Calorimetric determination of the upper critical fields and anisotropy of NdFeAsO1-xFx single crystals

We present heat-capacity measurements of the upper critical fields of single-crystal NdFeAsO1-xFx. In zero-magnetic field a clear step in the heat capacity is observed at T-c approximate to 47 K. In fields applied perpendicular to the FeAs layers the step broadens significantly whereas for the in-plane orientation the field effects are small. This behavior is reminiscent of the CuO2-high-T-c superconductors and is a manifestation of pronounced fluctuation effects. Using an entropy conserving construction we determine the transition temperatures in applied fields and the upper critical-field slopes of partial derivative H-c2(c)/partial derivative T=-0.72 T/K and partial derivative H-c2(ab)/partial derivative T=-3.1 T/K. Zero-temperature coherence lengths of xi(ab)approximate to 3.7 nm and xi(c)approximate to 0.9 nm and a modest superconducting anisotropy of lambda similar to 4 can be deduced in a single-band model.