Three major superconducting parameters can characterize a type II superconductor: critical transition temperature, Tc, upper critical magnetic field, Hc2, and critical current density, Jc. Because of the variety of crystal structures, chemical stoichiometrics, and microstructures of the materials, these superconducting parameters differ greatly from system to system.
It has been found that the critical transition temperature is closely related to the crystal structure and stoichiometry. Previous studies have shown that compounds with a high degree of symmetry tend to be more favorable to superconductivity. Compounds with a cubic structure, such as a bcc structure, usually have higher Tc values than compounds with a hexagonal lattice. Among conventional superconductors, the so-called A-15 structure is most favorable for high transition temperatures. It is well known that many high Tc superconductors such as YBa2Cu3Ox and Bi2Sr2CaCu2Ox have an orthorhombic structure with a high degree of symmetry.
The upper critical field, Hc2, is more complicated in terms of its relationship to structural characteristics. Experimental results have indicated that Hc2 is more closely related to the chemical stoichiometry and crystal structure than to the microstructure.
However, the critical current density, Jc, can vary tremendously in a compound with a given crystal structure. The change in critical current density is associated with the so-called flux pinning that arises from the interaction between the flux lines and crystal defects. Thus, Jc is determined mostly by the microstructure of the materials.