00098)

In this work, an approach toward reaching new insights into the mechanism of superconductivity in the iron-based compounds is presented.
The temperature dependence of the superfluid density and of the microwave conductivity of high quality Ba(Fe_1-xCo_x)₂As₂ and (Ba_1-xK_x)Fe₂As₂ single crystals has been obtained by a 8-GHz superconducting coplanar resonator technique. The proposed technique is suitable to obtain the absolute value of the penetration depth, and shows high sensitivity to small samples. The comparison of crystals with different shape factors allows deducing the ab-plane component of the penetration depth and achieving evidence of a coherence peak in the quasiparticle conductivity. Then, the experimental superfluid density and coherence peak are simultaneously fitted in a two-gap scheme, obtaining the temperature dependence of the energy gaps, with zero-temperature values which are in good agreement with literature.
This microwave technique has been used to study the effects of disorder, induced in the crystals by 250-MeV Au-ion irradiation. In fact, particle irradiation – increasing nonmagnetic scattering without contributing extra charge – is an optimal tool to validate pairing state models through its influence in suppressing the critical temperature and changing penetration depth and quasiparticle scattering time. The whole set of experimental results is compared to literature findings obtained by irradiations with other particles and to the expectations of the leading candidate for describing the pairing state in the iron based superconductors, i.e. the s± phase, with the possible presence of accidental nodes.

It is well know that due to the para-antiferromagnetic transition of oxygen at 45 K, the contamination with this element in the sample space may deeply affect, even under very low concentrations, measurements performed in high resolution magnetometers around this temperature. In fact, this important experimental difficulty is described by the manufacturers of these devices that recommend carrying out successive purge cycles in the sample space to mitigate these spurious effects [1]. By using a commercial SQUID-based magnetometer (Quantum Design, model MPMS-XL), we have first performed high resolution (up to 10^-8 emu) magnetization measurements around the superconducting transition in granular La_1.85Sr_0.15CuO₄ (LSCO) superconductors, with a critical temperature T_c of 38K and with an average grain diameter of 4 micrometers, which show that after successive purge cycles the O₂ signal decreases but saturates to a value that still competes with the intrinsic diamagnetic signal [2,3]. Then, we demonstrate the crucial role played by the O₂ physisorption in the extended surface of the LSCO grains by performing similar measurements after the grains were encapsulated into an epoxy resin (EPO-TEK 301), whose contribution to the total magnetic moment is almost temperature independent. Our results show that for the encapsulated sample the magnetic contribution of the O₂ para-antiferromagnetic transition becomes negligible as compared with the superconducting diamagnetism of the LSCO grains, in excellent agreement with the estimations of the effective surface reduction for the O₂ physisorption. In addition to its interest for granular high-T_c superconductors, the procedure proposed here may be useful for high precision magnetization measurements in other granular or porous materials having effective surface areas large enough for important oxygen physisorption.

[1] See, e.g., MPMS Application Note 1014-210, Quantum Design.

[2] J. Mosqueira, J. D. Dancausa, and F. Vidal, Phys. Rev. B 84, 174518 (2011).

[3] J. Mosqueira, L. Cabo, and F. Vidal, Phys. Rev. B 80, 214527 (2009).

We acknowledge funding from Spain's Xunta de Galicia project GPC2014/038. A.R.-A. acknowledges support from FPI grant BES-2011-046820.

Thermal expansion coefficients, α(T) were considered carefully in the 1.5-30 K temperature range on non-superconducting PrBa₂Cu₃O_6+x (PBCO) in order to determine if anomalies occur between the Néel temperature, T_N^Pr of the Pr antiferromagnetic ordering situated for compounds in both tetragonal and orthorhombic phases, respectively from 9 K to 20 K, and the so-called low-critical temperature, T_cr which has been observed at the same value of 4~5 K [1, 2]. The polycrystalline specimens have been prepared in the same conditions by the standard ceramic method. A set of microstructural characterizations such, X-ray diffraction, scanning electron microscopy and thermogravimetric analysis with a controlled oxygenation, has led to samples in two oxygenation states: an underdoped (UN; x = 0.44) and an optimally doped (OP; x = 0.95) with the value of T_N^Pr which increases respectively, from 9 to 14 K. Below T_N^Pr, both α(T)/T and α(T)/T² curves show two different thermal behaviors separated by the low-critical temperature, T_cr ~ 4~5 K. The previous spin reorientation phase transition (SRPT) at a T₂ temperature [3] is revealed with the occurence of a small anomaly near ~ 6-7 and 9-10 K, respectively for the UN and the OP states. The results are significantly different to those obtained previously on two ceramics in UN (x = 0.40) and OP (x = 0.90) states [4] where the SRPT at T₂ was not identified. The origin of these anomalous thermal properties is discussed in terms of Pr-Cu(2) magnetic coupling [5].
[1] W. Younsi, M. Lahoubi, M.-L. Soltani, D. Schmitt, J. Hejtmánek, J. Phys. Conf. Ser. 150 (2009) 052290.
[2] W.Younsi, M. Lahoubi, M.-L. Soltani, Journal of Low Temp. Phys. 166 (2012) 212.
[3] S. Uma, W. Schnelle, E. Gmelin, G. Rangarajan, S. Skanthakumar, J. W. Lynn, R. Walter, T. Lorenz, B. Büchner, E. Walker, A. Erb, J. Phys.: Condensed Matter 10 (1998) L33.
[4] P. Haen, M. Lahoubi, A. Harat, G. Fillion, D. Schmitt, H. Bioud, J. Hejtmánek, Physica B 284-288 (2000) 1035.
[5] A. T. Boothroyd, A. Longmore, N. H. Andersen, E. Brecht, Th. Wolf, Phys. Rev. Lett. 78 (1997) 130.

The author thanks Dr. J. Hejtmánek (Institute of Physics of ASCR, Praha, Czech Republic) for the preparation and the XRD analysis of the PBCO specimens. This work was supported in part by a previous collaboration between UBM Annaba, Algeria and UJF-Grenoble, France.

Fabrication of iron based superconductors has been achieved by means of two different techniques, the bulk sintering from powders and the electrochemical deposition of thin films. Iron-based superconductors are appealing for high magnetic field applications thanks to their promising superconducting properties, i.e. extremely high upper critical field and irreversibility field.
The sintering of FeSe and FeSeTe policrystalline powders and pellets was obtained in several steps; stoichiometric quantities of freshly polished powders shots, previously mechanically machined inside a mortar or a ball mill, were loaded into cleaned and dried silica tubes and afterwards sealed under vacuum. These tubes were then placed in a furnace where they underwent a specific heat treatment before being quenched in several different ways. After a first heat treatment, powders were mechanically machined again and then reloaded in silica tubes for a second and even a third heat treatment, to obtain the desired superconducting phase. Before the last heat treatment, powders were reduced to pellets of several diameters, from 0.5 up to 1cm, applying different pressure.
Beside the bulk fabrication, another technique for the production of superconducting FeSe has been attempted, which is the electrochemical deposition of FeSe superconducting films; this was achieved with a standard three-electrode method. The films have been deposited in an inert gas saturated solution at room temperature; different substrates have been used at different voltages, while for the counter electrode Pt was chosen. The solution employed for the deposition is produced by dissolving 0.03 M FeCl₂• 4H2O, 0.015 M SeO2 and 0.1 M Na₂SO₄ into distilled water. The mole ratio of Fe and Se is controlled by carefully tuning both the electric potential and pH value.

Using Impedance Spectroscopy one may propose physical models, for a device or material sample, consisting in suitable values and arrangements of electric properties (such as resistance, inductance, capacitance, absorbance, etc). In this work we present: (1) the equipment built to perform such measurements, using commercial apparatus as well ones developed in the Laboratory for Materials and Devices with Superconductors - LMDS at Federal Rural University of Rio de Janeiro - UFRRJ; (2) the impedance spectroscopy curves obtained in HTS samples and dielectric materials relevant for applied superconductivity, such as YBaCuO, NdBaCuO, PPLP, G10, etc, in room temperature and in cryogenic conditions; (3) the electric models achieved for those materials. The results contribute to optimize the mathematical modeling of such materials in order to perform better computational simulations.

P&D ANEEL D712: SUPERCABO Project, CEMIG D, CEMIG GT, CTEEP, TAESA, TBE, UFRRJ and FAPUR for financial support.

A small amount of excess Fe is trapped in interlayers of iron chalcogenides during the synthesis. Furthermore, this excess Fe is hard to remove during synthesis. It is well known that excess Fe suppresses superconductivity by supplying excess electrons to the superconducting layers [1].
Many attempts have been done to suppress the effect of excess Fe, then to induce superconductivity. However, these reactions progress slowly and/or are less tunable to induce superconductivity. More sophisticated methods to remove excess Fe, we focused on a removal of ion in battery reaction such as Li-ion battery. We expected an application of voltage to a sample could be a strong driving force to remove excess Fe.
Recently, we have succeeded in inducing superconductivity in FeTe_0.8S_0.2 by removing excess Fe via an electrochemical reaction which applying a voltage to the sample [2]. This is the first report of inducing superconductivity by removing excess Fe using an electrochemical reaction in the iron-based superconductors.
However, since the solvent is water, inevitably with increasing applied voltage, the electrolysis of water occurs. As a result, this produces an oxide film on the sample surface which interferes with the electrochemical reaction. Thus, the change to a water free solution is required to avoid the electrolysis of water. On this basis, we selected an ionic liquid which is water-free and has a good conductivity (e.g. 1-Butyl-3-methylimidazolium tetrafluoroborate) as a solution. With usage of ionic liquid, we have succeeded in inducing superconductivity in FeTe_1-xS_x electrochemically [3]. Additionally, we also succeeded in inducing superconductivity in FeTe_1-xSe_x, suggesting that this method has a high tunability and versability. Moreover, superconducting properties are enhanced by selecting the optimum condition.


References
[1] H. Okazaki et al., JPSJ 81 (2012) 113707
[2] A. Yamashita et al., Solid State Commun. 200 (2014) 29-31
[3] A. Yamashita et al., JPSJ 84 (2015) 034706

This work was partly supported by the Strategic International Collaborative Research Program (SICORP-EU-Japan).

In this work, RuSr2-xCaxGdCu2O8±Z system with 0.0 ≤x ≤ 0.20, was synthesized through solid-state reaction at ambient pressure in air with temperature between 960 °C and 1000°C. X-ray diffraction showed the formation of pure phases up to x = 0.15. The samples were annealed in oxygen flux for 24, 72, and 168 hours at 1060 °C. Rietveld-refinement analysis indicates that the all structure parameters of RuSr2-xCaxGdCu2O8±Z system were modified by Calcium concentration as well as the oxygen annealing time. The electrical resistance of the samples with 0.0 ≤x ≤ 0.10 shows superconducting behavior, whereas the samples with 0.15 ≤x ≤ 0.20 show semiconducting behavior. The structure parameters and temperature superconducting show a similar dependence on x, this results suggest that a strong interplay among electrical and structural properties are taken place in -superconducting RuSr2−x Ca xGdCu2O8±z system.

Precise measurements of magnetic moment relaxation of YBa₂Cu₃O_7-δ films at T=32 K in perpendicular low magnetic fields H up to 1600 Oe were carried out by original technique described in [1]. The experiments were performed for increasing field, when flux penetrates into film, and for decreasing field, when it leaves the film. Both partly and fully penetrated critical state was studied in increasing field.
From analysis of relaxation data the field dependences of glassy exponent μ and relaxation rates R=-dJ/dlnt, S=-dlnJ/dlnt of persistent current J were obtained. At low fields μ decreases with field, but at H>500 Oe it becomes independent of H having values 1 and 1.8 for two samples. The rate R has a maximum at H=500 Oe, the normalized rate S increases with field, but at H>500 Oe the increase slows down.
We used the Maley's method [2] to obtain activation energy from relaxation data as U=lnH-ln|dJ/dt|+C, here C is the constant independent of field and the term lnH accounts for field dependence of macroscopic time scale for creep [3]. From Maley's analysis we obtained dependence of critical current J_C and pinning energy U_C on field inductance B=H±H_d at film edge. Demagnetization field H_d=4πJd/2c induced by the persistent current adds to applied field as H increases and is subtracted from H as it decreases [4]. Here d is thickness of the film and c is velocity of light. We found that at H>500 Oe the pinning energy decreases very slightly with field. Obtained curves J_C vs. B coincide for both increasing and decreasing field and demonstrate power-law dependence with exponent -0.4 which is slightly less than the theoretical values -5/8 [5] or -1/2 [6].
1. I.I. Sannikov, et al. Bull. Lebedev Phys. Inst., 41 (2014) 215.
2. M.P. Maley, et al., Phys. Rev. B, 42 (1990) 2639.
3. Y. Abulafia, et al., Phys. Rev. Lett. 75 (1995) 2404.
4. J.R. Clem and A. Sanchez, Phys. Rev. B 50 (1994) 9355.
5. Yu. N. Ovchinnikov, B.I. Ivlev, Phys. Rev. B 43 (1991) 8024.
6. C.J. van der Beek, et al., Phys. Rev. B 66 (2002) 024523.

The work was supported by state-order No 3.1540.2014/K of the Ministry of Education and Science of the Russian Federation.

We report on dc magnetization data collected from a YBa₂Cu₃O_7-ð single crystal with a modified superficial pinning potential with the goal of to check the majority contribution of the flux compression scenario to the high field paramagnetic Meissner effect (HFPME). The additional superficial pinning centers (~200nm in depth) were introduced at ab plane central region of single crystals by FIB microscope Ga atoms beam. The dc magnetization measurements were performed in a SQUID device from quantum design and the field cooled warming, M_FCW(T), field cooled cooling, M_FCC(T) and zero field cooled, M_ZFC(T) dc magnetizations were recorded while magnetic fields, H up to 50kOe are applied parallel to single crystal c axis. The M_FCW(T) and M_FCC(T) data of our single crystal show the usual superconductor diamagnetic response to H < 0.5 kOe. In contrast to H > 0.5 kOe the M_FCW(T) and M_FCC(T) data behavior is characterized by a systematic reduction of superconductor diamagnetic response until it became predominantly paramagnetic as the applied magnetic field is increased. This M_FCW(T) and M_FCC(T) behavior is the signature of the HFPME in high temperature superconductors. The HFPME is identified in the M_FCW(T) and M_FCC(T) data just to T < T_irr(H) (magnetic irreversibility temperature) and its magnetization amplitude do not show tendency to a temperature saturation. We contrast the HFPME of our single crystal with those listed at the literature to the YBa₂Cu₃O_7-ð single crystal. We suggest that the enhancement of vortices compression scenario modulated by the flux pinning as the responsible to the HPME in your sample.

Acknowledgments: The authors would like to thanks to CNPq and CAPES Brazilian scientific agencies and PRONEM 03/2011 (under contract: 11/2042-7) scientific program for partially financing this work.

We report on isotherm dc magnetization hysteresis loops, M(H), at T = 77.5K, of a series of YBa_2-xSr_xCu₃O_7-δ (x = 0, 0.02, 0.1, 0.25, 0.37 and 0.5) single crystals with the aim of studying the influence of the Sr chemically introduced site disorder on the critical current density, J_C(H) and normalized flux pinning force density, f(h) of the YBa₂Cu₃O_7-δ single crystal. The Sr ion size chemical disorder is introduced at YBa2Cu3O7-δ structure by lattice distortion. The single crystals were grown by self-flux method and their crystallographic structure was analyzed by X-ray diffraction. The M(H) measurements were performed with a SQUID magnetometer to H ≤ 50kOe applied parallel to the c axis of the samples. The J_C(H) was determined from the application of the extended Bean critical state model. The magnetic irreversibility fields, H_irr(T) were obtained from M(H) loops as well as zero field cooled, M_ZFC(T) and field cooling colled, M_FCC(T) magnetizations. The f(h) behavior was determined from f = F_P.(F_P,max)^-1 versus h = H.(H_irr)^-1 plots where F_P is the pinning force density (F_P = Jxµ₀H). The transmission electron microscopy analysis of the YBa_1.75Sr_0.25Cu₃O_7-δ crystalline structure points out to a segregation of Sr atoms precipitates that could be associated to the enhancement of the flux pining potential of the Sr doped single crystals. The identification of the preponderant pinning mechanism of the samples was obtained from the application of the Dew Hughes scaling functions to f(h) plots. The preliminary J_C(H) results show that at T = 77.5 K the doped samples with x < 0.25 transport higher J_C(H) than the pure one. The core normal point is the preponderant pinning mechanism to those samples with x < 0.25 . On the other hand, the lower J_C(H) values verified to the Sr doped single crystals with x > 0.25 could be associated to the development of the pinning mechanism of Sr doped single crystals from the core normal point type to the core normal surface type.

The authors would like to thanks to CNPq and CAPES scientific agencies and PRONEM 03/2011 (under contract: 11/2042-7) scientific program for partially financing this work.

The present work portrays a detailed study of the magnetic irreversibility limit (Tirr ) and of the zero resistance point (Tc0) of three different top-seeding melt-textured YBa₂Cu₃O_7-d superconducting samples with well aligned c-axis and doped with a high density of non-superconducting Y₂Ba₁Cu₁O₅ (Y211) pinning centers. The measurements were performed for many magnetic fields up to 140 kOe and for the whole set of the different field-current configurations. The magnetization measurements were performed using a MPMS-XL SQUID magnetometer and a VSM both from Quantum Design. The electric transport measurements were made using a PPMS from Quantum Design. The goal of this exhaustive study is obtaining precise data about magnetic flux mobility along the various directions in the sample for the different field-current configurations, thereby defining the nature and effects due to the strength and anisotropy of the pinning mechanisms and disclosing the various physical mechanisms dissipating electric transport in these systems below the superconducting transition temperature. Our results are discussed in terms of the anisotropic flux pinning by the Y211 grains dispersed into the superconducting matrix.

This work was supported under the Brazilian grant MCTI/CNPq Universal 14/2012 (contract number 477506/2012-7). The experimental work at the HLD was supported by Euromagnet II (contract number 228043)

Pb_1-ySr₂Y_1-xCa_xCu_2+yO_7+δ(Pb1212) is a high-Tc superconductor which has single block layers and double CuO₂ layers similar to YBa₂Cu₃O_7+δ (YBCO). We have been successful in growing single-crystal Pb1212 thin films and clarified that Pb1212 is a low anisotropic cuprates (γ～10 [1]). Terahertz Time Domain Spectroscopy was carried out on Pb1212 epitaxial films with electric field polarization being parallel and perpendicular to the c-axis direction in order to investigate in-plane charge and Josephson tunneling dynamics in Pb1212.
The in-plane directional spectroscopy has revealed that under-doped Pb1212 is two-dimensional system explained by Berezinskii-Kosterlitz-Thouless theory [2] while optimum-doped Pb1212 is three-dimensional system. Superconducting fluctuation is observed at both doping levels.
By the c-axis directional spectroscopy, we have observed the Josephson plasma oscillation in Pb1212.
[1] Komori et al., Phys. Rev. B 89, 174509 (2014).
[2] J. M. Kosterlitz and D. J. Thouless., J. Phys. C 6, 1181 (1973).

Resistive current states in wide superconducting films with phase slippage have a nonstationary, nonequilibrium nature. Parameters that describe an electron system of superconductor oscillate with Josephson frequency in a core of the phase slip line (PSL). An absolute value of the order parameter amounts to zero periodically. It would appear reasonable that the electromagnetic and phonon radiation occur as a result of phase slippage.
We observed an interesting phenomenon for wide Sn films with phase slippage. Such samples demonstrate an instantaneous decrease the thermal flow from the film at voltages when a destruction of the stable phase-slip state occurs due to the thermal instability. This behaviour is suggested to take place due to a disappearance of the PSLs’ structure that operates as a source of phonons and change of phonon propagation mode from ballistic to diffusive one.
One can discuss the results with a view of the theoretical model predicting phonon fluxes from the active phase-slip regions. Every temporal change of the absolute value of the order parameter causes an exchange of phonons between the superconductor and thermostat. Phonon fluxes from the PSL’s core exhibit a pulse character and changes in direction. Nevertheless, the time-average power of the phonon flow into a surrounding medium is not equal to zero.
For the real parameters of PSLs, the power of the phonon flow could have the same order with the ohmic dissipation. As a result, disappearance of a phonon source could be appreciable on the background of total dissipation, even concerning the fact that ohmic term rises faster than the phonon one. The temperature probe is expected to register the thermal energy drop at a moment of the PSL structure's destruction locally. This value is suggested to correspond to the power of the phonon flow at the thermal instability voltage. Experimental and calculated dependencies are similar qualitatively.

Type II superconducting films, under certain circumstances, experience abrupt penetration of magnetic flux. Such events are consequences of thermomagnetic instabilities provoked by changes in one or more external parameters. These avalanches exhibit finger-like structures that invade the sample from the border toward its center. As a consequence of this kind of flux penetration, the shielding current decreases instantly, as detected by jumps on magnetization measurements. These occurrences are anomalous short-lived flux invasions that could induce noise in electronic circuits based in superconducting films. Hence, concerning applications it is important to prevent them. In this research we have studied ways to control flux avalanches, either inducing them, or stopping or even doing both. Under an external field, the application of a transport current can also induce avalanches. On the other hand, non-superconducting regions with appropriate format inserted along the way of the avalanches, are able to stop them.
The samples used were prepared with appropriate layouts, in which a curved and sharp path, as a needle, on the edge conducts electrical current pulses. When the transport and the shielding currents add up, an avalanche occurs. This happens because vortices, at this point, experience a Lorentz force higher than anywhere else on the sample. Such arrangement - the flux injector - makes it possible to drive flux avalanches at a specific place of the film. We have employed 200 nm thick Nb films shaped into squares of sides of 2.5 mm through use of optical lithography. Films were prepared with holes in front of the flux injector. The experiments were conducted in a Magneto-optical Imaging setup. Flux injectors are efficient to control the occurrence of flux avalanches and their interaction with structures in superconducting films. The effective arresting capability of each arrangement depends on the relative size of the hole as compared to that of the avalanches.

We have studied the functional behavior of the field-cooled (FC) magnetic relaxation observed in melt-textured YBa₂Cu₃O_7-d (Y123) samples with 30 wt% of Y₂Ba₁Cu₁O₅ (Y211) phase, in order to investigate the origin of the anomalous paramagnetic moments observed during the experiments. Magnetic fields from few Oe to tens of kOe were applied parallel to the ab plane and along the c-axis and several FC magnetic relaxation experiments were performed under different and controlled conditions, such as cooling rate, temperature and magnetic field, in order to study the time effects. Our results showed the known paramagnetic Meissner effect (PME), characterized by positive moments during FC experiments, and this effect, in our case related to the magnetic flux compression into the samples, has as a consequence a strong time dependence that could be verified by specific FC magnetic relaxation experiments, that showed a non conventional paramagnetic behavior that increases with time with no apparent tendency to saturation. After different attempts our experimental data could be adequately fitted by an exponential decay function with different relaxation times. We discuss our results suggesting the existence of different and preferential pinning mechanisms governing the anomalous FC paramagnetic relaxation in different time intervals. This work is one of the first attempts to interpret this controversial effect in a simple analysis of the pinning mechanisms acting during the time evolution of magnetic moment. However, the results obtained may be useful to develop models to explain this interesting and still misunderstood feature of the paramagnetic Meissner effect.

This work was supported under the Brazilian grant MCTI/CNPq Universal 14/2012 (contract number 477506/2012-7). The experimental work at the HLD was supported by Euromagnet II (contract number 228043).

The coexistence of two independent flux‐line lattices (FLLs) has experimentally been shown for structurally isotropic Nb films [1]. Within the surface superconductivity state, that is above B_c2 (the second critical field) and below B_c3 (surface superconductivity critical field), a specific arrangement of surface (Kulik) vortices [2] can appear in the superconducting surface sheaths. We show that these vortices undergo a decoupling transition from a coherent to an independent behaviour, similar to the behaviour observed for Giaever transformer [3]. At the transition, a feature in pinning properties is measured, which implies different pinning for the lattice of surface vortices coherently coupled through the normal layer between the surface sheaths and for two decoupled vortex arrays in these sheaths [4]. Thus, this surface superconductivity system resembles a structurally ideal trilayer (superconductor-normal metal-superconductor) with a well controlled coupling properties between vortex arrays in the two surface sheaths. Such the "trilayer" is technologically simple to manufacture, hence, it may be considered for potential applications.
[1] A. V. Pan, M. Ziese, R. Höhne, P. Esquinazi, S. Knappe, and H. Koch, Physica C 301, 72 (1998).
[2] I. O. Kulik, Zh. Eksp. Teor. Fiz. 55, 889 (1968) [Sov. Phys. JETP 28, 461 (1969)].
[3] I. Giaever, Phys. Rev. Lett. 15, 825 (1965).
[4] A. V. Pan and P. Esquinazi, Phys. Rev B 70, 184510 (2004).

This work is financially supported by the Australian Research Council (Discovery Project DP110100398).

The problem of Abrikosov vortex depinning at low temperatures from extended c-oriented linear defect in d-width superconducting film under the surface transport current influence is solved. This problem is considered in the framework of classical mechanics as behaviour of an elastic vortex string which is settled in the potential well of linear defect and exerted to exponentially distributed Lorentz force action within the screening layer of width London penetration depth
near the both sides specimen film surface. We search the mechanical stability problem for the vortex string inside the potential well created by extended linear defect and obtain conditions for the instability threshold as well as the critical current density j_c at which the instability leading to vortex depinning. The obtained results are also applied for calculation of critical current dependence on the width d of superconducting film. The results of suggested model is strongly supported by some recent experiments with HTS materials.

S. Khene expresses his thanks to Bernard Barbara of Néel Institute of National Center of Scientific Research of Grenoble (France) for his hospitality and his valuable conversations, and to his institute for the use of large scientific facilities.

We report on ac magnetoresistance, R(T,H) and field cooled cooling, M_FCC(T) and zero field cooled, M_ZFC(T) dc magnetizations measurements of YBa₂Cu₃O_7-y, YBa_1.98Sr_0.02Cu₃O_7-y and YBa₂Cu_2.97Zn_0.03O_7-y single crystals with the aim of to disclose the correlation between the zero resistance, T_R(H) and the magnetic irreversibility, T_irr(H) temperatures as a function of the superconducting inhomogeneous nature of the samples. The R(T,H) and M(T,H) measurements were performed respectively with a PPMS and a SQUID devices from quantum design. The R(T,H) and M(T,H) data were recorded while magnetic fields, H up to 50kOe were applied parallel to the c axis of single crystals. The low R(T) measurement current density was applied parallel to the samples ab plane. The contrasting of T_irr(H) and T_R(H) data profile of doped samples at H-T diagram shows that for H < 2.5kOe the T_R(H) data falls systematically underneath of the T_irr(H) data. We attributed this behavior to the establishment of a superconducting granular scenario provided by Sr and Zn doping. At this scenario the Josephson vortices dynamic is predominant and the T_irr(H) and T_R(H) do not depend of the same parts of the sample. While the T_irr(H) depends on well coupled grain clusters the T_R(H) depends on grain arrays traversing the whole sample. In contrast for H > 2.5kOe the T_R(H) data matches to the T_irr(H) data and the Abrikosov vortices dynamic become predominant. On the other hand for the YBa₂Cu₃O_7-y single crystal, which shows a superconducting granular character irrelevant, the matching of T_R(H) and T_irr(H) data is noted to the all H-T diagram extension as well as the Abrikosov vortices dynamic ascendancy. We suggests that the not matching of T_R(H) and T_irr(H) could be set as a limit to separate the predominance between the Josephson and Abrikosov vortices dynamics at H-T diagram of a superconductor.

Acknowledgments: The authors would like to thanks to CNPq and CAPES Brazilian scientific agencies and PRONEM 03/2011 (under contract: 11/2042-7) scientific program for partially financing this work.

Fast propagation, branching and inherent irreproducibility are typical features of flux avalanches which suddenly invade superconducting films under certain circumstances of temperature and applied magnetic field. The origin of these avalanches is associated with the occurrence of thermomagnetic instabilities [1], which lead to the abrupt propagation of magnetic flux rushing into the sample, while leaving behind a trail of overheated material. Avalanches in superconducting films constitute a rich and complex phenomenon which, however, might be detrimental to potential applications.
In the present work we report the occurrence of dendritic flux avalanches in films of V₃Si, grown from a stoichiometric target on a LaAlO₃ substrate, by use of pulsed laser deposition in high vacuum [2]. Magneto-optical imaging was employed to visualize the occurrence and spatial distribution of flux avalanches. The window of temperatures and applied magnetic fields at which the avalanches take place was determined using dc-magnetometry. Images of the penetrated flux profile indicate a clear anisotropic distribution of the critical currents, which we ascribe to the fact that the films studied here have a preferential growth direction (with the [210] axis of V₃Si parallel to the [001] of the LaAlO₃ substrate) [2] while, consistently, superconductivity is anisotropic in V₃Si single crystals [3]. Our results testify that V₃Si is part of the hall of superconducting thin films exhibiting flux avalanches [4] and, therefore, such a characteristic must be taken into consideration when films of this A15 superconductor are envisaged for possible applications.
[1] R. G. Mints and A. L. Rakhmanov, Rev. Mod. Phys. 53, 551 (1981).
[2] C. Ferdeghini et al., IEEE Trans. App. Supercond. 19, 2682 (2009).
[3] M. Pulver, Z. Physik 257, 22-28 (1972).
[4] F. Colauto et al., IEEE Trans. App. Supercond. 25, 7500704 (2015).

The dynamics of transient current distributions in YBa₂Cu₃O7-x thin films during and immediately following field penetration were investigated using both ultra-fast Magneto-Optical Imaging and computer simulation techniques.
As an external field is being increased the dynamic current profile is characterized by strong peaks, greatly diverging from the measured static current profiles and theoretical Bean profiles.
These peaks develop close to the sample edge during initial field ramping and subsequently move toward the centre of the sample while reducing in intensity as flux penetrates into the sample.
The simulated time-evolution of the current profile is seen to correspond most strongly to experimental results when considering a highly non-linear (hyperbolic) dependence of pinning potential on magnetic field.
After full penetration is reached or when magnetic field ramping is stopped relaxation effects are observed; the shape of the current profile relaxes to finally resemble the static Kim-model profile.

In the design of magnetic bearings including high Tc superconductors, as well as in MagLev systems [1] it is essential to determine the levitation force. As a general rule, this is done using numerical techniques [2,3]. The great advantage of these techniques is that they permit one to determine the force acting on the superconductors, if the distribution in space of the field generated by the magnets is known. Their disadvantage is that their implementation can be hard work.
We show in this contribution that using analytical expressions [4] it is possible to reproduce force measurements carried out on various YBCO samples at 77K in the vicinity of a magnet after field cooling as well as the trapped field along the axis of samples. Then, we investigate the force that can be expected at lower working temperatures i.e. while increasing the critical current density of the samples. As predicted by Sanchez [5], the force tends towards a maximum value. For YBCO samples, the calculated maxima are less than twice the force measured at 77K and show a very good agreement with the results of measurements carried out at low temperature in a cryo-cooler.


[1] G. G. Sotelo et al. Journal of Physics: Conference Series 234 (2010) 032054
[2] DH N Dias et al., Supercond. Sci. Technol. 23 (2010) 075013
[3] L. Alloui et al.., Physica C 475 (2012) 32
[4] E.H.Brandt, Phys.Rev.58, 6506 (1998)
[5] A.Sanchez et al.. J.Appl.Phys.105, 023906 (2009)

This work was partially funded by CIFRE contract N°232/2014 granted by Association Nationale de la Recherche et de la Technologie in France

We acknowledge funding from Spain's Xunta de Galicia project GPC2014/038. A.R.-A. acknowledges support from FPI grant BES-2011-046820.

The flux pinning in pulse magnetization for high-Tc bulk superconductor is important for the realization of power applications. We must evaluate state of the time evolution of the trapped magnetic flux density distribution of the pulse magnetization. In general, the scanning of the hall probe over the HTS bulk is used for the measurement of the magnetic field. We need long time before the measurement is finished because Hall element is moved mechanically. In this case, we are able to measure magnetic flux density distribution on the HTS bulk surface. However, information about dynamic magnetic flux density distribution is not obtained. In the other case, there is a method that several Hall element put on the HTS bulk surface measures the magnetic field. Each Hall element completes the measurement in a short time. By increasing the number of the Hall element, we are able to measure the magnetic flux density distribution. In the present study, we have developed the measurement system of which magnetic flux density distribution is continuously measured using a Hall element to be placed on a HTS bulk surface. NxN Hall element requires the measuring instrument system consisting of NxN pairs of constant current generator and A/D converter. On the other hand, our NxN two-dimensional magnetic sensor needs only 1xN pairs of measurement instruments by technique to reduce the wiring. This sensor which has up to 15x15=225 pieces Hall element is able to measure in a 70mm square range of HTS bulk surface with the instruments of 15-pair. Two-dimensional magnetic flux density distribution of the surface is measured every 1ms when the system operates fastest. It is useful for us to measure transient magnetic flux density distribution in a short time to elucidate the magnetic flux penetration, because our pulse magnetization includes a rise time of 5-15ms. We will obtain information about pinning mechanism by observing flux creep process in the HTS bulk magnet. In this presentation, we show two-dimensional behavior of the dynamic magnetic flux motion duration of the pulse magnetization. In addition, In addition, we will present the result of our waveform control pulse magnetization which was performed in order to trap the high magnetic flux density of Gd-Ba-Cu-O bulk superconductor.

This work has been supported by JSPS KAKENHI Grant Number 23360394, by Fundamental Research Developing Association for Shipbuilding and Offshore (REDAS), and by the Grant-in-Aid program of the Furukawa technology foundation.

Currently, the main construction material for levitation bearings and suspensions are bulk HTS samples. But bulk HTS has several drawbacks such as brittleness, the complexity of sample preparation, when samples of desired shape are needed. Also critical current density of the bulk samples is significantly less than it for tapes. Thus, an alternative way to construct levitation systems is to use HTS tapes stacks that has high strength, simplicity of thermal stabilization and the ability to vary the geometric parameters of the stack.
For the development of HTS bearings and suspensions important to have information about the stability of the system under different dynamic processes, as well as in magnetic fields of different orientations. One of the most important cases - the behavior of the system in a permanent magnetic field perpendicular to the plane of the stacks with the additional influence of the crossed alternating magnetic field different values of amplitude and frequency on it. In this case, AC field is parallel to the plane of the tape.
In this paper we present the results of measurements the magnetization of HTS tapes stacks and magnetic levitation force between the permanent magnet NdFeB and stacks composed of n = 2 ÷ 100 ReBCO tapes 12 mm x 12 mm size in the FC and ZFC in crossed magnetic fields. The alternating magnetic field was produced by a copper coil, the frequency was varied in the range from 1 to 400 Hz. Measurements were carried out at boiling temperature of liquid nitrogen.
The dependencies of the relaxation rate of the magnetization of a stacks and the levitation force on the magnitude and frequency of the alternating magnetic field were obtained. The results are compared with the characteristics of bulk superconductors. Experimental dependences was compared with the results of numerical calculations.

The work has been supported by state-order No 3.1540.2014/K of the Ministry of Education and Science of the Russian Federation

Trapped field magnets are usually constructed on the base of bulk YBCO superconductors. But resent achievements in production of long-length coated conductors opens the opportunity to use tape stack for this applications. Better thermal conductivity, mechanical stability in high fields and high critical current densities make this such systems favorable. Moreover, tape-form of the conductor allows to form different spatial structures, optimizing devise design.
Trapped field was measured on a stack of HTS tapes in this work. Industrial stabilized (with copper shield) tape, produced by SuperOx company with GdBa₂Cu₃O_7-x superconductor inside, was used in experiments. The stack was made of 12x12 mm² and the height was from 2 to 200 tapes. Magnetization was carried out by 8 T CRYOGENIC magnet on the 65-77 K temperatures.
To calculate the trapped field of a stack, field dependences and angular field dependences of critical current were measured on these tapes. Finite element method was used for numerical simulations. It is shown, that taking into account the magnetic field orientation in different areas of the stack leads to better agreement with the experimental measured trapped field values.

Research was supported by Russian Science Foundation (project №14-22-00098)

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.

The limiting effect of grain boundaries (GBs) on the global critical current density (J_c) is one of the main obstacles for producing HTS wires and tapes for high-field applications. GBs are inherent to every growth method and much research has been devoted to decrease the angle between adjacent grains to avoid the strong decrease in J_c at larger angles. However, many other questions remain about the physics and correlation of the local current flow in the material and the underlying GB network. Recent results on PLD-YBCO coated conductors (CC) based on cube-textured NiW substrate with oxide buffer layers and ABAD-YSZ coated stainless steel substrates with an additional CeO₂ buffer layer show a good transfer of the substrate texture to the superconductor. Large grain size of YBCO in the range of 40-80 µm is observed on the NiW template while the ion-beam textured tapes have YBCO grains with dimensions below 1 µm. It is found that the adjacent YBCO grains have larger misorientation angles in NiW tapes as compared to ion-beam textured template. The imperfect texture of the YBCO on NiW is reflected in the trapped field profile measured using a high-resolution scanning Hall probe device. It is found that local currents in the YBCO grains dominate the signal and the grain boundaries greatly inhibit the macroscopic current flow. The correlation of the grain misalignments and the local current percolation is studied. The results of YBCO on NiW tape are compared with those of the much finer grain structure on the ABAD template having a much smaller degree of misalignment. In-field measurements and trapped field profiles at different temperatures are analyzed to determine the extent of the current limitation by the GBs in these coated conductors.

The research leading to these results have received funding from the European Union Seventh Framework Programme [FP7/2007-2013] under grant agreement n⁰. NMP-LA-2012-280432.

We acknowledge funding from Spain's Xunta de Galicia project GPC2014/038. A.R.-A. acknowledges support from FPI grant BES-2011-046820.

The high values of the superconducting critical field (≈ 40T) and transition critical temperature (≈ 35−40K) of MgB₂ creates the possibility to use this material in substitution of Nb₃Sn and NbTi, due to the low cost of the raw materials and the possibility of working with cryocoolers around 20K. In other hand, since the discovery of graphene, the bidimensionality of already known bulk materials have been exploited, such as for MgB₂. Based on this, the present work studied the electronic band structure of MgB₂ to compare the bulk material with few unit cell height thin films. 'Ab initio' simulations were done via Density Functional Theory with the software Quantum Espresso using a Vanderbilt ultrasoft pseudopotential with Perdew-Burke-Ernzerhof exchange-correlation factor obtained in the software’s database. The results suggest a significant increase in the density of states (DOS) near the Fermi level. Besides its relatively high critical temperature, superconductivity in MgB₂ can be well explained by the BCS theory, leading to the well-known fact that an enhancement of the DOS near the Fermi level would improve the critical temperature of the material. Besides that, it is possible to observe the crossing points of the Pi and Pi* bands for bulks happening above the Fermi energy, crossing the Fermi level towards lower energies when calculated for few MgB₂ layers. Depending on the number of MgB₂ layers, the crossing points could happen closer to the Fermi energy. The implications of this study can be directly applied to the optimization of MgB₂ properties and characteristics, leading to optimization of applications.

The authors wish to acknowledge the financial support by FAPESP, CNPq, and CAPES, Brazil. DRJ is a CNPq researcher.

Shielding a sensitive device against DC or AC stray magnetic fields can be realized very efficiently with high-Tc superconductors. A simple magnetic shield can be obtained with a tube enclosing the device, which is protected against external fields until the magnetic flux diffuses either across the tube wall or through the end openings. A better shielding vessel is obtained by using superconducting end caps. In the simplest design, the end caps, consisting of superconducting disks placed at the extremities of the tube, are making a 90 degree angle with the tube wall. Because of demagnetization effects, this arrangement results in a 'weak spot' for the penetrating magnetic flux. In this work, we investigate numerically different shapes for the end caps so as to avoid sharp angles and obtain a smoother magnetic flux penetration. The external magnetic induction is applied parallel to the shield axis. The magnetic flux distribution and the shielding currents are calculated with a finite element approach. The shielded volume is defined as the region where the local magnetic flux density is attenuated below a given fraction of the external field. It is studied as a function of the shape of the cap, its superconducting properties, the aspect ratio of the assembly, and the presence of a cabling hole. Curved and elongated caps, which could in principle be obtained by melt cast processing or different coating techniques, are shown to substantially increase the shielded volume.

We would like to thank Dr. Eamonn Maher from 3-Cs Ltd. who provided us with the YBCO coated conductor cylinders.

Nd_2-xCe_xCuO_4±d (NCCO) epitaxial thin films have been deposited on (100) SrTiO₃ substrates by dc sputtering technique in pure Argon atmosphere. The as-grown samples show a peculiar insulating logarithmic dependence of the in-plane resistivity at low temperature, with a metal-insulator crossover at a characteristic temperature T_min. It is commonly accepted that the higher anisotropic properties of NCCO crystalline cell with respect the hole doped YBaCuO and LaSrCuO, and the conduction mainly confined in the CuO₂ plane, strongly support the two-dimensional (2D) character of the electric transport in this system. This pronounced 2D behaviour, together with its strongly sensitivity to the oxygen content, points toward the 2D localization induced by disorder as the origin of the insulating state at low temperature. In this system, the magnetoresistance is also expected to show an unusual behaviour. In fact, theoretical calculations suggest a negative magnetoresistance in perpendicular field, and a zero magnetoresistance in parallel field. Results on resistance and magnetoresistance, as well as on Hall effect of epitaxial NCCO thin films in the overdoped region of the phase diagram, x≥0.15, are presented and discussed.

Nowadays magnetic bearings are used for design motors, generators, separating platforms and flywheel energy storage systems. HTS magnetic bearing construction is based on YBCO bulks through its ability to capture and hold significant magnetic field for a long time. However, that material requires external mechanical reinforcement and has low value of thermal conductivity what provides additional restrictions on practical application. Modern HTS tapes do not have such disadvantages. Its structure provides better rigidity and thermal stability in comparison with HTS bulk. One more significant advantage of tapes is simplicity of cutting to required form. In order to construct magnetic bearings from stacks of HTS tapes it is necessary to have data about stability of system stack-magnet.
In this work relaxation of levitation force was researched in both static and vibration mode with different oscillation frequencies. The measurements were performed for stacks with different amount of tapes in a stack at temperature T=77 K. Both FC and ZFC cooling methods were realized. Different displacements of stack from field axis were investigated. Relaxation rates of levitation force were measured and characteristic relaxation times of levitation force were calculated for various conditions. Comparison of relaxation rate of levitation force and relaxation rate of trapped field in presence of external magnetic field was made.

The work has been supported by state-order No 3.1540.2014/K of the Ministry of Education and Science of the Russian Federation

First principal investigation of FeSe under pressure
Mesfin Asfaw AFRASSA^1,2 , Fabio Bernardini^3
1Addis Ababa University, Department of Physics, Ethiopia
² Dipartimento di Fisica, Universita di Cagliari, 09042 Monserrato, Italy
³ CNR/IOM and Dipartimento di Fisica, Universita di Cagliari, 09042 Monserrato, Italy
Resistance, magnetism, and superconductivity of FeSe show a rich behavior under pressure[1, 2]. The non-monotonic behavior of the superconducting critical temperature Tc is puzzling. By first principles Density Functional Theory DFT approach we investigated the magnetic and structural phase diagram of FeSe under pressure. Our findings help to understand the experimental evidences.
Reference
[1] Taichi Terashima et al, arXiv:1502.03548v1
[2] Yoshikazu Mizuguchi et al, Appl.Phys.Lett. 93, 152505 (2008)



Mesfin A.A acknowledge support from DREAMACP program.

The dependence of the critical current (I_c) of a type II superconductor on the angle of a magnetic field applied in-plane with the transport current is commonly held to follow a variable Lorentz force behaviour in which it varies trigonometrically from some enhanced value in the force-free configuration of field parallel to current to its minimum value in the maximum Lorentz force configuration of field and current perpendicular, whereby the reduction in I_c is considered to result from the increasing depinning force exerted by the current on the flux lines.
Here we show, for contemporary RBCO-based coated conductor wires, that this presumed behaviour is only apparent at high temperatures, notably including 77 K. At temperatures as high as 60 K in moderate magnetic fields (8 T), significant deviations from the expected behaviour begin to occur, becoming stronger to lower temperatures and higher fields. An intermediate state of doubled periodicity is observed, and around 30 K, 3 T this resolves into a fully inverted behaviour, with maximum I_c obtaining in the “maximum Lorentz force” condition and minimum I_c in the “force-free” condition.
These experimental results have important implications for the design and operation of superconducting coils and magnets at low temperatures and intermediate to high fields. In particular, it is commonly assumed that the maximum Lorentz force geometry of current perpendicular to field is universally the geometry of minimum I_c. In fact, we show that this is not the case, with I_c values in excess of 35% lower being observed here in the force-free orientation.

Intrinsic supercurrent and extrinsic pinning current flow together within a superconductor, share the same boundary. How they respond together, electromagnetic description on perfect conductivity within superconductivity has never been answered. Here, we report that both currents do interact with each other indirectly via the boundary share. Irreversible and reversible magnetizations are not separable quantities. If we assume the boundary share can be decided by the number of vortex, then typical features of magnetization curves can be easily understood. Moreover, the boundary sharing also gives us a natural explanation for the weird paramagnetic Meissner effect. Further sophisticated works on this interplay are anticipated. Precise determination of fundamental physical parameters, such as the lower critical field, or accurate AC loss estimation for engineering applications can be possible.

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

A large-size and high-J_c bulk superconductor can trap a high magnetic field by a field cooling method, while a magnetic flux is difficult to penetrate the bulk due to strong magnetic shield by pulsed field magnetization (PFM). If a large magnetic field is applied, a large amount of heat is generated due to pinning loss, and, consequently, a trapped field is decreased because J_c is lowered. We have proposed to decrease magnetic shield by opening small holes and evaluated the trapped field performance varying the size and number of holes. In our previous research, small holes were drilled at the growth sector region (GSR) based on the idea that magnetic flux was supplied through GSR part with low J_c and they was trapped at the growth sector boundary (GSB) with high J_c. When the holes are arranged at GSB part, on the other hand, heat generation caused by pinning loss at flux penetration is suppressed, and it can be expected to improve the trapped field. In this paper, a trapped field performance is investigated in the bulk which processed a hole into GSB part, and it is compared with the case processed into GSR part.

This work was supported by JSPS KAKENHI Grant Number 24560343.

Abstract: Ever since the discovery of superconductivity in Y1Ba2Cu3O7 exhaustive study has been done on the substitution at the Cu site which has helped in understanding the mechanism of superconductivity. Studies over a last two decades, have shown that the other rare earth based systems such as RE1Ba2Cu3O7 (RE = Nd, Sm, Eu, Gd, Dy etc. ), have better applicability compared to Y-123 system [1,2]. Extensive studies have been done to understand the combined effect of the partial substitution of Pr ion at the RE site along with the substitution of Fe, Co, Zn and Ni at the Cu site [3,4], primarily to understand the interaction between Pr 4f electrons and the CuO2 valence band by varying the electronic state of the CuO2 planes. Study of transport properties of high temperature superconductors has been important not only for understanding the theoretical aspects, but also for their technological applications. In the present work, measurements of thermal conductivity, thermoelectric power and specific heat have been carried out on pure and co-doped samples of Nd0.95Pr0.05Ba2(Cu0.98 M0.02)3O7-d system [M = Fe, Co, Ni &Zn]. The electrical resistivity data demonstrates that all the samples exhibit metallic behavior. The thermal conductivity results show that for the pure sample of NdBa2Cu3O7-d, the expected well-defined hump in thermal conductivity is observed below the transition temperature. The hump is suppressed with 5% Pr-doping and this is further suppressed with co-doping. The thermoelectric power measurements demonstrate that the pure sample exhibits hole-like behavior and this continues for doped as well co-doped samples. Pristine sample shows the expected jump in specific heat and with Pr-doping a slope change is observed near the transition temperature.

We have studied experimentally the electrical conductivity near the normal-superconducting transition of two Y_0.25Gd_0.25Er_0.25Nd_0.25Ba₂Cu₃O_7-δ polycrystalline samples with different oxygen contents. The oxygen content for one of the samples is close to the optimum. The oxygen content of the other samples is slightly less than the optimum. The results show that resistive transition proceeds in two stages as seen by the temperature derivative of the resistivity near T_C. Decreasing the temperature, we first observe a pairing transition, at T_C, which stabilizes superconductivity within the grains. Near the zero-resistance state, at T_CO, occurs the coherence transition, where the phases of the order parameter in individual grains become long-range ordered. Fluctuations in electrical conductivity were investigated by the determination of the logarithmic temperature derivative of the conductivity. In the normal phase, above T_C, Gaussian fluctuation regimes were identified. On approaching the zero resistance state we observed, for both samples, a power law behavior that corresponds to a phase transition from a paracoherent to a coherent state of the granular array. In the poorly oxygenated sample we have identified a temperature region, below and near T_C, where the dissipative state could be related with a flux-creep mechanism induced by transport current. In general, our results indicate that partial substitution of Y by Gd, Er and Nd, in polycrystalline YBa₂Cu₃O_7-δ, produces or magnifies granularity effects at microscopic length scale in a manner relevant to the criticality. Also, the results indicate that the degree of oxygen is relevant below T_C, where predominate the effects related to the grain coupling phenomenology.

The conditions for single-grain growth of YBCO bulk superconductors by top seeded infiltration growth were tested. It is shown that the interaction of the melt, formed from BaCuO₂ + CuO + Y₂O₃ precursor, with the Sm123 seed causes dissolution of the seed and it is also shown that the diffusion rate of Sm in the melt and consequently the maximum overheating temperature is controlling this process. The conditions for epitaxial growth of single-grain YBCO bulk were found and they are supported by measurement of epitaxial crystal growth rate from the seed, the size and number of parasitic grain as a function of growth temperature. The influence of the way of CeO₂ addition to the system on the microstructure of the bulks is shown.

This work was realized within the framework of the projects: New Materials and Technologies for Energetics (ITMS 2 6220220061), Research and Development of Second Generation YBCO Bulk Superconductors (ITMS 26220220041), APVV No. 0330-12, VEGA No. 2/0090/13 and Stefanik Project Sk-Fr-2013-0025.

This work was realizad within the framework of the project: New Materials and Technologies for Energetics (ITMS 2 6220220061), Reseach and Development of Second generation YBCO Bulk Superconductor (ITMS 2 62202200041), APVV No. 0330-12, VEGA No. 2/0090/13 and Stefanik Project Sk-Fr-2013-0025.

The employment of bulk YBa₂Cu₃O_y "Y-123" high-T_c superconductors (HTS) as quasi-permanent magnets is based on the fact that they can trap magnetic fields several orders of magnitude higher than those of Fe-Nd-B magnets. The high-T_c material can be used for very powerful, compact superconducting super-magnets, which can be operated at liquid nitrogen temperature. To utilize these materials in several industrial applications, an uniform superconducting performance is essential, which can be achieved with the infiltration-growth (IG) technique. In this presentation, we adapted the top-seeded IG technique and produced several Y-123 samples with addition of Y₂Ba₁Cu₁O₅ "Y-211" secondary phase particles in varying sizes by the sintering process. All Y-211 disks were sintered at temperatures ranging from 900 to 1100 ^oC and used for the production of Y-123 material for IG process. We studied the superconducting performance, critical current density (J_c), and trapped field performance at 77 K. Magnetization measurements showed a sharp superconducting transition with an onset T_c at around 92 K, irrespective of the sintering temperature. However, the trapped field and critical current density (J_c) values were dependent on the sintering temperature and we found that the best temperature was around 925 ^oC. The improved performance of Y-123 material at 925 ^oC can be understood in terms of homogeneous distribution of fine secondary phase particles.

　　For producing large REBa₂Cu₃O_y (RE123) melt-solidified bulks, multiple-seeding is known to be an effective method, while multi-seeded bulks generally show lower critical current density (J_c) at grain boundaries than in each domain. We have attempted to increase J_c of grain boundaries by Ca doping to promote carrier overdoping. Calcined powders of Y_1-xCa_xBa₂Cu₃O_y (x = 0, 0.01, 0.02, 0.05) and Y211 were mixed into a molar ratio of 7:3 and 0.5 wt% of platinum powder was added to the each mixture. The mixtures were uniaxially pressed at 100 MPa into cylindrical shapes with dimensions of 20 x 8 mm^t or 30 mmf x 10 mm^t. One or two single crystals of Nd123 were put on each disk with its c-axis vertical to the disk surface, melt-solidified in air, and annealed in flowing oxygen at 450 ºC. Systematic susceptibility studies for various parts of the bulks and our preliminary results on the change in T_c of Y123 sintered bulks by Ca-doping suggested that Ca was substituted in the whole melt-solidified bulks and the substitution level near the seed crystal was higher than that at edges of the bulks. Although Ca doping largely decreases T_c, a sample cut from the a-growth region of a Ca 5% doped Y123 bulk exhibited high J_c ～ 5 x 105 A cm^-2 at 40 K in low fields. Enhancement of J_c at grain boundaries of a bulk grown from two seeds was confirmed even for a 2% Ca doped sample. Magneto-optical images of the remanent magnetic field distribution for multi-seeded bulks and effect of oxygen composition control on superconducting properties will be shown.

It is well-known that the very small regions where RE elements partially substitute for Ba sites in melt-solidified bulks and single crystals of REBa₂Cu₃O_y (RE123) act as effective pinning centers in magnetic fields when the substitution level of RE is moderately low. In our previous study, RE-mixing was found to be an effective way to control RE/Ba substitution levels below 1% and J_c-B properties[1]. In the present study, (RE’,RE’’)123 bulks were synthesized from precursors with RE’123 : RE’’211 = 7 : 3 (molar ratio, RE’ and RE’’ = Gd, Dy, Y, Ho and Er). As an index of RE/Ba substitution level, orthorhombicity (= 1000 (b - a) / (b + a)), where a and b are the lattice constants, was calculated because RE/Ba substitution monotonically decreases the orthorhombic distortion of RE123 crystals. The relationship between the maximum values of pinning force density F_p,max (= (J_c × B)_max) and orthorhombicity indicated that suitable orthorhombicity for higher F_p,max increases with a decrease in temperature. It was suggested that bulks with lower RE/Ba substitution level are more promising for high-field applications at lower temperatures. In addition, the microstructural observation revealed that RE-mixing can change the size of RE₂BaCuO₅ particles dispersed in the RE123 matrix, which affects pinning force in low magnetic fields. It can be explained by slight difference in peritectic temperatures of RE123.

[1] Y. Setoyama et al., Supercond. Sci. and Technol. 28 (2015) 015014.

A modified top seeding melt texture growth processing (TSMTG) has been presented, which produced higher quality of YBCO single domains. In the new processing, Y-211 particles distribution within Y-123 pellet matrix was artificially set into an un-uniformed pattern. YBCO single domain bulks fabricated by this modified approach have illustrated an enhanced trapped magnetic field. For a YBCO single domain with 35mm in diameter, growing in air with cold seeding method, the maximum trapped field of 1.2 T can be reachable under 77K, and, additionally, a batch of single domain bulks with high trapped field can also be realized by using this new processing.

Growth and Superconducting Properties of Multi-seeded Quasi (RE)BCO Single Grains
Yunhua Shi, Anthony R Dennis, Namburi Devendra Kumar, John H Durrell and David A Cardwell

Abstract
Single grain, (RE)BCO (rare earth–barium–copper oxide) bulk superconductors in large or complicated geometries are required for a variety of potential engineering applications. Unfortunately, (RE)BCO single grains have intrinsically very low growth rates, which represent a limitation to the sample size that may be achieved for practical growth times. Multi-seeding (i.e. the use of two or more seeds to control the nucleation and subsequent grain growth) has been developed to fabricate larger samples and to reduce the time taken to melt process. However, the formation of grain boundaries in samples grown by the multi-seeding technique appears to be unavoidable, even when the seeds employed are aligned well. The work presented here describes novel a technique to grow multi-seeded, quasi single grain bulk (RE)BCO superconductors for which the effect of grain boundaries on the applied properties of the samples is decreased significantly. The multi-seeded grain growth procedures for the fabrication of YBCO and GdBCO-Ag samples of different sizes are described and the superconducting properties of the resultant quasi single grains presented.

In this study, large grained YBCO rings were produced by a crystal growth technique for levitation and SFCL applications. First, YBCO-123 nominal composition was calcined at 920°C for 10 h. Then, calcined powder were mixed with some amount of YBCO-123 single crystals which crushed into small pieces to create many seed points to grow large crystals in the ring materials. Therefore, the mixtures were compacted into ring shape with the sizes outer and inner diameters 50 mm and 35 mm under 8 tons axial pressure. The crystal growth process were performed as follow: the pressed ring material was put into an Al₂O₃ plate and some amount of Y₂O₃ powder were poured between crucible and YBCO ring material to prevent the liquid phase not to distribute inside the crucible. Finally, a standard crystal growth temperature-time scale were applied to have large grained YBCO-123 in the ring shape. Consequently, high quality YBCO-123 in the ring shape can be successfully fabricated in the different sizes for the levitation and SFCL applications. The levitation force measured below transition temperature indicates high quality ring superconductors can be produced for the large scale applications.

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

The top-seeded melt texture growth of GdBCO single bulks using the mixture precursor powder of GdBa2Cu3O7-δ (Gd123) and Gd2BaCuO (Gd211) pressed as the precursor pellet has been considered to be an effective way to yield high critical current density. However, these two different precursor powders should be prepared previously by the conventional solid state reaction method. Hence, for reducing the fabrication time and cost, we proposed a new economic method without preparing any precursor powder in this study. The single domain GdBCO bulks were successfully fabricated in air with the initial raw materials of Gd2O3, BaO and CuO according to different ratios. After detailed investigations in the morphology, levitation force and trapped magnetic field of the as-grown samples with different ratios, it is found that the modified precursor phase source can be used to fabricate the single domain GdBCO bulk superconductors more efficiently, which can combine the crystal growth procedure and the precursor powders preparation process as a whole at some extent.The top-seeded melt texture growth of GdBCO single bulks using the mixture precursor powder of GdBa2Cu3O7-δ (Gd123) and Gd2BaCuO (Gd211) pressed as the precursor pellet has been considered to be an effective way to yield high critical current density. However, these two different precursor powders should be prepared previously by the conventional solid state reaction method. Hence, for reducing the fabrication time and cost, we proposed a new economic method without preparing any precursor powder in this study. The single domain GdBCO bulks were successfully fabricated in air with the initial raw materials of Gd2O3, BaO and CuO according to different ratios. After detailed investigations in the morphology, levitation force and trapped magnetic field of the as-grown samples with different ratios, it is found that the modified precursor phase source can be used to fabricate the single domain GdBCO bulk superconductors more efficiently, which can combine the crystal growth procedure and the precursor powders preparation process as a whole at some extent.

This work was supported by the Key grant Project of Chinese Ministry of Education (No.311033),the Outstanding Doctoral Thesis Foundation Project of Shaanxi Normal University (No.X2011YB08, X2012YB05), and the National Natural Science Foundation in China (No.50872079, 51167016).

As a synthesis method of REBa₂Cu₃O_y (RE123) phase, liquid phase growth process is expected to reduce the cost. However, traditional liquid phase growth process needs a high temperature up to approximately 1000 ºC. This is one of the problems for applications. We synthesized RE123 in a nitrogen atmosphere at 500~700 ºC by molten KOH flux method. For RE123 with heavy rare-earth elements (HRE123; HRE=Yb, Er, Ho, Y, Dy) synthesized by this method, the critical temperature (T_c) showed about 90 K. By contrast, T_c of RE123 with light rare-earth elements (LRE123; LRE=La, Nd, Sm, Eu, Gd) was lower than that of HRE123. For RE123 system, it is known that T_c degradation and c-axis length shrinkage are caused by the partial substitution of Ba site by RE (RE/Ba substitution), and the RE/Ba substitution is suppressed by reducing oxygen partial pressure. Hence it is supposed that lower T_c of LRE123 synthesized by the molten KOH flux method is caused by RE/Ba substitution. In the molten KOH flux method, it is necessary to control the RE/Ba substitution to synthesize high-quality RE123 single crystals. We investigate whether it is possible to control the RE/Ba substitution in the molten KOH flux method. At various temperatures and RE elements, we studied the change of T_c and c-axis length of RE123 which was synthesized by changing the oxygen partial pressure.
For Eu123 samples, T_c and c-axis length increased with either decreasing oxygen partial pressure or increasing synthesis temperature. The highest T_c samples synthesized at oxygen partial pressure of 1×10^-4 atm at 700 ºC showed T_c=92.5 K and c-axis length similar to the stoichiometric EuBa₂Cu₃O₇ phase. The correlation between T_c and c-axis length of Eu123 samples which were changed by the oxygen partial pressure is the same as Eu123 which Eu/Ba substitution occurred. Consequently, it is possible to control the RE/Ba substitution of the Eu123 single crystal by controlling the oxygen partial pressure in the molten KOH flux method.

(RE)Ba2Cu3O7-x bulk high temperature superconductors fabricated in the form of large, single grains can generate magnetic fields that are an order of magnitude higher than those achievable using conventional permanent magnets. It is now well established that the microstructure of these materials plays a dominant role in governing the superconducting properties such as critical current densities and resultant trapped fields.
Seeded Infiltration and Growth (SIG) process is an alternative to the conventional Top Seeded Melt Growth (TSMG) process for growing (RE)BCO bulks as it overcomes drawbacks such as porosity, sample shrinkage and inhomogeneity in the distribution of RE-211 content across the volume of the samples which are inherent in the TSMG process. However, due to the complexity in the growth process, it is not trivial to make single grain samples with size > 20 mm in dia by the SIG process.
In the present work, we report the development of a 2-step TSIG technique which makes the growth process more straightforward and very reliable. We will show how an appropriate liquid phase reservoir is chosen for the successful infiltration of liquid phases it into RE-211 preform which later enables a smooth growth of RE-123. We will then discuss the influence of the two step process (Infiltration and Seeded Growth) on the microstructures and superconducting properties of YBCO samples fabricated by this novel technique

Authors are grateful to the King Abdulaziz City of Science and Technology (KACST), Saudi Arabia and to the Engineering and Physical Sciences research Council (EPSRC) for financial support in the form of research projects.

In this work two different processes were used to synthesize superconducting YBa2Cu3O7-δ polycrystalline ceramic samples, using a new acetate method and a conventional solid state method. A detailed analysis of the correlation between structural and physical properties in solid solution compounds, such as YBa2Cu3O7-δ, requires samples with a high degree of chemical and physical homogeneity. Studies based on different synthesis techniques suggested that the sample homogeneities depended on the synthesis conditions. YBa2Cu3O7-δ samples were explored and compared in order to determine the optimum conditions to obtain superconductor state. The structural and superconducting properties of the samples were studied by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), AC-electric characterization and DC-magnetization. The experimental results are discussed and compared. Stoichiometric quantities of powders (99,9% purity) of Y2O3, BaCO3 and CuO were mixed in a solution of 50% glacial acetic acid, heated and dried in order to produce the starting powder to obtain the superconducting samples. Same amounts were used in a conventional solid state mixing and reaction to obtain reference samples for comparison. The results of XRD measurements showed the presence of a small amount of the green-phase Y2BaCuO5, as an impurity phase, coexisting with YBa2Cu3O7-δ. Orthorhombic space group Pmmm was assumed for both samples, and the data fitted using the Rietveld method for structural refinements showed that only those two phases were present in that samples. AC electrical resistance as a function of temperature measurements and DC magnetization measurements showed that both samples had superconducting transition with TC » 91 K. SEM provided information of the grains size and homogeneity. This result confirms that the proposed acetate method saves some preparation time without change in the YBa2Cu3O7-δ sample quality, and therefore it would help in the production of large quantities of ceramics superconductors.

CBPF’s X-ray Laboratory, resistivity and magnetic measurements, UFRRJ-LMDS’s SEM characterization facilities. P&D ANEEL D712: SUPERCABO Project, CEMIG D, CEMIG GT, CTEEP, TAESA, TBE, UFRRJ, FAPUR and FAPERJ for financial support.

NdBa2Cu3O7-δ (Nd-123) ceramic samples were sintered by both a new wet-chemical or acetate method and conventional solid state method. The proposed method of preparation of a superconducting ceramic compound through this wet-chemical method is primarily intended to save time maceration of the precursor reagents in order to obtain the samples. In the proposed acetate method, the reagents Nd2O3, BaCO3 and CuO were added to a solution of 50% glacial acetic acid, heated and dried in order to produce the starting powder to produce the superconducting samples. Characterizations were performed by X-ray powder diffraction and the refinement of the diffraction pattern was made by Rietveld method. The electrical characterization of AC electrical resistance as a function of temperature was performed in the absence of applied magnetic field by the conventional four probe method. The equipment used in those measurements was an AC resistance bridge that supplies electrical current to the samples inside a closed circuit recycling helium cryostat, connected to a temperature controller. Zero Field Cooling and Field Cooling magnetic moment measurements as a function of temperature were performed using a Vibrating Sample Magnetometers from Quantum Design. The samples were also observed by Scanning Electron Microscopy (SEM) providing information of the grain growth. The results have shown superconductivity with critical temperatures of about 60 K for the Nd-123 acetate sample, orthorhombic crystalline system, spatial group Pmmm with lattice parameters a=3.878 Å, b=3.848 Å and c=11.647 Å. All results obtained were compared with those for samples prepared using conventional solid state method. Samples prepared by the acetate method presented in this work have shown the best results in all characterization methods applied. Those results suggest that the proposed acetate method leads to superconducting samples in shorter time without change in sample quality, and therefore it would help in the production of large quantities of those superconductors.

CBPF’s X-ray Laboratory, resistivity and magnetic measurements, UFRRJ-LMDS’s SEM characterization facilities. R&D ANEEL D712: SUPERCABO Project, CEMIG D, CEMIG GT, CTEEP, TAESA, TBE, UFRRJ, FAPUR and FAPERJ for financial support.

1. S.K. Nikoghosyan et al, The appearance of semiconducting (dielectric) conduction in polycrystalline high-temperature superconducting cuprates after heat treatment, Armenian Journal of Physics, 2015, vol. 8, issue 1, pp. 1-6.

The influence of RE (RE = Sm or Yb) substitutions on microstructure and superconducting properties of YBa₂Cu₃O_7-δ (YBCO, Y123) bulk superconductors have been studied for different RE concentrations and their combination. YBCO bulk superconductors with REO₂ addition were prepared by the optimized Top Seeded Melt Growth (TSMG) process in the form of single crystals. Microstructure of prepared samples was studied by polarized light microscope, scanning electron microscope (SEM) equipped with Energy Dispersive Spectrometer (EDS) and Wavelength Dispersive Spectrometer (WDS) as well as by X-ray Powder Diffraction (XRD). The superconducting properties were measured using a VSM magnetometer in magnetic fields up to 6 T at 77 K. The effect of RE substitutions on Y₂BaCuO₅ (Y211) particle size, critical temperature, T_c and critical current density, J_c is reported.

This work was realized within the framework of the projects: New Materials and Technologies for Energetics (ITMS 26220220061), Research and Development of Second Generation YBCO Bulk Superconductors (ITMS 26220220041), APVV No. 0330-12, VEGA No. 2/0090/13 and Stefanik Project Sk-Fr-2013-0025.

The effect of Nd3.6Ba2.4Cu1.8O5-d [Nd(422)] dopant on the microstructure and superconducting properties of Nd0.9Ba2.1Cu3O7-d [Nd(123)] superconductor melt-processed in air has been studied using various techniques. It has been confirmed that the addition of Nd(422) phase prior to melt-processing, significantly enhance the superconducting prosperities of Nd(123) superconductor when melt-processed in air atmosphere. Microstructural analysis using EPMA revealed the presence of Nd(422) inclusions in the 123 matrix and these inclusions are better dispersed and more refined in the doped sample. Compared to the undoped sample, the Nd(422)-doped sample showed slight improvement of current density at low fields and high temperatures, with largely depressed fishtail peak effect at high temperatures. These results are ascribed to the effect of Nd(422) phase which yet unoptimized to serve as efficient pinning centers and also allow the introduction of sufficient field induced pinning centers in the sample.
The effect of Nd_3.6Ba_2.4Cu_1.8O_5-d [Nd(422)] dopant on the microstructure and superconducting properties of Nd_0.9Ba_2.1Cu₃O_7-d [Nd(123)] superconductor melt-processed in air has been studied using various techniques. It has been confirmed that the addition of Nd(422) phase prior to melt-processing, significantly enhance the superconducting prosperities of Nd(123) superconductor when melt-processed in air atmosphere. Microstructural analysis using EPMA revealed the presence of Nd(422) inclusions in the 123 matrix and these inclusions are better dispersed and more refined in the doped sample. Compared to the undoped sample, the Nd(422)-doped sample showed slight improvement of current density at low fields and high temperatures, with largely depressed fishtail peak effect at high temperatures. These results are ascribed to the effect of Nd(422) phase which yet unoptimized to serve as efficient pinning centers and also allow the introduction of sufficient field induced pinning centers in the sample.

The present work was supported by the Ministry of Education and Science of Russia and by the Ural Branch of RAS (Project No 15-17-2-22).

We report the effects of the edition of As₂O₃ on the electric and magnetic properties in Hg-based high temperature superconductors. Samples with composition HgBa₂Ca₂Cu₃As_xO_8+δ (x=0.0-1.2 wt.%) were prepared by the two-step solid-state reaction method. In the first step highly pure powders of BaO, CaCO₃ and CuO were mixed in the stoichiometric ratio Ba:Ca:Cu=2:2:3 and then they were mixed and ground carefully in an agate mortar. The resulting powder mixture was calcined in an alumina crucible in air with four intermediate grindings at 900 ^oC for 60 h. In the second step the Ba–Ca–Cu–O precursor was mixed with HgO and As₂O₃ powders according to the composition HgBa₂Ca₂Cu₃As_xO_{8+ δ} (where x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0 and 1.2 wt.%) and After final grinding the powder was pressed into a disc-shaped pellet 6 mm in diameter, and 2 mm thick, by using a hydraulic press under a pressure of 5 ton/cm². The pellets were put in a quartz tube and from quartz tube was evacuated up to 10^-4 Torr and sealed. Thereafter, quartz tube was inserted into a programmed muffle furnace and heated to 860^oC and held at this temperature for 12 h. Finally, the samples were oxygenated in furnace at 300^oC in air 15 h.
The prepared patterns were characterized by X-ray diffraction (XRD, Dron-3M) with CuKa radiation. Ac susceptibility and high harmonic response of un-doped and Arsenic-doped samples are measured in weak ac and dc magnetic fields (10⁻² ≤ h ≤1 Oe, 0 ≤ H ≤ 12 Oe.). Critical state models are used to explain the nonlinear magnetic response. As a result we could conclude that, the low level Arsenic doping enhances the transport critical current densities J_c.

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

The discovery of iron based superconductors (FBS) opened a new possibility for high magnetic field applications owing to their high T_c and H_c2. Among the various FBSs, BaFe₂As₂ (Ba122) system is one of the most promising candidates for applications because of its small electromagnetic anisotropy. Especially, a phosphorus doped Ba122, BaFe₂(As,P)₂, thin film has recorded very high self-field J_c over 10⁷ A/cm² enough to attract great interests^[1]. In the present study, we have synthesized BaFe₂(As_1-xP_x)₂ polycrystalline bulks with systematically varied phosphorus doping level (x = 0-1) and evaluated their microstructures and electromagnetic properties in order to clarify the current transport properties at grain boundaries. Highly pure elemental metals, i.e. Ba, Fe, As, P, were mixed and ground by planetary ball-milling apparatus under argon atmosphere. The milled powder was uniaxially pressed into pellet, and then sealed in an evacuated quartz ampoule followed by heat-treatment at 900˚C for 48 h. For comparison, Ba122 polycrystalline bulks were also synthesized from BaFe₂As₂ and BaFe₂P₂ powders. Powder XRD analysis revealed that 122 phase was formed as main phase for all the samples. Electrical resistivity measurements showed that the highest T_c^onset = 29.7 K was obtained for a sample with x = 0.35. Well-connected, fine Ba122 grains were observed for under-doped and optimally doped samples, however, Fe₂P impurity phase was observed in most of the samples regardless of synthesis conditions, especially in the samples with large x. Generation dopant containing impurities is a particular problem in phosphorus doped Ba122. Remanent magnetization studies revealed that a sample with x = 0.40 has highest J_c^global ~4×10³ A/cm² at 5 K, which is comparable to that of cobalt doped Ba122.
[1] A. Sakagami et al., Physica C 494 (2013) 181-184.

Recently, a new series of Fe-based superconductors (Ca,RE)FeAs₂ ((Ca,RE)112) were reported. These new phase are composed of stacking of FeAs layers, Ca(RE) layers, and As chain layers. RE substitution for Ca site is necessary to form this phase, and we have succeeded in synthesizing (Ca,RE)112 phase with RE = La-Tb. Substitution of RE with small ionic radii (Sa, Eu, Gd, Tb) for Ca site were performed using high pressure synthesis.
Superconductivity was observed in all (Ca,RE)FeAs₂ samples, and we found that transition metals (Co, Ni) co-doping improved superconducting properties of (Ca,RE)112. For example, Co doped polycrystalline sample showed T_c = 38 K, relatively high T_c as Co doped iron-based superconductors. Furthermore, Co doped (Ca,La)112 showed relatively high J_c approximately 2 x 10⁴ Acm^-2 as polycrystalline sample.
T_c of Co doped (Ca,RE)FeAs₂ decreased according to the decrease of ionic radii of RE³⁺, from 38 K (RE = La) to 28 K (RE = Tb) though (Ca,Eu)112 (T_c = 21 K) did not follow with this trend. (Ca,Eu)112 has larger interlayer distance of Fe-Fe plane than (Ca,Sm)112 and (Ca,Gd)112, suggesting the existence of Eu²⁺ which has larger ionic radii (1.25 Å) compared to Eu³⁺ (1.07 Å). Low T_c of (Ca,Eu)112 can be explained by low carrier concentration due to co-existence of Eu²⁺ and Eu³⁺.
We are now trying to synthesize (Ca,RE)112 phase with other transition metals such as Rh and Ir, and we will show results of these research at presentation.

The new kind of iron-based superconductor FeSe was discovered in 2008, has about 8 K of Tc. To realize the anisotropic property, the single crystal and thin film technology of FeSe has been widely studied. In our study, the low porosity poly-crystal with (110) preferred orientation was grown by the Bridgman method [1]. With the analysis of 2D-XRD (two dimension X-ray diffraction), less than three orientations are indexed, implied the highly preferred orientation of these samples as well as the existence of twin structures. From the image of optical microstructure, the cellular like morphology was observed, with the nano-rod-like precipitation which is identified as γ-iron phase by energy dispersive spectrum (EDS) analysis. In the differential thermal analysis (DTA) with the different heating rate, our samples showed the typical diffusion-less behavior [2]. In this report, these crystallographic data will be summarized and compared with that of superconducting properties and phase transformation of preferred orientated FeSe samples.
[1] C. M. Yang, P. W. Chen, J.C. Kou, P. Diko, I. G. Chen, and M. K. Wu, IEEE Transactions on Applied Superconductivity, 21, 2845–2848 (2011)
[2] P. Diko, V. Antal, V. Kavecansky, C. M. Yang, I. Chen, Physica C, 476, 29–31 (2012)

This study was supported by the Ministry of Science and Technology, Taiwan, under Contract MOST 103-2811-M-006-046 and NSC 102-2112-M-006-001-MY3. The two dimensions X-ray diffraction is measured in Establishment of the Instrument Development Center of NCKU.

This study investigated the chemical doping mechanism of Li in the Fe(Se,Te) system. With the addition of Li, T_c of FeSe_1-xTe_x was noticeable increased (1–1.5 K). For the FeSe_0.5Te_0.5 samples with additions of Li₂CO₃ ranging from 0 to 10 wt%, the XRD refined structure analysis indicate Li ions are inserted into FeSe_0.5Te_0.5 lattice, causing a large distortion and pushing Se away from the original position [1]. For the FeSe_1-xTe_x samples (x = 0.5~0.9), with 1 wt% additions of Li₂CO₃, the lattice elongation becomes complicated, which may resulted from the different chemical interactions of Te and Se with Li doping respectively. In this report, the detailed XRD data of this series of Li doped FeSeTe samples will be summarized and compared with that of superconductivity and Raman analysis [2].
[1] Chia-Ming Yang , Po-Wei Chen , In-Gann Chen, Xiao-Ding Qi, Der-Chung Yan and Maw-Kuen Wu, Superconductor Science and Technology, 25, 095010 (2012)
[2] Chia-Ming Yang, In-Gann Chen, Der-Chung Yan and Maw-Kuen Wu, submitted for Superconductor Science and Technology(2015)

This study was supported by the Ministry of Science and Technology, Taiwan, under Contract MOST 103-2811-M-006-046 and NSC 102-2112-M-006-001-MY3.

The superconducting transition temperature up to 44 K in K_xFe₂Se₂(NH₃)_y compounds has attracted great attention. However, Tianping Ying et. al (J. Am. Chem. Soc. 2013, 135, 2951−2954) has pointed out that: ”NH₃ has little, if any, effect on superconductivity”. On the other hand, D. M. Wang et. al (PHYSICAL REVIEW B 2011, 83, 132502) also mentioned the existence of a 44 K transition in K_0.8Fe₂Se₂. In the present paper, three different K_xFe_2-ySe₂ samples with nominal compositions of K_0.8Fe₂Se₂ were prepared using one-step (FeSe-0) and two-steps (FeSe-1, FeSe-2) self-flux method employing different FeSe precursors. The onset superconducting transition temperature around 44 K of sample FeSe-1 and FeSe-2 are much higher than that of FeSe-0, while the transition width of them broadening to 12 K. This might results from the decreasing of iron vacancy. It was demonstrated that the FeSe precursor gave rise to significant effects on the intercalating of K ions and the homogeneous of sample, afterwards influenced the behavior of the resistance curves and crystal quality.

Interpreting measurements of the physical properties of new iron-based superconductors is made difficult by the frequent coexistence of secondary phases, sometimes magnetic. For our physical properties studies and possible applications, we are particularly interested in the superconducting phase with the simplest structure, i.e. the ß-Fe_1+∂Se tetragonal phase superconducting for interstitial iron of ∂ ~ 0.01, and non superconducting for ∂ > 0.03. In order to obtain high quality single crystals of tetragonal iron selenide, we have optimized their growth by using a chemical vapor transport method based on an AlCl₃/KCl eutectic.

By coupling magnetization, transport measurements and X-ray diffraction, we determined the optimal synthesis conditions, that is to say the nominal composition of the Fe-Se-AlCl₃-KCl mixture and the thermal gradient along the two zones furnace allowing the transport of Fe and Se species in the chloride vapor. With this technique, we obtained excellent single crystals with a critical temperature of 8.7 K and a very narrow diamagnetic transition of 0.7 K. We will detail our single crystal growth method and explain the properties of the resulting single crystals.