CRITICAL-CURRENT FREE-ENERGY RELATIONS IN HIGH-TC SUPERCONDUCTORS

Free-energy surfaces of a series of Fe-doped YBa2Cu3O7- have been measured in order to look for a relation between the free energy per unit length of a vortex and the intragranular critical current density. Data for grain-aligned samples with the magnetic field parallel to the c axis show that Fe additions lead to large changes in the thermodynamic variables measured in the reversible region of the H-T plane and correspondingly large changes in the critical current density measured in the irreversible range. Both the free energy per unit length for the vortices and the critical current density fall exponentially with Fe concentration and are roughly proportional to one another. © 1990 The American Physical Society.