Lower critical fields and the anisotropy in PrFeAsO_1-y single crystals

We report the lower critical fields H_c1 measurements using a miniature Hall-sensor array on superconducting iron-pnictide PrFeAsO _1-y single crystals. The local magnetic induction measured by the sensor at the edge of the crystal clearly resolves the first flux penetration from the Meissner state. The temperature dependence of H_c1 for H _c is well scaled by the in-plane superfluid density determined by the penetration depth measurements. The anisotropy of H_c1 at low temperatures is estimated to be ≃2.5, which is smaller than the anisotropy of the upper critical fields H_c2, indicating the multiband superconductivity in this system where the active band for superconductivity is more two-dimensional.