Intrinsic flux pinning and flux creep in oxide superconductors

Abstract

After a brief review of unusual flux creep phenomena in high $T_c$ superconductors, evidence for intrinsic flux pinning by the periodic lattice is summarized. The authors then calculate the lattice periodic part of vortex energy for a vortex parallel to the Cu-O plane using a realistic Ginzburg-Landau like functional, finding its amplitude to be 33 K per unit cell (4 A), about half the core energy. They also estimate the pinning energy per unit cell for a vortex perpendicular to the plane as about 20-30 K per unit cell (10 A). Local defects in individual vortices, in particular kink-antikink pairs (K-K) are shown to be metastable beyond a certain size, with an activation energy in the range 0.05 to 0.2 eV. It is shown that these results, namely intrinsic pinning and local defects in vortices as causes of creep, can rationalize many properties of oxide superconductors at low temperatures. Some of these are: size, distribution, magnetic field and orientation dependence of flux creep activation energies; lack of dependence of creep on angle between vortex and the electric current when both are in plane. Finally, they speculate on the possibility of quantum creep.