Superconducting phase diagram and fluctuations in SmFeAsO$_{0.85}$F$_{0.15}$ single crystals

We use micro-calorimetry to investigate the anisotropic phase diagram and effects of superconducting fluctuations in sub-micro-gram single crystals of SmFeAsO$_{0.85}$F$_{0.15}$. Our measurements reveal that SmFeAsO$_{0.85}$F$_{0.15}$ is characterized by a large anisotropy of $\Gamma \sim $8 and a short in-plane Ginzburg-Landau coherence length of $\xi _{ab}$(0) $\sim $ 1.3 nm. These materials parameters promote strong superconducting fluctuations which are seen in the zero-field specific heat as clear upwards curvature in C/T at temperatures below T$_{c}$ = 49.5 K and long tails above T$_{c,}$. The resulting anomaly is cusp-shaped with height of $\Delta $C/T$_{c}$ = 24 mJ/moleK$^{2}$, which can be fitted with 3D-Gaussian fluctuations. The transition shows pronounced broadening in magnetic fields applied along the c-axis. The field evolution in fields higher than 3 T is well described in the frame of 3D lowest-Landau-level scaling of fluctuations using an upper critical field slope of -4.4 T/K. We will compare these characteristics to the behavior of other members of the FeAs-family. This work was supported by DOE-BES under Contract No. DE-AC02-06CH11357.