STM study of the electronic structure and superconductivity in heavily hole-doped RbFe₂As₂

The heavily hole-doped iron-pnictide superconductors AFe₂As₂ (A=K, Rb, Cs) exhibit unusual properties such as only hole band presented at E_F, large Sommerfeld coefficient and high sensitivity to nonmagnetic impurities, which indicate they have strong electron correlation and possibly different pairing mechanism with respect to other iron-based superconductors. Here we present an ultra-low temperature (20 mK) STM study on RbFe₂As₂ single crystal. We observed a typical “V” shaped SC gap which can only be fit with a nodal gap function. The gap is found to be suppressed near the surface Rb vacancies and atomic step edges along As-As direction, which consists with a d-wave pairing. Furthermore, quasi-particle interference (QPI) measurements reveal that the band structure has significant two-fold symmetry, which breaks the four-fold symmetry of the lattice. Particularly, such C4-C2 symmetry breaking happens along (π, π) direction rather than (π, 0). Our observation suggests d-wave pairing coexists with a new type of electronic nematicity in RbFe₂As₂.