Scanning tunneling spectroscopy of SmFeAsO0.85: Possible evidence for d-wave order parameter symmetry

We report a scanning tunneling spectroscopy investigation of polycrystalline SmFeAsO0.85 having a superconducting transition at 52 K. On large regions of the sample surface the tunneling spectra exhibited V-shaped gap structures with no coherence peaks, indicating degraded surface properties. In some regions, however, the coherence peaks were clearly observed, and the V-shaped gaps could be fit to the theory of tunneling into a d-wave superconductor, yielding gap values between 8 to 8.5 meV, corresponding to the ratio 2D/KTc ~ 3.55 - 3.8, which is slightly above the BCS weak-coupling prediction. In other regions the spectra exhibited zero-bias conductance peaks, consistent with a d-wave order parameter symmetry.