Lower critical fields of superconducting PrFeAsO1-y single crystals

We have studied the lower critical fields Hc1 of superconducting iron oxipnictide PrFeAsO1-y single crystals for H parallel and perpendicular to the ab planes. Measurements of the local magnetic induction at positions straddling the sample edge by using a miniature Hall-sensor array clearly resolve the first flux penetration from the Meissner state. The temperature dependence of Hc1 for Hc is well scaled by the in-plane penetration depth without showing any unusual behavior, in contrast to previous reports. The anisotropy of penetration lengths at low temperatures is estimated to be 2.5, which is considerably smaller than the anisotropy of the coherence lengths. This is indicative of multiband superconductivity in this system in which the active band for superconductivity is more anisotropic. We also point out that the local induction measured at a position near the center of the crystal, which has been used in a number of reports for the determination of Hc1, might seriously overestimate the obtained Hc1 value.