The lower critical fields of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.95}$ 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 ${\mathit{H}}_{\mathit{c}1}$ below about 0.5${\mathit{T}}_{\mathit{c}}$, with ${\mathit{H}}_{\mathit{c}1}$(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 ${\mathit{H}}_{\mathit{c}1}$ data to results of London penetration-depth measurements, the vortex core energy is found to be linear with temperature.

• Received 18 April 1994

DOI:https://doi.org/10.1103/PhysRevB.50.4212