Lower critical fields, critical currents, and flux creep inPb2Sr2R1−xCaxCu3O8+y(R=Y,Dy) single crystals

The lower critical field ${\mathbf{H}}_{\mathit{c}1}$(T), temperature and field dependences of the critical current ${\mathit{j}}_{\mathit{c}}$(T,H), and the normalized magnetization relaxation rate d(lnM)/d(lnt) have been studied for two orientations of the magnetic field H∥c and H⊥c in ${\mathrm{Pb}}_2$ ${\mathrm{Sr}}_2$ ${\mathit{R}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Ca}}_{\mathit{x}}$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{8+\mathit{y}}$ (R=Y,Dy) single crystals having practically a cubic shape with characteristic dimensions 0.6×0.6×0.5 ${\mathrm{mm}}^3$. ${\mathbf{H}}_{\mathit{c}1}$(T) shows for H∥c a positive curvature with decreasing temperature without the usual low-temperature saturation found for the H⊥c orientation. This anomalous ${\mathbf{H}}_{\mathit{c}1}^{\mathrm{\parallel }\mathit{c}}$(T) behavior is related to the modification of the character of the field penetration in a layered structure consisting of a stack of superconducting and nonsuperconducting planes. The temperature dependence of the normalized relaxation rate S=d(lnM)/d(lnt) shows a maximum at temperatures where the largest anisotropy of the critical currents is observed.