Comparison of specific heat and magneto-resistance measurements in the same SmFeAsO$_{1-x}$F$_{x}$ crystals

We have performed resistivity and specific heat measurements in the same sub-microgram single crystals of an iron-based superconductor SmFeAsO1-xFx (T$_{c} \approx $ 50K). This allowed for the first direct comparison of Hc2 curves from thermodynamic measurements with estimates from the magneto-resistance at commonly used criteria (10, 50, 90{\%} $\varrho _{N})$. A criterion of 40-50{\%} $\varrho_{N}$ well describes H$_{c2}$(T) for both in and out of plane fields. We attribute the low field dependence of the criterion to filamentary superconductivity. The challenging heat capacity measurement on microscopic crystals (50$\mu$m in diameter, 10$\mu$m thick) was done using a commercially available gas-nanocalorimeter. The thermodynamic data was in good agreement with previous experiments performed on crystals from the same batch by other groups. H$_{c2}$ slopes of 1.6 T/K for fields parallel to the c-axis and 12.3 T/K in the ab-plane were found yielding a $\xi $ anisotropy $\gamma \approx $ 7. This demonstrates that our experimental technique is both relatively fast to set up and furthermore reliable in fields up to 6T.