Superconducting phase diagram of Sm-based oxy-pnictide single crystals at high magnetic fields

The recently discovered class of high Tc superconducting pnictides has led to very intense research efforts over the last year. Despite the efforts, there is still no consensus about such fundamental problems as the gap symmetry or the pairing mechanism. Measurements of the critical field Bc2 can help to find an answer. In practice, accurate measurements of Bc2 near the zero temperature limit are often hampered by the large critical fields, which promotes these systems as good candidates for high field experiments. One method to obtain Bc2 is by measuring electric transport in magnetic fields. So far, most studies of this kind are performed on polycrystaline samples where the interpretation of the resistance curves is not straightforward and assumptions about the anisotropy are based on the width of the transition. We present transport measurements of Bc2 in fields up to 60 T on single crystals of F-doped SmFeAs(O,F) and oxygen-deficient SmFeAsO1-δ. From the measurements for different orientations we were able to extract the anisotropy in the upper critical field, which show a significant temperature-dependence below Tc. We compare our results with data obtained on polycrystaline samples and discuss our data in terms of superconducting gap symmetries