Nematic superconducting state in iron pnictide superconductors

摘要

Nematic order often breaks the tetragonal symmetry of iron-basedsuperconductors. It arises from regular structural transition or electronicinstability in the normal phase. Here, we report the observation of a nematicsuperconducting state, by measuring the angular dependence of the in-plane andout-of-plane magnetoresistivity of Ba0.5K0.5Fe2As2 single crystals. We findlarge twofold oscillations in the vicinity of the superconducting transition,when the direction of applied magnetic field is rotated within the basal plane.To avoid the influences from sample geometry or current flow direction, thesample was designed as Corbino-shape for in-plane and mesa-shape forout-of-plane measurements. Theoretical analysis shows that the nematicsuperconductivity arises from the weak mixture of the quasi-degenerate s-waveand d-wave components of the superconducting condensate, most probably inducedby a weak anisotropy of stresses inherent to single crystals.