Scanning tunneling spectroscopy of SmFeAsO_(0.85): Possible evidence for d-wave order-parameter symmetry

摘要

We report a scanning tunneling spectroscopy investigation of polycrystalline SmFeAsO_(0.85) 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 fitted to the theory of tunneling into a d-wave superconductor, yielding gap values between 8 and 8.5 meV corresponding to the ratio 2Δ/K_BT_C ～ 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.