Condensation energy density in Bi-2212 superconductors

Abstract

The relationship between the condensation energy density and the anisotropy parameter,γa, has been derived for Bi-2212 superconductors in various anisotropic states by analysingthe critical current density due to columnar defects introduced by heavy ion irradiation.The critical current density depended on the size of the defects, determined by the kindand irradiation energy of the ions. A significantly large critical current density of17.0 MA cm−2 was obtained at 5 K and 0.1 T even for the defect density of a matching field of 1 T in aspecimen irradiated with iodine ions. The dependence of the critical current density on the sizeof the defects agreed well with the prediction from the summation theory of pinning forces,and the condensation energy density could be obtained consistently from specimens irradiatedwith different ions. The condensation energy density obtained increased with decreasingγa over the entire range of measurement temperature, and reached about 60% of the value forthe most three-dimensional Y-123 observed by Civale et al at 5 K. This gives thereason for the very strong pinning in Bi-2212 superconductors at low temperatures.The thermodynamic critical field obtained decreased linearly with increasingtemperature and extrapolated to zero at a certain characteristic temperature,T*, lower than thecritical temperature, Tc. T*, which seems to be associated with the superconductivity in the block layers, was highestfor the optimally doped specimen. This shows that the superconductivity becomes moreinhomogeneous as the doped state of a superconductor deviates from the optimumcondition.