Temperature dependence of anisotropic lower critical fields in (${\mathrm{La}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${)}_2$${\mathrm{CuO}}_4$

The anisotropic lower critical fields ${\mathit{H}}_{\mathit{c}1}$ are determined from a novel procedure based on the Bean’s critical-state model to pick up the onset of flux penetration in broad zero-field-cooled magnetization curves in (${\mathrm{La}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Sr}}_{\mathit{x}}$ ${)}_2$ ${\mathrm{CuO}}_4$ (x≊0.07) single crystals. The values of ${\mathit{H}}_{\mathit{c}1}$ at T=0 K are estimated to be 70 G for H∥ layer and 300 G for H⊥ layer. The temperature dependence of ${\mathit{H}}_{\mathit{c}1}$ is such that it tends to saturate towards the lowest temperature measured (2 K) in contrast to the other recent results which report the abnormal upturn of the ${\mathit{H}}_{\mathit{c}1}$-T curve at low temperatures. Instead the results indicate that the critical current density in plane has a strong temperature dependence even in the temperature range well below ${\mathit{T}}_{\mathit{c}}$ and becomes ∼${10}^6$ A/${\mathrm{cm}}^2$ at 2 K.