Type: Article
Publication Date: 2008-10-30
Citations: 61
DOI: https://doi.org/10.1103/physrevb.78.140510
We present heat-capacity measurements of the upper critical fields of single-crystal ${\text{NdFeAsO}}_{1\ensuremath{-}x}{\text{F}}_{x}$. In zero-magnetic field a clear step in the heat capacity is observed at ${T}_{c}\ensuremath{\approx}47\text{ }\text{K}$. 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 ${\text{CuO}}_{2}$-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 $\ensuremath{\partial}{H}_{c2}^{c}/\ensuremath{\partial}T=\ensuremath{-}0.72\text{ }\text{T}/\text{K}$ and $\ensuremath{\partial}{H}_{c2}^{ab}/\ensuremath{\partial}T=\ensuremath{-}3.1\text{ }\text{T}/\text{K}$. Zero-temperature coherence lengths of ${\ensuremath{\xi}}_{ab}\ensuremath{\approx}3.7\text{ }\text{nm}$ and ${\ensuremath{\xi}}_{c}\ensuremath{\approx}0.9\text{ }\text{nm}$ and a modest superconducting anisotropy of $\ensuremath{\lambda}\ensuremath{\sim}4$ can be deduced in a single-band model.