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We developed a non-destructive evaluation (NDE) system using an HTS SQUID array in order to examine rare-earth (RE)-123 HTS coated conductors striated into multi-filamentary lines. The 5-channel HTS SQUID gradiometer array was composed of ramp-edge junctions with LaErBaCuO and SmBaCuO electrode layers, and fabricated by using an HTS multi layer fabrication technique. The planar gradiometers with 1 × 1 mm 2 pickup loops and a baseline of 1 mm detected the vertical element of magnetic field gradient induced around defects by an eddy current. The gradiometer array cooled by thermal conduction from a liquid nitrogen bath was placed above the coated conductor on the main stage with a lift-off of about 1.5 mm. A coated conductor was fed from a reel to reel, and cooled blow its T c by stages connected to Gifford–Mcmahon (GM) coolers. By employing a 3 kHz induction current generating the maximum field of 0.14 mT, we could identify a distribution of defects in a long-length non-striated conductor. Furthermore, we could detect and distinguish three kinds of defects, existence of a spotty normal-state region, electrical short between striated filaments, and delamination of the superconducting layer from the Hastelloy tape for each filamentary superconducting line at a high speed up to 30 m/h.

We have developed a low-cost, short time manufacturing process using a novel method, starting from a mixture of Er2BaO4, BaCuO2, and CuO, with an excess of Er at expense of Ba. Several micrometers thick films were prepared by the screen-printing technique, spreading the mixture paste on silver substrates, processed then in Ar–1% O2 atmosphere. Using a double-step annealing process, we obtained good quality Er-123 thick films on silver substrates, with Tc (onset) 92 K. Formation of large flat grains was observed by scanning electron microscope. According to XRD analysis, the films were preferentially c-axis oriented. The magnetic (induced), self-field Jc measured at 77 K was above 32 kA/cm2. We observed a strong dependence of the transport current, Ic, on the grain size. The maximum Ic at 77 K as high as 10 A was observed in the sample with the average grain size around 40 μm. The present results indicate that our technology, enabling preparation of large-area superconducting thick films on Ag-based substrates in short processing times, is relatively cheap and appropriate for utilization in long-length RE-123 silver sheath wire production for commercial applications.

The critical current density J c and the apparent pinning potential U 0 ∗ in a single crystal NdBa 2 Cu 3 O y superconductor which shows a broad peak effect are investigated by measuring a DC magnetization and its relaxation. The field-induced pinning mechanism does not explain the temperature dependence of peak field B p and dip field B d . The experimental results of J c and U 0 ∗ are compared with the theoretical analysis based on the flux creep-flow model, taking the distribution of the flux pinning strength into account. The number of flux lines in the flux bundle ( g 2 ), the most probable value of pinning strength ( A m ) and distribution width ( σ 2 ) are determined so that a good fit is obtained between the experimental and theoretical results. The behavior of these parameters is discussed in correspondence to the disorder transition of flux lines.

Torque measurements on single crystals of YBa 2 Cu 4 O 8 show multiple peaks as a function of angle θ ca between the c -axis and the magnetic field H in the c – a plane. We propose that the first peaks at θ ca = (90 − 10)° and (90 + 10)° arise from a long-range spiral spin density wave through the matching pinning between vortices and spins. We construct a contour map for a competing order parameter in YBa 2 Cu 4 O 8 which starts at a finite magnetic field and is located far from the upper critical field line. We extended our torque measurements up to 15 T. The regime of the proposed spin density wave goes to higher temperatures when field increases. The first peak does not appear when the field direction is scanned in the c – b plane. We interpret that the origin of the first peak is due to the Zeeman effect between spiral spins and vortices. The NMR line width of the c -axis aligned YBa 2 Cu 4 O 8 powders in H perpendicular c shows broadening at temperatures below 30 K, and may be caused by the proposed spin density wave. We consider that these features are able to be observed only for the cleanest underdoped cuprate.

We have performed local magnetization measurements on Bi 2 SrCa 2 Cu 2 O 8+ y (Bi2212) single crystals grown by TSFZ method using Bitter decoration technique and MO imaging technique, which provides information of static vortex structures at the surface of superconductors. MO image revealed the presence of micron-scale inhomogeneities of critical currents, which is ascribed to the chemical fluctuation as analyzed by WDS. We could successfully observe nanometer-scale line defects with Bitter decoration technique performed at 65 K. The observed structure was largely disordered, presumably due to thermally activated motion during the decoration.

The temperature dependence of Cu nuclear spin-lattice relaxation rate ( T 1 T ) −1 and frequency spectra have been measured in Y 1− x Ca x Ba 2 Cu 4 O 8 ( x =0, 0.05 and 0.075) and YBa 2− y La y Cu 4 O 8 ( y =0.05) by a zero field Cu nuclear quadruple resonance (NQR) spin-echo technique. The carrier doping dependence of pseudo-spin-gap was observed through ( T 1 T ) −1 at the planar Cu site but no appreciable change in ( T 1 T ) −1 at the chain site Cu. Cation substitution without oxygen deficiency is demonstrated to result in substantial carrier doping on the CuO 2 plane.

We investigated the applicability of Cu as sheath materials for MgB2 wires prepared by in situ PIT (powder-in-tube) method in comparison with stainless steal. Since the critical current density of MgB2 increases with TiH2 doping, we prepared TiH2 doped MgB2 Cu sheath wires 40 m in length and ∅ 1.0 mm or 0.5 × 1.0 mm2 in cross section by rotary swaging, drawing, and two-axial rolling under cold working. We then annealed the samples at 600–850 °C for 1–2 h in Ar gas atmosphere. The critical current of TiH2 (6%) doped MgB2/Cu short sample annealed at 650 °C reached 208 A (Jc=230 kA/cm2) at 4.2 K and self-field. We also fabricated several coils using these wires. The Ic value was ∼100 A at 4.2 K for a coil prepared with use of a 5 m length wire. These results suggest that it is possible to fabricate Cu sheathed MgB2 wires with good performance by using the in situ PIT method.

We revisited the Cu nuclear spin–lattice relaxation of 63 Cu-enriched high- T c superconductors La 2− x Sr x CuO 4 with x =0.13 (an underdoped sample) and 0.18 (an overdoped sample). From application of the impurity-induced NMR relaxation theory, we observed a remnant of the pseudo-spin-gap effect on the host Cu nuclear spin–lattice relaxation rate 63 (1/T 1 T) host and the inhomogeneous slow fluctuation effect via the impurity-induced relaxation rate 63 (1/τ 1 ) . The magnetic phase diagram with the pseudo-spin-gap temperatures T s is presented.

We have measured the magnetic torque of YBa 2 Cu 4 O 8 in the a -axis and b -axis rotation for revealing the intrinsic physics in the underdoped cuprates. So far, we have shown that the first peak below 40 K in b -axis rotation appears at θ ca ∼90°±10°, where θ ca is the angle between the c -axis and the field direction towards the a -axis. We consider that the origin of first peak is the spiral spin density wave along a -axis with a long period. We have also measured the magnetic torque of YBa 2 Cu 4 O 8 around the [1 1 0] axis. The first peaks are again confirmed to appear in the torque curve. The NMR spectra are systematically investigated by using the field-aligned powder sample. The NMR line width shows an appreciable broadening at temperatures below 30 K when H ⊥ c axis. We consider that this might be subsequent evidence for the occurrence of the spin density wave in YBa 2 Cu 4 O 8 .

The polarized Raman spectra of NdBa 2 Cu 3 O 7− δ single crystals heated at ∼900 °C under various oxygen partial pressures ( P O 2 ) have been investigated. It was found that the linewidth of Ba mode sharpens and the relative intensity of this mode is enhanced with increasing P O 2 . These observations show that such a high-temperature heat-treatment changes the distribution of Nd-rich 123 clusters.

We have measured the transport properties in Ca and Zn substituted NdBa 2 Cu 3 O 7− δ (Nd123) single crystals grown by the travelling solvent floating zone (TSFZ) method. The normal state resistivity decreased with Ca doping and the electronic state changed into an over-doped state, which was revealed by the fact that the temperature dependence of resistivity turned into super-linear from the T -linear behavior. In contrast, a normal state resistivity increased with Zn doping presumably due to the fact that Zn acted as a scattering center. It was found that a combined addition of Ca and Zn was effective in increasing the irreversibility field and the critical current density simultaneously through the optimization of carrier concentration and pinning performance.

Ti, Zr and Hf doped MgB2 polycrystalline samples have been prepared by using a hot pressing technique. In Ti doped MgB2, it was found that the optimum concentration of Ti for the critical current density Jc was 5% and Jc value was improved from 1.9 × 105 A/cm2 (Ti 0%) to 5.6 × 105 A/cm2 (Ti 5%) in self-field at 10 K. Jc values were also improved to 5.0 × 105 A/cm2 with 2% Zr doping and to 4.8 × 105 A/cm2 with 1% Hf doping. We found that the improvement of the irreversibility field Birr with Zr and Hf doping was larger than that of Ti doping.

We have fabricated the MgB 2 superconducting wire with a copper sheath by using an in situ synthesis powder in tube (PIT) method. The diameter of MgB 2 wire was reduced down to 0.5 mm. MgB 2 coil with 27 mm∅ inner diameter and 3.5 m length was made using this wire. In the first current raising process, the critical current ( I c ) of the coil was 76 A at 4.2 K in self-field which corresponded to J c ≈170,000 A/cm 2 and J e ≈40,000 A/cm 2 was achieved in magnetic field of H ≈0.14 T. In the second run, the coil did not quench at the maximum current of 200 A, which corresponded to J c ≈440,000 A/cm 2 , J e ≈100,000 A/cm 2 and H ≈0.37 T.

We studied the main and the satellite signals of plane-site Cu(2) nuclear quadrupole resonance (NQR) spectra in optimally doped YBa 2 (Cu 1− x Zn x ) 3 O 7− δ . The Cu nuclear spin–lattice relaxation time is found to distribute in the broadened Cu(2) NQR spectra. This is in contrast to nearly frequency independent relaxation in slightly oxygen-deficient YBa 2 Cu 3 O y . The Cu nuclear spin–lattice relaxation time of the Zn-induced satellite signal is shorter than that of the main signal, suggesting a locally enhanced magnetic correlation by Zn. From the analysis of Zn-induced Cu NQR relaxation, the origin of pair-breaking mechanism due to Zn impurity is discussed.

The degradation of the superconducting properties, such as the diamagnetic magnetization ( M dia ), the critical current density ( J c ), and the irreversibility field ( H irr ) for MgB 2 exposed to water has been investigated. The time evolution of the degradation obeys an exponential law with a different decay-time constant for each of these properties. M dia shows the most rapid decay with exposure time, whereas H irr exhibits the slowest degradation. By doping with Ti, the degradation of these superconducting properties was significantly suppressed, although the decay time still obeyed an exponential law. The experimental results have been reasonably explained in terms of a grain-boundary-degradation model. Our study suggests that the degradation of M dia is related to the degradation of the relatively-high-angle grain boundaries near the sample’s surface, whereas the degradation of J c is associated with the degradation of the relatively lower-angle grain boundaries. The degradation of H irr is probably due to the reduction of the surface pinning force.

Ti-doped MgB2 superconductors with different doping levels were prepared by solid-state reaction at ambient pressure. Jc of the samples changes with the doping level, with the best result achieved at x=0.1. At 5 K, the Jc reaches 2×106 A/cm2 in the self-field. In addition, degradation in water of Jc and irreversibility field (Hirr) of MgB2 bulks is significantly reduced by Ti doping. Microstructural analysis reveals that Ti mainly forms a thin TiB2 layer in the grain boundaries of MgB2. At the same time MgB2 grains are greatly refined, forming a strongly coupled nanoparticle structure. Our results show that the unique microstructure of the MgB2 nanoparticles with TiB2 nanograin boundaries may take the responsibility for the enhancement of the flux pinning and the chemical stability against water.

The polarized Raman spectra of Zn or Ca doped NdBa2Cu3O7−δ single crystals have been investigated. It was found that the linewidth of Ba mode in Zn doped Nd 1:2:3 is broader than those in pure Nd 1:2:3. This result suggests that the Nd substitution on Ba sites occurred in Zn doped Nd 1:2:3. In x(zz) x geometry the relative intensity of the O(4) mode in Zn doped Nd 1:2:3 is smaller than those in pure or Ca doped Nd 1:2:3. The difference between Zn doped and Ca doped Nd 1:2:3 in the relative intensity of the O(4) mode may be produced by the disordering in the CuO2 plane.

The doping effect of Ti and Zr on the superconducting properties and the microstructure of sintered MgB 2 bulk materials has been studied. The transition metal is found to work as a sintering assistant, which significantly enhances the grain connection of the MgB 2 bulk materials during sintering processing. With this technique, we have fabricated Ti-doped MgB 2 bulk materials consisting of fine 10 nm scale particles and achieved a high J c over 1 MA/cm 2 in 0 T at 20 K. The large enhancement of J c is explained by the excellent connection between grains and a high density of pinning centers served by grain boundaries and MgO nanoparticles.

The magnetic properties in NdBCO single crystal fabricated by flux growing method in controlled oxygen atmosphere have been investigated using both SQUID magnetometer and magneto-optical (M-O) technique before and after heavy ions irradiation. An external magnetic field was applied parallel to the sample c-axis up to 7 T at different temperatures. Critical current densities were obtained using the extended Bean model from the hysteresis loops as measured by SQUID. The M-O images were collected using a special facility equipped with a superconductor magnet able to reach 8 T. The indicator film was produced in ISTEC. The M-O images were associated to the data obtained by analysing the magnetization curves. It was found that the irradiation strongly affects the magnetic behaviour. The direct and detailed visualization of the field behaviour provided by magneto optical images allows a more precise understanding of the magnetization loops.

We report on the observation of the first-order melting transition of the flux line lattice in a twinned NdBa 2 Cu 3 O y single crystal by transport measurements. The magnetization J c does not exhibit peak effect in crystals grown up to 0.05% oxygen partial pressure. The observed melting transition is a strong function of O 2 partial pressure during growth. The first-order melting transition becomes continuous and peak in critical current density first emerges with oxygen partial pressure≥0.07%. Our results demonstrate the existence of a first-order melting transition in twinned RE-123 system containing rare-earth magnetic ions. We have shown that twin planes do not suppress the melting transition in the absence of defects due to Nd–Ba substitution.

We report zero field chain-site 63 Cu NQR studies of the low-lying magnetic excitations of CuO zigzag double chains in low- T c and non-superconducting YBa 2 (Cu 1− x Zn x ) 4 O 8 , YBa 2 (Cu 1− x Ni x ) 4 O 8 and Y 1− x Pr x Ba 2 Cu 4 O 8 . For the non-superconducting YBa 2 (Cu 1− x Zn x ) 4 O 8 with x = 0.03, we observed a non-vanishing behavior of a 63 Cu nuclear spin-lattice relaxation rate divided by temperature 1/ T 1 T at low temperatures, that is, the absence of an energy gap in a magnetic excitation spectrum. The gapless magnetic excitation spectrum of the zigzag chains is characteristic of a quasi-one-dimensional Tomonaga-Luttinger liquid. The Zn-, Ni- and Pr-induced stretched exponential 63 Cu nuclear spin-lattice relaxation times suggest that the Zn-induced edge state has different magnetic correlation from Ni and Pr impurities.

The temperature dependences of the 63 Cu nuclear spin-lattice relaxation rate T 1 -1 and the Knight shift K ab at the planar Cu(2) site in Ca substituted YBa 2 Cu 4 O 8 have been measured by the nuclear quadrupole resonance (NQR) and the nuclear magnetic resonance (NMR) techniques. The notation “ ab ” means that the external field is perpendicular to the c -axis. The temperature dependence of K ab in the Ca substituted sample becomes weaker compared with the non-substituted sample, indicating that the electronic state in the CuO 2 plane turned more metallic with Ca substitution. On the other hand, ( T 1 T ) -1 in the non-substituted sample increased rapidly with increasing temperature and showed a maximum value at 160 K, the so-called spin-gap temperature, T S . Subsequently, ( T 1 T ) -1 decreased with increasing temperature above the spin-gap temperature as in the Curie-Weiss law. While the behaviors of ( T 1 T ) -1 in Ca substituted samples were similar to that in the non-substituted YBa 2 Cu 4 O 8 , the spin-gap temperature decreased with increasing Ca (hole carrier) content. From the decrease of T S with the carrier concentration, we conclude to the existence of a negative linear relation between the spin-gap temperature and the superconducting transition temperature.

A nuclear magnetic resonance (NMR) study has been carried out to investigate the magnetic properties of polycrystalline samples of superconducting Nd2CuO3.8F0.2. It is found that not only the shift but also the width of the 19F-NMR line decreases as temperature increases from 4.2 to 200 K. Since there exists no indication of peak splitting, the F ions were considered to be substituted into a certain oxygen site. Furthermore, the Cu-NMR spectra are quite similar to those for Nd2−y Ce y CuO4. In addition, the zero-field frequency spectra at 1.3 K indicates that the Nd2CuO4 phase is absent but an unknown metallic- antiferromagnetic-like phase exists, as was reported for Nd1.85Ce0.15CuO4.

The temperature dependences of the 63Cu nuclear spin-lattice relaxation rate 1/T1 and Knight shift K in YBa2Cu4O8 have been measured by nuclear quadrupole resonance (NQR) and nuclear magnetic resonance (NMR) techniques. In the normal state, both the 1 T 1 for the Cu(2) sites and the in-plane component of the Knight shift K2⊥ increase rapidly with temperature from Tc up to about 250 K. Approximate T-linear temperature dependence of 1 T 1 was observed in a wide temperature range between 250 and 680 K, that is in general accord with previous analyses.

We report a Cu NQR study of Ni impurity spin fluctuations in high- T c cuprate superconductors, YBa 2 (Cu 1− x Ni x ) 4 O 8 and YBa 2 (Cu 1− x Ni x ) 3 O 6.92 , from measurement of Ni-induced Cu nuclear spin–lattice relaxation times. We found that the temperature dependence of the Ni spin correlation time is different from that of Kondo impurity in a conventional metal, e.g. Mn or Fe in Cu.

The temperature dependences of the Cu nuclear spin-lattice relaxation rate (1/ T 1 ) and the Knight shift at the planar Cu site in the as-sintered HgBa 2 CuO 4+δ with T c ≈ 84K have been measured by the nuclear quadrupole resonance(NQR) and the nuclear magnetic resonance(NMR) techniques. The as-sintered sample seems to be a lightly doped material because of the increase of T c by annealing in the oxygen atmosphere. From the broadness of the NQR spectrum and the distribution of the (1/ T 1 ), we conclude that the as-sintered sample has an inhomogeneity of the atomic location, especially oxygen. The temperature dependence of (1/ T 1 ) behaves as T n ( n ≈ 3−4) and there is no Hebel-Slichter peak below T c , which is re-confirmed the common feature in high- T c cuprate superconductor.