High-quality single crystals of the unconventional superconductor NdFeAsO_{1 − x}F_x were grown. We developed a new optimized flux technique to overcome the difficulties in single-crystal growth and the sample quality limitations of NdFeAsO_{1 − x}F_x. 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 NdFeAsO_{1 − x}F_x (x = 0.1). The anisotropy ratio [γ_Hc1 (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 H_c1(T) curvature were observed, which are attributed to the multi-band superconductivity.

中文翻译：

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氧化物中下临界场的单晶生长和小各向异性：NdFeAsO1-xFx

生长了非常规超导体NdFeAsO _{1- x} F _x的高质量单晶。我们开发了一种新的优化通量技术，以克服单晶生长中的困难以及NdFeAsO _{1- x} F _x的样品质​​量限制。从室温冷却后，掺F样品的正常状态表现出简单的金属行为，随后出现急剧的超导转变。x的剩余电阻率比（RRR）的值为3.2、6.4和10.3分别为0.1、0.15和0.2。大的RRR和狭窄的超导过渡都标志着晶体的高质量。我们研究了平面内和平面外的下临界场，以及涡旋穿透NdFeAsO _{1- x} F _x（x = 0.1）样品的场。随着温度从0.8 Tc升高到Tc ，各向异性比[ _γHc1（0）]略有增加。在平行于c轴和_a轴的静磁场H下，获得第一涡旋穿透场的温度依赖性，并且在H _{c 1}（T观察到曲率，这归因于多频带超导性。