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[en] The superconductor industry is demanding new methodologies to manufacture km-long, high quality coated conductors at high growth rates, using cost-effective, scalable processes. We report on the fabrication by an all-chemical deposition method of highly textured, thick (0.9 μm) inkjet-printed YBCO films, using a Ce0.9Zr0.1O2 (CZO) capping layer deposited by MOD, on top of robust, buffered ABADYSZ/SS substrates. Thinner, 0.25 μm spin-coated YBCO films were also analyzed for comparison. The structural study performed by x-ray diffraction, optical, AFM, SEM and TEM microscopy demonstrates the success of the capping layer for enhancing the planarity of the as-received tape and obtaining highly homogeneous and well-textured YBCO films. DC magnetometry granularity analysis was used to determine the mean superconducting grain diameter, ∼2.5 μm, and the intra- and intergranular critical current densities of the coated conductors (CCs). For the thin, spin-coated sample, high self-field intragrain critical currents were measured (JcG=4 0, 3.3 MA cm−2 at 5, 77 K). For the thick, inkjet-printed tape JcG was reduced by ∼30%, but, notably, the percolative critical current, JcGB=1 2.5 MA cm-2, was only ∼10% smaller at 5 K, thanks to good preservation of the texture. At 77 K, JcGB=1.3 MA cm-2 was achieved, implying a critical current of Ic = 117 A/cm-width. AC susceptibility measurements allowed us to demonstrate the high homogeneity of the fabricated CCs, and investigate the magnetic vortex-pinning phase diagram. Remarkably, the thick, inkjet-printed sample showed comparable irreversibility line (IL) and activation energy for thermal depinning, Ue(H), to the thin sample. The present results open new perspectives for the fabrication of high quality-to-cost ratio, all-chemical CCs with yet higher Ic values by inkjet printing multideposition of thicker YBCO layers. (paper)
[en] In this study Te-rich iron chalcogenide (FeSexTe1−x) thin films with a composition close to antiferromagnetic ordering have been deposited on SrTiO3 (STO) substrates. The superconducting critical transition temperature (Tc) of the FeSe0.1Te0.9 thin film on STO substrate ranges from ∼12.5 to ∼13.3 K. The upper critical field is as high as 114 T, which is much higher than that of the FeSe0.5Te0.5 thin film on STO substrate (∼49 T). The self-field critical current density (Jcsf) at 2 K of 1.8 × 105 A cm−2 is much higher than that of the FeSe0.5Te0.5 thin film, and the FeSe0.1Te0.9 thin film also demonstrates superior pinning properties under applied magnetic field. Compared to FeSe0.5Te0.5, which was considered as the optimum composition, FeSe0.1Te0.9 presents even more promise for high field applications because of its high upper critical field and high critical current density. (fast track communication)
[en] We study the microwave properties of a superconducting tunable coplanar waveguide (CPW). Pairs of Josephson junctions are forming superconducting quantum interference devices (SQUIDs), which shunt the central conductor of the CPW. The Josephson inductance of the SQUIDs is varied in the range of 0.08–0.5 nH by applying a dc magnetic field. The central conductor of the CPW contains Josephson junctions connected in series that provide extra inductances; the magnetic field controlling the SQUIDs is weak enough not to influence the inductance of the chain of the single junctions. The circuit is designed to have left- and right-handed transmission properties separated by a variable rejection band; the band edges can be tuned by the magnetic field. We present transmission measurements on CPWs based on up to 120 Nb–AlOx–Nb Josephson junctions. At zero magnetic field, we observed no rejection band in the frequency range of 8–11 GHz. When applying the magnetic field, a rejection band between 7 GHz and 9 GHz appears. The experimental data are compared with numerical simulations. (paper)
[en] The scattering through a Josephson junction (JJ) interrupting a superconducting line is revisited including power leakage. We also discuss how to make tunable and broadband resonant mirrors by concatenating junctions. As an application, we show how to construct cavities using these mirrors, thus connecting two research fields: JJ quantum metamaterials and coupled-cavity arrays. We finish by discussing the first nonlinear corrections to the scattering and their measurable effects. (paper)
[en] In this work we demonstrate that the use of striated tapes from coated conductors (CCs) significantly reduces the dissipation of a cable made of tapes wound helically on a round core when it is exposed to AC magnetic field. The coupling loss can vanish provided that the striations ensure electrical insulation between filaments and the cable length corresponds to an entire number of lay pitches. In our study we compare the magnetization loss in two cable models exposed to magnetic field perpendicular to their longitudinal axis. The overall geometry of the models was identical: each consisted of three tapes 4 mm wide that were placed with a pitch of 50 mm in a single layer on the 8 mm diameter round core. The cable length was designed to reach two complete tape pitches. In the first cable (the reference cable) tapes without striation were used; the second cable was prepared using similar tapes but striated to five filaments by laser processing. The AC loss was measured for cables without terminations as well as with low resistance terminations; this latter configuration simulates the conditions in a magnet winding. Our experiments have clearly shown the loss behavior expected in the regime of uncoupled filaments. In particular, at AC fields of 0.1 T amplitude the loss in the cable from striated tapes is five times lower than in the reference cable. Numerical models have explained the experimentally observed cable behavior in the whole range of AC fields. (paper)
[en] Roebel cables are one possible option for use in the manufacture of high current conductors for various technical applications. In this work the behaviour of critical current under transverse loads up to 90 MPa was studied in coated conductor Roebel cables. First the critical current evolution under transverse load was measured on 4 mm wide coated conductor tapes with Hastelloy substrate (copper surround stabilizer) and on punched tapes, which are used for the assembling of Roebel cables. It was found that they were able to withstand large transverse loads (>100 MPa) with less than 2% degradation of the critical current at 77 K. In contrast, with tapes, a significant reduction of the critical current (>20%) was observed in Roebel cable samples at pressure as low as 10 MPa. Marks of mechanical defects were observed in the strands extracted from the cable. Measurements of the local critical current on extracted strands confirmed that in the regions which were visually damaged the critical current was also reduced. The defects are attributed to pinching (stress concentration), which takes place in the cable. The results suggest that the integration of coated conductor Roebel cables into large systems should be carefully evaluated. (paper)
[en] Thin superconducting NbTiN and NbN films with a few nm thickness are used in various device applications including in hot electron bolometer mixers. Such thin films have lower critical temperature (Tc) and higher resistivity than corresponding bulk materials. In an effort to improve them, we have investigated an effect of the AlN buffer layer between the film and the substrate (quartz or soda lime glass). The AlN film is deposited by DC magnetron sputtering, and the process condition is optimized so that the x-ray diffraction intensity from the 002 surface of wurtzite AlN becomes the highest. By use of this well-characterized buffer layer, Tc and the resistivity of the NbTiN film with a few nm thickness are remarkably increased and decreased, respectively, in comparison with those without the buffer layer. More importantly, the AlN buffer layer is found to be effective for NbN. With the AlN buffer layer, Tc is increased from 7.3 to 10.5 K for the 8 nm NbN film. The improvement of Tc and the resistivity originates from the good lattice matching between the 002 surface of AlN and the 111 surface of NbTiN or NbN, which results in better crystallization of the NbTiN or NbN film. This is further confirmed by the x-ray diffraction measurement.
[en] One issue with the micro-superconducting quantum interference device (micro-SQUID) magnetometer system is the magnetic flux detected by the SQUID loop cannot be converted to sample magnetization. Herein we present a technique for absolute quantification of magnetization of a microcrystal sample using a micro-SQUID magnetometer. The field dependence of the magnetic flux from a microcrystal of a known molecular ferromagnet, RbMn[Fe(CN)6], was measured. Then, by combining the experimental data and a simple dipole approximation calculation, a highly accurate molar effective magnetic moment was obtained.
[en] Quantitative analysis of the near-surface dislocation content performed using electron backscattered diffraction for samples cut from niobium cavities limited by the 'high field Q-slope' effect show a high dislocation content, as well as a pronounced change in the near-surface dislocation content caused by the 100-120 0C annealing when such annealing has a healing effect on the high field Q-slope. We propose a model for the sharp degradation of the cavity Q at high field for niobium cavities based on dislocation-assisted magnetic flux entry at surface magnetic fields lower than those expected for ideal niobium. It is proposed that the elimination of the high field Q-slope by 100-120 0C annealing is caused by vacancy-assisted dislocation climb in the near-surface region.
[en] We report the synthesis of a polycrystalline bulk (Ba, K)Fe2As2 superconductor by a combined process of melting and deformation. The sample has fewer impurity phases and a higher density compared to samples prepared by a conventional sintering process. Magneto-resistance and magnetization measurements were carried out on the sample in order to evaluate superconducting critical properties. The temperature dependence of the upper critical field, - dHc2(T)/dT, is as large as ∼ 9 T K-1, which corresponds to the value of a single crystal in a magnetic field parallel to the a-b plane. The temperature dependence of irreversibility fields, - dHirr(T)/dT, also has a steep slope at about 10 K below Hc2(T), suggesting that (Ba, K)Fe2As2 superconductors have a good potential for magnetic applications at medium temperatures, like MgB2. However, the magnetization hysteresis loop shows a very small dependence on sample size and the global Jc estimated is as low as ∼ 2 x 103 A cm-2 at 5 K in applied magnetic fields, while the local Jc estimated from the powdered sample is about two orders of magnitude higher. Those results indicate that the current path is still largely limited in the bulk sample despite its high density, and a weak link at the grain boundary is the most plausible origin of the current limitation in the bulk sample.