Exploration of the superconducting properties of Y_3Ba_5Cu_8O_(18) with and without Ca doping by magnetic measurements

摘要

The superconducting Y_3Ba_5Cu_8O_(18) (Y-358) and Y_3Ba_5Ca_2Cu_8O_(18) (YCa-358) compounds have been synthesized by using the sol—gel method. Hence, the influence of doping of Ca into the compound Y-358 has been studied by comparing the resistivities, DC magnetizations both M(H) and M(T),flux pinning properties, AC susceptibilities and critical current densities of the undoped and doped compounds, at low temperatures. The AC susceptibility and resistivity measurements showed that the superconducting transition temperature, T_c, is suppressed (about 6 K) with the addition of Ca into the main compound. The hysteresis loops of YCa-358 show peak like projecting parts at temperatures below 45 K and around zero applied field that may be due to the local modulation of the composition in YCa-358. Such a behavior has been observed for the first time. The critical current densities, J_c, determined from the hysteresis measurements decrease with the addition of Ca into Y-358. At 15 K, the maximum values of J_c, for the compounds Y-358 and YCa-358 are found to be 8 × 10~4 A/cm~2 and 4.5 × 10~4 A/cm~2, respectively. The flux pinning force, F_p calculated from the field dependence of the J_c values shows that the irre?versibility line shifts to lower magnetic fields with the doping of Ca into Y-358. Furthermore, the measurements of the inphase and out off components of the ACsusceptibilities clearly demonstrate that the superconducting volume fraction of Y-358 decreases with the addition of Ca.