Lower critical fields in an ellipsoid-shaped YBa2Cu3O6.95 single crystal

The lower critical fields of YBa2Cu3O6.95 have been determined by magnetization M(H) measurements using an ellipsoidal-shaped crystal and a thin platelet for H∥c and H⊥c, respectively. Careful data analysis rules out the effects of surface barriers. The data show a linear temperature dependence of Hc1 below about 0.5Tc, with Hc1(0)=1100 Oe and 200 Oe for H∥c and H⊥c, respectively. The anisotropy factor γ is almost temperature independent and has the value 5.6±0.3. By comparison of the Hc1 data to results of London penetration-depth measurements, the vortex core energy is found to be linear with temperature.