Temperature dependence of lower critical fields in Y-Ba-Cu-O crystals

Abstract

The temperature dependence of the anisotropic lower critical fields ${H}_{c1}$ in single-crystal Y-Ba-Cu-O is determined from the onset of flux penetration in zero-field-cooled dc magnetization versus temperature from ${T}_{c}$ down to $0.5{T}_{c}$. An extended Bean-critical-state model is developed to define this onset consistently. The ${H}_{c1}'\mathrm{s}$ are linear in temperature near ${T}_{c}$ as in Ginzburg-Landau theory and consistent with a clean local-limit BCS form with extrapolated ${H}_{c1}(T=0)=180$ Oe for $\mathrm{H}\ensuremath{\perp}\stackrel{^}{c}$ and 530 Oe for $\mathrm{H}\ensuremath{\parallel}\stackrel{^}{c}$.