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The ceramic YBa2Cu4O8 superconductor composed of submicron grains is considered a random Josephson-coupled network containing the so-called π junctions and shows successive phase transitions. With decreasing temperature, first the intragrain superconductive transition occurs inside each grain at T c1 and then the chiral-glass transition occurs among the grains at T c2 (< T c1). The third transition at T c3 (< T c2) is the intergrain superconducting transition. We measured the nonlinear susceptibility and resistivity of the ceramic YBa2Cu4O8 superconductor to determine the field dependences of the transition temperatures T c2 and T c3. The phase diagram of the intergrain ordering is discussed in light of the result predicted by Kawamura., 29th International Symposium on Superconductivity (ISS2016), 13–15 December 2016, Tokyo, Japan

The magnetic memory phenomena in a ceramic YBCO superconductor are studied by zero-field-cooled and thermoremanent magnetization after specific cooling protocols. The ceramic YBCO is considered as random Josephson-coupled networks of 0 and π junctions and shows successive phase transitions. The magnetic glass behavior, similar to those of spin-glasses, is observed below the lower transition temperature. The memory effect occurs strongly in an intermediate stop temperature and suppressed with an increased probing field. The results can be understood by the screening effect of the ceramic superconductor with a finite self- inductance.

Superconductivity caused by oxygen reduction treatment for sintered Pr2Ba4Cu7O15–δ (Pr247) synthesized using citrate pyrolysis precursor method is examined experimentally. The sintered sample tends to be heterogeneous structure including Pr124 phase and stacking fault structure rich in CuO single chains. In this work, such a heterogeneous sample and nearly pure phase one were prepared, and the electric conductive behaviors were examined comparatively for various reduction grade sample series. In enough reduced stages, superconductivity with onset temperature 25–28 K was observed for both heterogeneous and purer samples. At transition process by the reduction, superconductivity of the heterogeneous samples can arise from lower oxygen-reduction% than purer Pr247 material. The results may possibly suggest superconductivity by CuO double chains besides Pr247 crystal in our heterogeneous ceramic system.

Sintered material of Pr2Ba4Cu7O15−δ (Pr247) was synthesized at ambient pressure condition by using citrate pyrolysis precursor method, and the superconductivity caused by oxygen reduction treatment has been confirmingly observed and examined. By resistivity and magnetization experiments, it has been found that the critical temperature Tc of the reduced samples could reach about 10 K higher than previously reported data for pure Pr247. Besides, from TEM observations, the present material was found to be heterogeneous system containing Pr247, Pr124 and novel stacking structure phases rich in CuO single chains. These results have suggested that oxygen–reduction at such region rich in the CuO single chains may affect the superconductivity of the adjacent crystal region containing CuO double chains, and might enhance the superconductive critical temperature.

Ceramic high-T c superconductors may be viewed as a weakly coupled random Josephson network containing π-junctions, and a chiral-glass transition is predicted to be occurred. Grain-size and pressure dependences of the chiral-glass transition have been investigated on YBa 2 Cu 3 O 7-y ceramics. With decreasing temperature, a superconducting order occurs at first inside each grain at T c1 and furthermore among the grains at T c2 (< T c1 ). The increase in T c2 with increase of synthesis temperature is considered as the change of strength of inter-grain Josephson coupling accompanying the growth of grain size. The T c2 also increases with increase of pressure, however, the sharpness of the transition at T c2 does not change. In spite of difference in T c2 's, the increasing rates of t c2 by pressure are nearly the same.

We investigated the effects of oxidation and reduction on the successive superconducting transition in a ceramic material composed of YBa2Cu4O8 (Y124) submicron grains by linear and non-linear resistivity measurements. This material shows two-step superconducting transition and the lower one is identified as an inter-grain ordering. A sintering process fuses and joins the adjacent grains, so the lower transition was suppressed with increasing sintered time. Although Y124 is a stoichiometric ceramic, a reduction treatment suppressed the temperature of zero resistivity and broadened the non-linear resistivity peak. This suggests an oxygen vacancy was induced on the grain surface.

A superconductive sign found in electric resistivity behavior of mixed ceramic Pr124/Pr123 is searched and discussed, to advance preceding discussion about the superconductive region induced by intergrain charge transfer effect in ceramic Pr247 containing the related impurity phases. Observed reduction effects for largely impure ceramic of Pr247 with Pr124 and Pr123 and for simply dual-phase sample of Pr124/Pr123 inform us that the oxygen-reduction brings not only superconductive electron doping but also normal resistivity enhancement. Milder heat treatment condition is expected to assist possible attainment of zero-resistivity for Pr124/Pr123.

We have succeeded in observing the de Haas–van Alphen effect in LaOs 4 P 12 which is a superconductor with the transition temperature T C =2.0 K and a reference compound for PrOs 4 P 12 . We have found the multiply-connected and almost spherical Fermi surfaces whose cyclotron effective masses are ranging 1.1–4.7 m 0 . The Fermi surface topology is close to that of PrOs 4 P 12 , indicating the localized nature of 4 f -electrons in PrOs 4 P 12 . From the comparison with LaOs 4 P 12 , the large mass enhancement in PrOs 4 P 12 has been clarified.

Superconducting characteristics of borocarbide intermetallic compounds have been investigated by the measurement of nuclear-spin lattice relaxation rate 1/T1 of 13C on YNi2B2C. The present NMR study has revealed that borocarbide compounds are characterized as a gap function with point nodes described in terms of s+g-wave superconductor.

The ceramic YBa 2 Cu 3 O 7– δ samples composed of sub-micron size grains are considered as random Josephson-coupled networks of 0 and π junctions and show successive phase transition. With decreasing temperature, a transition occurs at first inside each grain at T c1 (just the same with the bulk superconducting transition temperature T c ) and successively another transition occurs among the grains at T c2 ( T c1 ). The various ceramic samples were prepared with not only changing the calcination and the sintering temperature but also changing the pressure in pelleterizing process between calcination and sintering, using sub-micron size powder prepared by the coprecipitation method. With increasing sintering temperature (equal to calcination temperature), T c2 increases and both the absolute values of linear resistivity and the divergence of nonlinear resistivity (just above T c2 ) decrease. With increasing the density of sample by increasing the pressure in the pelleterizing process, the T c2 and the absolute value of conductivity increase monotonously. These effects may be considered as the changes in Josephson-couple strength between superconducting grains in the ceramic specimens.

We report the results of 31P NMR measurements on YFe4P12, a new member of the family of the filled skutterudite compounds, which displays superconductivity below T c = 7 K . In the normal state, the nuclear spin-lattice relaxation time T 1 obeys the Korringa relation T 1 T = constant below 40 K, but it changes its behavior at higher temperatures. The Korringa constant ( T 1 T ) - 1 for YFe4P12 is smaller than that for LaFe4P12, suggesting that the density of states at Fermi energy N ( E F ) for YFe4P12 is lower than the case of LaFe4P12. In the superconducting state, T 1 - 1 ( T ) shows a small coherence peak just below T c , followed by an exponential decrease below 4 K revealing on energy gap 2 Δ ( 0 ) / k B T c = 3.3 . Our results indicate that YFe4P12 is a conventional type s-wave superconductor.

The Pr content x dependence of the superconducting transition temperature Tc is examined for various oxygen concentrations by quenching the samples of the Pr-substituted Ln1−xPrxBa2Cu3O7−y, where Ln is Y, Nd and Gd. In the case of Ln = Y, the initial rate of decrease of Tc by Pr substitution (−dTcdx) decreases with decreasing oxygen concentration for 6.6 < 7 − y < 7.0, then becomes constant for 7 − y < 6.6. In contrast, the effect of Tc-suppression by Pr in the case of Ln = Nd does not depend upon the oxygen concentration, and is much stronger than that in the case of Ln = Y. In the case of Ln = Gd, intermediate behavior between those of Y and Nd is observed, corresponding to the ionic size of Ln.

The superconductive ordering of a YBa 2 Cu 4 O 8 ceramic with a single phase and of homogeneous sub-micron size grains, is studied by systematic DC magnetic measurements under weak magnetic fields. Successive phase transitions of a hierarchical nature are identified to occur at two temperatures T C1 (≃ 80 K) and T C2 (≃ 37 K). The transition at T C1 is that from the normal into the intra-grain superconductive state. The transition at T C2 is that into the inter-grain superconductive state and results in an infinite superconductive network over the whole sample. Taking a possible Josephson junction type coupling into account, the second transition at T C2 is speculated to be a long range ordering among the superconductive grains and similar to that for a three-dimensional XY ferromagnet.

The NQR spectra and the nuclear spin-lattice relaxation rate 1/T1 associated with the Cu(2) sites in Y1−xPrxBa2Cu3O7 with x = 0.1 and 0.2 have been investigated. The spectrum of Y0.8Pr0.2Ba2Cu3O7 at 4.2 K is found to be broadened in the high-frequency side, and the half-width becomes two times of that for pure YBa2Cu3O7. It is also noted that several distinct lines contribute to the spectrum with increasing Pr concentration. These spectral features are quite different from the case of oxygen-depleted YBa2Cu3O7−δ, in spite of the similar behavior in the decrease of Tc. The temperature dependence of 1/T1T for x=0.1 shows a maximum around 106 K. This temperature is much higher than Tc = 86 K, indicating that the Pr doping induces the pseudo-spin-gap behavior. From the Curie-Weiss fit of 1/T1T(T), it is concluded that the Pr doping is considered to enhance the 2D AF spin fluctuations from the decrease of the Weiss temperature.

We report on the results of the 195Pt NMR measurements for M Pt 4 Ge 12 ( M = Sr,Ba ) to clarify the microscopic electronic states at low temperatures. In SrPt 4 Ge 12 and BaPt 4 Ge 12 , the isotropic component of the Knight shift, that is, the contribution from the Pt 5 s states is smaller than that in LaPt 4 Ge 12 . In the superconducting state, 1 / T 1 shows a small coherence peak just below T C , followed by an exponential decrease at low temperatures revealing on energy gap Δ / k B T C = 1.6 , which suggests a conventional BCS superconductor.

Ceramic high- T C superconductors may be viewed as a weakly coupled random Josephson network containing the so-called π-junctions, and a chiral-glass transition is predicted to be occurred in such d-wave ceramics. In the present work, pressure dependence of the chiral-glass transition have been investigated on YBa 2 Cu 3 O 7− y and YBa 2 Cu 4 O 8 ceramics. With decreasing temperature, a superconducting order occurs at first inside each grain at T C1 and furthermore among the grains at T C2 ( T C1 ). The T C2 increases with increase of pressure, while the sharpness of the transition at T C2 does not change. In spite of differences in not only T C2 ’s but also T C1 ’s, the increasing rates of T C2 by pressure are nearly the same.

Dynamical properties of the ceramic YBa 2 Cu 3 O 7− δ have been investigated by magnetic measurements. We measured ac-susceptibilities at various frequency under zero external field and carried out dynamic scaling analysis near inter-grain transition temperature. The results exhibit scaling behavior for the critical exponents zν =8 and β =0.5. These values are fairly close to the ones for the chiral-glass estimated by Monte Carlo simulations.

Intergrain ordering nature of weak-sintered ceramic of YBa2Cu4O8 (Y124) is studied experimentally by ultra precise electric resistivity observation together with magnetic measurements for linear and nonlinear susceptibilities. For this purpose, a new method to detect pure linear resistivity down to the order of 1 nΩm has been developed with the help of combination technique of pulse delta method and FFT treatment. Making use of this experimental system for several weak-sintered Y124 samples, it has been revealed that linear resistivity first drops toward zero steeply with decreasing temperature, and then turns abruptly upward to forms faint maximum at lower temperature. Such peaking of electric resistivity has been found to occur below the intergrain ordering temperature that was estimated by the magnetic behavior. This newfound phenomenon, to say 'revival of resistivity', is being regarded as general nature for the weak-sintered ceramics, and considered to be an intrinsic frustration effect for actual d-wave type ceramics of irregular network structure.

Superconducting behaviours in reduction-treated mixtures of fine Pr124 and Pr123 ceramics synthesized using citrate pyrolysis precursor method are examined by electrical resistivity and magnetic measurements. With increasing the ratio of Pr123, the temperature dependence of ρ(T) changes to be semiconductor-like behaviour hugely, and there exist no broad maximum for as-sintered Pr124/Pr123 (1:1.5) and (1:2) samples contrary with the case of (1:1). By reduction treatment, the ρ(T) curve shows the maximum according to the increase of carrier concentration in the CuO double chain in Pr 124 grains. By subtracting the Pr3+ paramagnetic contribution from the temperature dependence of magnetization, the remnant magnetization shows weak diamagnetism below ~17 K, indicating small Meissner effect. The diamagnetic susceptibility is independent of magnetic field in the measured field range below 20 Oe.

The d-wave ceramic YBa2Cu4O8 superconductor composed of sub-micron size grains is considered as random Josephson-coupled network of 0 and π junctions and shows successive phase transitions. The upper transition occurs inside each grain at Tc1 = 82 K and the lower transition occurs among the grains at Tc2 = 66 K. We measured the temperature dependence of the current-voltage characteristics of the ceramic YBa2Cu4O8 and derived the linear and nonlinear resistivity. The nonlinear resistivity ρ2 and ρ4 have finite values between Tc1 and Tc2 and have the peak at the same temperature Tp = 70 K above Tc2. The result agrees with the theoretical one obtained by Li and Dominguez. They interpreted Tp as the crossover temperature from the normal state phase to a chiral paramagnetic one.

We report on the results of the 73 Ge-NQR measurements for M Pt 4 Ge 12 ( M = Sr, Ba) to clarify the microscopic electronic states and the relationship between superconductivity and rattling. In the normal state, 1/ T 1 is proportional to temperature, showing a Korringa relation 1/ T 1 T = 0.020 and 0.016 (sK) -1 for SrPt 4 Ge 12 and BaPt 4 Ge 12 , respectively. Also, in the superconducting state, 1/ T 1 shows a distinct coherence peak just below T C , which suggests a conventional BCS superconductor.

Thermoelectric power are measured on high-T c oxide superconductors such as quenched YBa2Cu3O7−y (Y-(123)), Bi2Sr2CaCu2O8+y (Bi-(2212)) and Bi2Sr2Ca2Cu3 O1 0+y (Bi-(2223)), and Pr- or Ca-doped YBa2Cu 4 O8 (Y-(124)). There exists a universal relationship between T c and Seebeck coefficient (S) on the above superconductors. The T c shows its maximum at S∼ 0. The S is considered to be related with the carrier concentration which has influence upon the superconductivity. In the quenched Y-(123) and Bi-(2223), however, carrier concentration in the different type Cu-O structures (chain Cu-O and plane Cu-O without apical oxygen, respectively) is also considered to change in the higher S region.

The temperature dependence of 13C spin-lattice relaxation time T1 in YNi2−xPtxB2C (x=0 and 0.8) has been measured. With decreasing temperature the relaxation rate 1/T1 forms a Hebel–Slichter peak just below Tc and after that it decreases exponentially with the gap parameter 2Δ 0 =4.8 k B T c . Below about 4 K it starts to vary linearly with T, where the values of 1/T1T were proportional to H0, the observing magnetic field. This means that the spin diffusion to normal cores is important for the relaxation. These results support the model of the phonon-mediated superconductivity in this system.

Ceramic high-Tc superconductor has intra-grain superconducting transition at TC1 and the inter-grain one at TC2. We investigated the effects of pressure on the ceramic superconductor YBa2Cu3O7-y in the pressure region up to 5.3 GPa, through the AC susceptibility measurements using a miniature diamond anvil cell (mDAC). The TC1 increases against the initial pressure, whereas it turns to decrease at around 2.5 GPa. The TC2 also exhibits similar pressure dependence. The up-and-down change of TC1 has been already observed in the previous single crystal experiment, whereas that of Tc2 was observed through the present study for the first time. From the viewpoint of measurement technique, we also point out some problems on the high pressure AC susceptibility measurements using a SQUID magnetometer.

Superconductive characteristics of Pr2Ba4Cu7O15-δ (Pr247) ceramics with crystalline phase inhomogeneity for the stacking structures is examined experimentally, using reference observations for multi-phased ceramic sample consists simply of PrBa2Cu4O8 (Pr124) and PrBa2Cu3O7-δ (Pr123). After reduction treatment by vacuum-heating, the reference multiphased sample shows onset of abrupt electric resistivity dropping and also weak Meissner magnetization below ~20 K. The results suggest that superconductivity at CuO double chains in Pr124 grains is caused by charge transfer from neighbouring Pr123 grains. Such a charge transfer effect is thought to occur also in Pr247 sample including phase inhomogeneity.

We have synthesized the polycrystalline samples of Ge-based filled skutterudites R Pt 4 Ge 12 ( R =La, Ce, Pr, Nd) and investigated the lattice constant, electrical resistivity, magnetic susceptibility and nuclear magnetic resonance (NMR) systematically. LaPt 4 Ge 12 is confirmed to be a conventional BCS-type superconductor by the NMR measurements. Additional contribution ascribed to the rattling motion of the La ions has been observed in the nuclear relaxation at the La site at high temperatures. Temperature dependence of the magnetic susceptibility of CePt 4 Ge 12 shows a broad peak around 80 K, which is a typical feature of valence fluctuation systems, whereas PrPt 4 Ge 12 and NdPt 4 Ge 12 show the well-localized nature of 4 f -electrons. The NMR measurements in CePt 4 Ge 12 revealed that the nuclear spin–lattice relaxation rate 1/ T 1 shows an activation-type temperature dependence 1/ T 1 ∝exp (-Δ/ k B T ) with an energy gap Δ/ k B =150 K above 100 K, whereas it is proportional to the temperature belo...

13 C and 11 B NMR studies have been made on the quaternary superconductor Y(Ni1−xPtx)2B2C with x=0 and 0.4. Temperature dependence of Knight shift and spin-lattice relaxation rate (SLRR) in the normal state was explained consistently among all the observed nuclei by Korringa mechanism without taking account of antiferromagnetic spin fluctuation. Bellow the superconducting transition temperature we observed a Hebel–Slichter peak in case of 13 C SLRR. These results mean that the borocarbide is a normal metal and a phonon-mediated superconductor

13 C NMR study has been made of the quaternary superconductor YNi 2 B 2 C. Temperature dependence of T 1 and K in the normal state showed normal metal behavior. Temperature dependence of T 1 in the superconducting state strongly suggests that the quaternary borocarbide is a conventional BCS superconductor.

The Pr contentx dependence of the superconducting transition temperatureT c is examined for various oxygen concentrations in Ln1−x Pr x Ba2Cu3O7−y , where Ln is Nd, Gd, Ho, Y and Yb. In the case of Ln≠Nd, ionic radii of which are smaller than that of Pr, the initial rate of decrease ofT c by Pr substitution (-dT c /dx) decreases with decreasing oxygen concentration for 6.6

Pulsed NMR study of13C has been carried out in the quaternary borocarbide superconductor YNi2B2C. The13C nuclear spin-lattice relaxation rate, 1/T 1, in an unoriented powder sample has been measured between 20 and 300K in the normal states. TheT 1 T=constant behavior is found between 20 and 100K which suggests the existence of density of states at Fermi level at carbon site. Furthermore, an anormalous decrease in 1/T 1 T with increasing temperature similar to11B NMR is found above 100K.

Successive superconductive transitions in single phase YBa 2 Cu 4 O 8 ceramics composed of homogeneous grains are examined by AC susceptibility at zero DC magnetic field. In a series of the ceramics with different sintering temperatures intrinsic critical behaviors of X ′ and X ″ are derived from the limits of X ∗ − T behavior when AC field amplitude tends to zero. Compared with the previous DC magnetization measurements, the inter-grain coupling intensity is concluded to change the location of the lower critical point but does not the essential ordering characteristic at all.

The oxygen concentration dependence of T c is examined in Y 1−x Pr x Ba 2 Cu 3 O 7− y . The T c decreases with decreasing the oxygen concentration 7− y in not only Ortho I (6.75≦7− y ≦7.0) but also the region between Ortho II and Tetra (7− y ≦6.6) for the Pr contents of 0≦ x T c increases conversely with decrease of 7− y for 0.3≦ x y ≦6.75). The Pr content dependence of T c for Ortho II is quite different from that for Ortho I, and is preferably similar to that in the stoichiometric oxide superconductor Y 1−x Pr x Ba 2 Cu 4 O 8 .

Superconducting properties are examined in Y 1−x Pr x Ba 2 Cu 4 O 8 synthesized by the citrate pyrolysis method followed by sintering in a one atm oxygen flow. Lattice constants increase linearly, while the orthorhombicity decreases linearly with increasing x. Meissner effect measurements indicate that the onset T c decrease monotonically with the Pr substitution, and disappears at x =0.75. This is caused by the change in the hole concentration with the substitution of Pr for Y.

The citrate pyrolysis method is used to investigate the synthesis process and the superconducting transitions of the YBa 2 Cu 4 O 8 (124) phase. DC susceptibility measurements show that the intrinsic Tc, both for the one bar sintering route and the 90 bar treatment, is close to 82 K. In the former case, however, the reaction temperature must not exceed 800°C, and χ ec and resistive transitions are severely degraded due to the extremely small grain size.

Structure and electrical property are studied in the quenched Bi 2 Sr 2 Ca 1-x Nd x Cu 2 O 8 + δ system. The quenched samples with 0≦x≦1 have a single phase of the Bi 2 Sr 2 CaCu 2 O 8 + δ structure, as do the furnace-cooled samples. The oxygen concentration decreases slightly by about 0.05 in the chemical formula upon quenching from 800°C into liquid nitrogen. The T c remains a constant (∼90K) for x≦0.15, and decreases linearly with x in the range of 0.15≦x≦0.45, and superconductivity disappears for x≧0.45. The result for the quenched systems, if x is shifted by +0.1 in it, agrees very well with that for the furnace-cooled Bi 2 Sr 2 Ca 1-x Nd x Cu 2 O 8 + δ and Bi 2 Sr 2-x Nd x CaCu 2 O 8 + δ systems. The relationship between the T c and the hole concentration in [Cu-O] is confirmed to be universal in both the quenched and the furnace-cooled Bi 2 Sr 2 CaCu 2 O 8 + δ systems.

Samples of the Bi2-x Pb x Sr2CaCu2O8+δ system are prepared to be almost a single phase of the Bi2Sr2CaCu2O8+δ structure for a region of 0≦x≦0.15. Substitution of Pb for Bi ions is observed by EPMA at grain level. The T c decreases linearly with x in this region. Comparison with the results for the Bi2Sr2Ca1-x Nd x Cu2O8+δ shows that the hole concentration in [Cu-O] plays a most significant role in determining the T c in the Bi2Sr2CaCu2O8+δ system. The universal relationship between the hole concentration in [Cu-O] and T c is determined in a wide range of concentration in this system.

Structure, resistivity and susceptibility are studied in the quenched Bi2Sr2Ca1-x Nd x Cu2O8+δ system. The quenched samples with 0≦x≦1 have a single phase of the Bi2Sr2CaCu2O8+δ structure, as do the furnace-cooled samples. The oxygen concentration decreases slightly by about 0.05 upon quenching from 800°C to liquid nitrogen temperature. The T c remains a constant (∼90 K) for x≦0.15, and decreases linearly with x for 0.15≦x≦0.45, and superconductivity disappears in the samples with x≧0.45. Referring to the previous results for the furnace-cooled Bi2Sr2Ca1-x Nd x Cu2O8+δ system, the relationship between the T c and the hole concentration in [Cu-O] is confirmed to be universal in both the quenched and the furnace-cooled Bi2Sr2Ca1-x Nd x Cu2O8+δ systems.

The superconductivity and structural transformation from cubic to tetragonal phase have been investigated in near-equiatomic M-Ru (M = V, Nb and Ta) alloys through the measurements of X-ray diffraction, electrical resistivity and magnetic susceptibility. The transformation is possibly a second-order phase transition. The electronic specific heat coefficient estimated from the temperature dependence of upper critical field for a transformed sample is smaller than that for a non-transformed sample. The superconducting transition temperature and magnetic susceptibility at 77 K for V-Ru show a peak at the boundary composition between the cubic and tetragonal phases, while the values for Nb-Ru and Ta-Ru vary monotonically without a peak at the boundary composition. Using the McMillan's formula, it is found that the electronic density of states for V-Ru is about 2–4 times larger than the values for Nb-Ru and Ta-Ru, while the electron-phonon interaction is smaller.

Structure, resistivity and susceptibility have been studied in the Bi2Sr2Ca1-x RE x Cu2O8+δ (RE=Nd and Pr) system. The samples with 0≦x≦1 have a single phase of the Bi2Sr2CaCu2O8+δ structure. The oxygen concentration, 8+δ, is estimated as about 8.3 which is independent of the RE composition x. The T c increases slightly with x in the range of x≦0.10 and remains constant (∼85 K) for 0.10≦x≦0.25, then decreases linearly with x for 0.25≦x≦0.55. Superconductivity disappears for the samples with x≧0.55 in both cases. The Nd and Pr ions in the samples were both determined to be trivalent from the susceptibility data. The results show that the hole concentration in [Cu–O] plays a most significant role in the occurrence of superconductivity in the Bi–Sr–Ca–Cu–O system.

Structure, resistivity and susceptibility have been studied in the Ba2Nd1-x Pr x Cu3O7-δ system. The samples with 0≤x≤1 have a single phase of the orthorhombic Ba2YCu3O7-δ structure. The T c decreases linearly with increasing x and superconductivity is not observed for the samples of x≥0.3. The valences of Nd and Pr ions in the samples were determined from the susceptibility measurement as Nd3+ and Pr4+, respectively. It is supposed naturally that the valence of Cu2 ions (Cu ions in the two-dimensional Cu–O plane) is affected strongly by the neighboring rare earth ions. The results show that the valence of Cu2 ions is significant for the superconductivity in the Ba2RECu3O7-δ system.

The ErBa2Cu3O7-δ sample which is of a single phase of the orthorhombic YBa2Cu3O7-δ structure has shown the value of T c=90.0±0.7 K and the Meissner flux expulsion up to 50% at 79 K. The critical current density was 350 A/cm2 at zero magnetic field, while it decreased rapidly with increasing field. The magnetic susceptibility in normal state is explained by the Curie-Weiss law with the effective moment µeff=9 µB which is in good agreement with the one for the free ion of Er.

Superconducting properties are studied through electrical resistivity, magnetic susceptibility and X-ray diffraction on the high temperature superconductor Y0.4Ba0.6CuO3-x. The X-ray powder diffraction pattern of the sample indicates almost an orthorhombic phase. The onset superconducting transition temperature Tc(onset) and the transition width ΔTc are sensitive to the sintering temperature. The best superconducting properties are obtained when the sample is sintered at about 1000°C in oxygen flow; Tc(onset)=91.2K, ΔTc=1.4K and the critical current density at 77K Jc≧60A/cm2.