Caloric Determination of the Upper Critical Fields and Anisotropy of NdFeAsO1-xFx Single Crystals

TL;DR

This study measures heat capacity of NdFeAsO1-xFx single crystals to determine upper critical fields and anisotropy, revealing fluctuation effects similar to high-Tc cuprates and providing key superconducting parameters.

Contribution

It provides the first detailed calorimetric analysis of upper critical fields and anisotropy in NdFeAsO1-xFx single crystals, highlighting fluctuation effects and deriving critical superconducting parameters.

Findings

Heat capacity step at Tc = 47K observed in zero field.

Pronounced fluctuation effects similar to cuprates.

Upper critical field slopes: dHc2,a = -0.72 T/K, dHc2,ab = -3.1 T/K.

Abstract

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 Tc = 47K . 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-Tc 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 dHc2,a = -0.72 T/K and dHc2,ab = -3.1 T/K. Zero-temperature coherence lengths of xiab = 3.7 nm and xic = 0.9 nm and a modest superconducting anisotropy of gamma ~ 4 can be deduced in a single-band model.