M.F. Tai

Critical current densities in K3C60/Rb3C60 powders determined from AC/DC susceptibility measurements

Abstract We report the magnetic J c ( T ) in K 3 C 60 and Rb 3 C 60 powder samples for temperatures from 5 K to T c . J c near T c was obtained from AC measurements whereas J c at low temperatures was obtained from DC measurements. In the AC measurements, two χ″ peaks were observed on some of the samples. This is attributed to weak links in granular superconductors. The intragranular J c is much larger than the intergranular J c . The J c ( T ) data are fitted by J c ( T ) = J c (0)(1 − T / T c ) γ , with J c (0) and γ as fitting parameters. In the DC measurements on Rb 3 C 60 , J c depends approximately linearly on temperature. Extrapolating the zero-field DC data to 0 K yields J c (0) = 3.1 × 10 6 A/cm 2 . This is a rather large J c for a powdered superconductor, suggesting strong pinning in these materials. Finally, the values of γ for the intergranular and intragranular components are compared with the predictions of theories.

Scaling law for vortex dynamics in Rb3C60 superconductor

Abstract AC susceptibility of the Rb3C60 superconductor shows a weak dependence on ac magnetic-field frequency f. The temperature, Tp where χ″ is maximum, rises with increasing frequency and the effect is enhanced as the dc magnetic-field amplitude Hdc is increased. The temperature is found to obey a scaling law (1 − T p /T c ) 3 2 = 1.9 × 10 −4 H dc [ ln (f 0 /f) + 23] + 8 × 10 −5 [ ln (f 0 /f) + 23] , where f0 (≈ 1010 s−1) is a characteristic frequency and Hdc is in tesla. The scaling relationship is in good agreement with the predicted equation based on the model of thermally activated flux motion.

Superconductivity and magnetism of the (Hg0.7Bi0.3)Sr2(Ca1−xPrx)Cu2O6+δ system

Abstract The (Hg0.7Bi0.3)Sr2(Ca1−xPrx)Cu2O6+δ system for 1.0 ≧ x ≧ 0.25 has been prepared. The transition temperature Tc always decreases as the rare-earth substitution parameter x increases. We observed the maximal Tc around x = 0.25 with 91 K. From the viewpoint of doping the ionic size and analytic results of the Pr valence in the x = 1.0 sample, we think that Pr exhibits the trivalent state in this system. No anomalous rare-earth Pr antiferromagnetic (AFM) ordering was observed in the temperature dependence of the susceptibility down to 5 K for the Ca free sample.

Studies of magnetic characterization and critical current density in Rb3C60

Abstract Temperature-dependent magnetization and magnetic-hysteresis curves for the zero-field cooled and the field-cooled processes of Rb 3 C 60 superconducting fullerene with T c of 30.5±0.1 K were measured between 5 and 30 K in magnetic fields up to 1 T. Small field-cooled diamagnetism and flux jumps were observed at low temperatures and in low fields. Both results strongly suggest the existence of a rather large pinning strength in our sample. This suggestion is further confirmed by the field-cooled hysteresis curve and high critical current densities, J c ( H , T ), at low temperatures. Based on the major hysteresis loops and the Bean critical-state model, the J c at T = 5 K are 2.1 × 10 6 A/cm 2 for H = 0 T and 5.2 × 10 5 A/cm 2 for H = 0.9 T. These J c values are larger than other powdered superconductors with comparable sample sizes, except for YBa 2 Cu 3 O y . For T >10 K, J c versus H agrees well with the expression: J c ( H , T )= k 1 /(1+ H / H 01 ) n . On the other hand, for T J c ( H , T ) may be expressed as J c ( H , T )= k 2 exp(− H / H 02 ). Here k 1 , k 2 , H 01 , H 02 and n are temperature-dependent parameters. The dependence of J c on temperature thus obeys the empirical scaling relation, J c ( H , T )= J c0 ( H )(1− T / T c ) m , where J c0 and m are field-dependent parameters. For 0.4 T H m equals about 2.

Magnetic properties of Pr(Ba2−xSrx)Cu3Oy single crystals

Abstract The high quality Pr(Ba 2−x Sr x )Cu 3 O y (0 ≤ x ≤ 0.6) crystals with isostructural to R Ba 2 Cu 3 O y (R =Y or rare earth) compounds have been successfully prepared using a stoichiometrically melting growth method without flux. The x-ray diffraction patterns and rocking curves showed the well c-axis-oriented single-crystal nature of these specimens. The lattice parameter, degree of orientation, and crystallinity decrease with the substitution of Sr for Ba. No magnetic ordering was apparently observed in the series (x ≤ 0.6) by the all magnetization data between 5 K to 200 K and within ± 1 T. The χ ab −1 ( T ) with the applied field normal to c -axis have been fitted by simple Curie-Weiss law without considering the crystal field effect and shown the effective moments μ eff of 3.537 μ B for x = 0 and 3.572 μ B for x = 0.2, which is closely the Curie-Weiss moment value of free ion Pr 3+ .

The trend of magnetic irreversibility for Rb3C60 and its implications

Abstract The irreversibility magnetic field ( H irr ) as a function of the irreversibility temperature ( T irr ) for two-dimensional superconductors,3e.g., the ones with a perovskite structure, can be characterized by an exponential law. However, with increased thermal agitation toward T c , the coupling strength between adjacent superconducting (i.e., CuO 2 ) layers would impart a three-dimensional like character. As a result, the exponential law may be approximated by a simpler power law. The fullerene superconductors are intrinsically isotropic so their H irr trends appear to follow the power law throughout the entire temperature range. For example, our powdered Rb 3 C 60 sample showed such a trend with a highly three-dimensional character. Moreover, a well crystallized Rb 3 C 60 sample exhibits an ideal isotropic feature as manifested by the linear dependence of H irr on T irr . Furthermore, this linearity may be represented by two superimposed straight lines that intersect the T irr axis at two T 0 values (30.5 K and 60.0 K). For most superconductors, T 0 is indicative of T c . The cause of two T 0 values for our Rb intercalated fullerene crystals is not known. If they represent two T c values of superconducting phases, then, the phase with T c = 30.5 K corresponds to the well-known composition of Rb 3 C 60 . However, the phase with T c = 60.0 K may possess a different composition designated as Rb x C 60 with x possibly higher than 3.

Synthesis and superconductivity of (Hg0.7Bi0.3)Sr2(Ca1-xLax)Cu2O6+σ system

The problems of making phase-pure (Hg0.7Bi0.3)Sr2(Ca1-xLax)Cu2O6+σ system for 1.0≧x≧0.0 are discussed in detail. This system with the tetragonal HgBa2CaCu2O6-type (1212) structure was found only for x=0.25, 0.5 and 0.75. The a and c lattice parameters increase with the increasing La concentration due to the decreasing in oxidation state of Cu and the replacement of smaller Ca+2 ions (1.12 Å) by larger La3+ ions (1.16 Å). The transition temperature Tc decreases as the La substitution parameter x increase. We observed the maximal Tc at x=0.25 with 87K and 72K for x=0.5.

Magnetic relaxation and pinning potential in superconducting Rb3C60 powder

Magnetic relaxation in various magnetic fields and at different temperatures was carried out in Rb3C60 powder withTc=30.5 K, and its pinning potential as functions of temperature and field is determined from these relaxation measurements. ForH=5 kOe, the apparent pinning potential,Uo, is 12 meV at 5 K, and increases to about 47 meV with temperature up to 12.5 K. Then it keeps at 45 meV betweenT=12.5 K and 20 K, and at last decreases with temperature. For 3 kOe, a maximumUo of 35 meV was observed atT=20 K. AtT=15 K, theUo rapidly decreases from ≈60meV to 24 meV as field increases from 1 kOe to 2 kOe, then it shows field-independent and always remains at 24 meV in our measuring field range.

Enhanced diamagnetism in YBa2Cu3O7−δ by hydorgen-plasma treatment

Abstract We report that the diamagnetism of as-grown YBa 2 Cu 3 O 7−δ powders can be enhanced by treatment with RF-generated hydrogen plasma. The field-cooling susceptibility at 5 K is found to increase by up to 24%. The enhancement is reproducible for all five samples studied. The lattice parameter c decreases after hydrogenation. For a sample treated for the shortest period, its pinning potential,obtained from magnetic relaxation measurements, is reduced following H-treatment.

Influence of hydrogen absorption on magnetization in superconducting YBa2Cu3O7−δ powders☆

Abstract Hydrogen (H) absorption by YBa 2 Cu 3 O 7− δ (YBCO) was carried out by exposing the superconducting powders to rf-generated H plasma. We find that both the field-cooled (FC) and zero-field-cooled (ZFC) susceptibilities increase after hydrogenation. The largest enhancement is about 24% for the FC susceptibility at 5K.