Search

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.

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.

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.

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.

Far infrared measurements on detwinned single crystals at 2 K give an anisotropic absorptivity for the a and b polarizations. A ≊ 0.5% for the axis from ≊ 100cm −1 up to the optical edge at 400cm −1 . The a axis conductivity shows low frequency absorption bands of electronic strength.