Effect of short coherence length along the c-axis on the irreversibility line in high- Tc superconductors

Abstract

It has been empirically known that the irreversibility line in quasi-two-dimensional high- T c superconductors is farther from the phase boundary, B c2( T), than in three-dimensional ones. Such a difference was investigated in terms of the flux creep theory from the viewpoint of the coherence length along the c-axis, ξ⊥. For superconductors with short ξ⊥ in the field normal to the c-axis, the flux bundle size is small due to the small shear modulus C 66 of the fluxoid lattice, resulting in small pinning potential depth. Hence, even if strong pinning centers such as fine non-superconducting particles are successfully introduced into the materials, their effect on the irreversibility line is not as large as that in three-dimensional superconductors. However, this inherent inferiority can be overcome up to a sufficient level for practical applications by improvement of the flux pinning characteristics. The case is also treated where the magnetic field is applied parallel to the c-axis. In this case, the irreversibility line is restricted to a low level because of the small B c2 value.