Effect of a Zn impurity on T_c and its implications for pairing symmetry in LaFeAsO_1−xF_x

GENERAL SCIENTIFIC SUMMARY Introduction and background. The pairing symmetry of superconducting states is crucial for understanding unconventional superconductivity. For iron-based superconductors, theory proposes a possible anti-phase s-wave pairing, where the superconducting order parameters in hole and electron pockets have opposite signs. The effect of non-magnetic impurities, such as Zn atoms, on the transition temperature depends crucially on the pairing state, and may be used to probe the pairing symmetry.

Main results. The effect of non-magnetic Zn impurity on superconductivity in LaFe_1−yZn_yAsO_1−xF_x system is reported systematically for the under-doped, optimally doped and over-doped regimes. In the presence of Zn impurity, the superconducting transition temperature increases in the under-doped regime, remains unchanged in the optimally doped regime, and is severely suppressed in the over-doped regime. Our results strongly suggest a switch of the symmetry of the superconducting order parameters from an s-wave to s± or d-wave states as the charge carrier doping increases (see figure).

Wider implications. We report for the first time a doping-dependent, non-magnetic Zn impurity effect on superconductivity in FeAs-based superconductors. Our findings suggest a switch of the pairing symmetry of the superconductivity from an s-wave state to a Zn-impurity sensitive anti-phase s-wave or d-wave state in the overdoped regime, which should have significant implications for the further development of theoretical models, and is likely to generate further experimental and theoretical activities in this field.

Figure. Superconducting transition temperatures versus Zn content in LaFe_1−yZn_yAsO_1−xF_x. Solid and open symbols refer to T_C determined from the measurements of resistivity and susceptibility, respectively.