Single-crystal growth and small anisotropy of the lower critical field in oxypnictides: NdFeAsO1−xFx

Abstract

High-quality single crystals of the unconventional superconductor NdFeAsO1 −xFx were grown. We developed a new optimized flux technique to overcome the difficulties in single-crystal growth and the sample quality limitations of NdFeAsO1 −xFx. The normal state of the F-doped samples exhibits simple metallic behavior upon cooling down from room temperature, followed by a sharp superconducting transition. The values of residual resistivity ratio (RRR) is 3.2, 6.4, and 10.3 for x = 0.1, 0.15, and 0.2, respectively. Both the large RRR and the narrow superconducting transition signpost the high quality of the crystals. We have examined the in-and out-of-plane lower critical fields, and the field at which vortices penetrate the sample of NdFeAsO1 −xFx (x = 0.1). The anisotropy ratio [γHc1 (0)] increased slightly with increasing temperature from 0.8 Tc to Tc. The temperature dependence of the first vortex penetration field was obtained under the static magnetic field, H, parallel to the c-and ab-axis, and pronounced changes in the Hc1(T) curvature were observed, which are attributed to the multi-band superconductivity.