Magnetic Anisotropy in A High-Tc Superconductor YBa2Cu3O7-δ Single Crystal below Tc

Abstract
We have measured the magnetic field dependence of magnetization of single-crystal YBa2Cu3O7-δ in detail in fields up to 5 kOe in the temperature range below T c, i.e., 93 K. Lower critical fields, H c1's, at which the magnetization deviates from linearity, are very anisotropic; H c1 (0) (perpendicular to the a b-plane) is 305 Oe and H c1 //(0) (parallel to the a b-plane) is 104 Oe. Both H c1 and H c1 // decrease linearly with increasing temperature up to about T c. It is suggested that random oxygen defects in the Cu-O planes and chains are important for the high-J c type II superconductor YBa2Cu3O7-δ.