[en] The existence of the irreversibility line is one of the most intrigue features of high temperature superconductors. The irreversibility line is resulted from the short coherence length, large anisotropy and high working temperature of high-T_c oxides. For low temperature superconductors, however, there has been experimental evidence of the existence of the irreversibility line even though it is previously thought that the irreversibility line is too close to the upper critical field line to be observed. In this work, systematic measurements of dc magnetization, ac susceptibility and magnetoresistivity as a function of applied field and temperature have been performed on Nb alloy samples in order to investigate the irreversibility line in low temperature superconductors. The results show that there exits an observable region below the mean-field critical field, H_c2, where the magnetization is reversible during a cycle of increasing and decreasing field. Furthermore, results obtained on other low temperature superconductors such as the Chevrel phase superconductor PbMo₆S₈, Ba_0.25Pb_0.75BiO₃, the newly discovered intermetallic compound MgB₂ and overdoped cuprate Tl₂Ba₂CuO_6+δ (T_c∼20 K) are also discussed

[en] We have carried out structure, magnetic and specific heat measurements on aluminum doped magnetism diboride samples Mg_1-xAl_xB₂ in order to investigate possible superconductivity at the x=0.5 concentration. A diamagnetic signal was observed in magnetization measurements accompanied by a decrease in resistivity. However, the diamagnetic signal was extremely small as compared to what expected from full diamagnetism. Also, the transition both in magnetization and resistance was very broad. We propose that the diamagnetism is due to a very small amount of superconducting phase such as MgB₂ and the resistive transition is due to the percolation behavior. Furthermore, we performed specific heat measurements, which are considered as a tool to investigate the bulk nature of superconducting transition, on the x=0.5 sample to verify the existence of superconductivity. We observed no evident superconducting transition in the entire temperature region from 2 to 300 K. The undistinguishable data between 0 and 5 T magnetic fields also indicated the absence of bulk superconductivity in the x=0.5 sample

[en] We have proposed a novel type of high-T_c superconducting quantum interference device (SQUID) planar gradiometer that could measure the mixed partial derivative of magnetic fields ∂²B_z/∂x∂y. The device consists of two parts, one is the signal pickup antenna formed by four square pickup loops, and another one is a SQUID directly coupled to the common line which separates the antenna loops. The four pickup loops were arranged to a two-dimensional array. The loops in the diagonal direction are connected in series. The total current in the common line in this configuration is directly proportional to the mixed partial derivative of the magnetic field. We have made a device of such design on a STO bicrystal substrate and tested the device in gradient field

[en] We have investigated the structural and superconducting properties of a series of Al doped MgB₂ samples. The results show that the superconductivity of MgB₂ is destroyed when Al is alloyed into MgB₂ as reported previously. A structural instability was observed from the data of X-ray diffraction. Further study by TEM shows that the Al substitution leads to an ordering occupation on Mg sites. This corresponds to a superstructure with a repeat period of 2c along the c-axis. The superstructure was identified to be a form of MgAlB₄ structure which intergrows with MgB₂. Measurements on the low doping level samples showed that the upper critical field B_c2, irreversible field B_irr and the critical current density J_c of these samples were reduced as the Al content was increased. Analysis of the upper critical field data suggests that the density of states near the Fermi surface N(E_F) is reduced by Al doping. This is consistent with the decrease in the superconducting transition temperature T_c. The field dependence of pinning force is also discussed for these samples

[en] Ba_0.1Sr_0.9TiO₃/YBa₂Cu₃O_7-δ heterostructure films have been fabricated by a pulsed-laser deposition method on vicinal LaAlO₃ substrates. The effect of the growth mode of YBCO thin films on the dielectric properties of BSTO thin films was studied. The BSTO thin films grown epitaxially on YBCO thin films in step-flow mode showed high dielectric constant and large tunability. The dielectric constant and loss tangent measured were 1100 and 0.005, respectively, and a tunability of 55% was achieved at ±50 V/μm electric field at 77 K and 1MHz for a BSTO thin film grown on 120 nm YBCO thin film

[en] Two characteristic transition temperatures have been observed in superconducting YBa₂Cu₄O₈ ceramic by means of an ac inductive method. Our susceptibility data from a powdered sample, and a bulk sample at different magnetic field strengths, show that the two temperatures correspond to superconducting transition at the intergrain region Tc_(inter) and at the intragrain region Tc_(intra); the latter gives a weak diamagnetic signals. We found that Tc_(inter) was gradually lowered when the sample was subjected to increasing quenching temperatures, while Tc_(intra) was unaffected by such treatment, the latter being consistent with the oxygen stability of YBa₂Cu₄O₈. The small susceptibility signals occuring at Tc_(intra) will be discussed. (orig.)

[en] Magnetic measurements have been carried out on the hot-isostatically-pressed Chevrel-phase superconductor PbMo₆S₈ at temperatures from 4.2 K to T_c and for magnetic fields up to 12 T. The results show that for the PbMo₆S₈ compound there is a wide magnetically reversible region, between the irreversibility field B_irr and the upper critical field B_c2, on the isothermal magnetic hysteresis curves. The B_irr(T) line, i.e., the irreversibility line, was found to obey a power-law expression: B_irr=B^*(1-T/T_c)^α with α∼1.5. Magnetic relaxation measurements revealed that the flux-creep effect in the material studied is substantial and is greater than those observed in conventional metallic alloys, but smaller than in high-temperature superconductors. The existence of the irreversibility line and pronounced flux-creep effect in PbMo₆S₈ is attributed to the short coherence length of the material. From the reversible magnetization data, the values of the penetration depth, the coherence length, and the critical fields are obtained together with the Ginzburg-Landau parameter κ. At 4.2 K, the critical current density J_c is 10⁹ A m^-2 at zero field, and decreases to 2x10⁸ A m^-2 at 10 T. Pinning force curves measured at different temperatures obey a Kramer-scaling law of the form: F_p(=J_cxB)∝b^1/2(1-b)², which indicates that the J_c is limited by one predominant flux-pinning mechanism

[en] A series of Al-doped Mg_1-xAl_xB₂ samples have been fabricated and systematic study on structure and superconducting properties have been carried out for the samples. In addition to a structural transition observed by XRD, TEM micrographs showed the existence of a superstructure of double c-axis lattice constant along the direction perpendicular to the boron honeycomb sheet. In order to investigate the effect of Al doping on flux pinning and critical current properties in MgB₂, measurements on the superconducting transition temperature T_c, irreversible field B_irr and critical current density J_c were performed too, for the samples with the doping levels lower than 0.15 in particular. These experimental observations were discussed in terms of Al doping induced changes in carrier concentration

[en] The effects of Eu substituting for Ba on structural and superconducting properties were investigated for a series of superconductors Eu_1+xBa_2-xCu₃O_7-δ (x=0, 0.02, 0.04, 0.06, 0.08, 0.1, 0.12, 0.14, 0.16, 0.18, and 0.2). With Eu content increasing, the random substitution of excessive Eu for Ba ion forms a Eu-Ba non-stoichiometric distribution on Ba site, which deviates from the ideal 123 phase stoichiometry. The analysis of crystallographic structure shows that the solid solubility x of Eu on Ba site is over 0.2. Eu_1+xBa_2-xCu₃O_7-δ take on an orthorhombic-tetragonal (O-T) phase transition from x=0.06, and end in x=0.18. Corresponding to the O-T phase transition, superconducting critical temperature T_c rapidly decreases with Eu content increasing, which is attributed to the decline of carrier density and the proximity effects of non-superconducting clusters. The positron annihilation measurements show anomalous changes for lifetime spectra and Doppler-broadened radiation spectra when the O-T phase transition occurs. By the model of excessive oxygen defects in Cu-O chain regions, we discuss effects of Eu-Ba non-stoichiometric distribution on structural and superconducting properties

[en] This paper reports that x-ray precession technique has been utilized to study the reciprocal space of the superconducting single crystal Bi₂Sr₂CaCuO_y. An average structure with space group of Pnnn and a real structure with the superspace group of P₁₁₁^Pnnn or P_1S1^Pnnn have been found for this one dimensional modulated incommensurate material. As well, a basic structure with the space group of Fmmm is derived from the experimental data