Evidence of nematic order and nodal superconducting gap along [110] direction in RbFe2As2

TL;DR

This study reveals a novel nematic electronic state and a nodal superconducting gap along the [110] direction in RbFe2As2, highlighting the interplay between nematic order and superconductivity in iron-based superconductors.

Contribution

It provides direct STM evidence of a distinct ({ extpi}, { extpi}) nematic state and a nodal gap structure in RbFe2As2, advancing understanding of nematicity's role in superconductivity.

Findings

Identification of a ({ extpi}, { extpi}) nematic state with symmetry breaking.

Observation of a V-shaped, nodal superconducting gap.

Surface doping suppresses nematicity and enhances superconductivity.

Abstract

Unconventional superconductivity often intertwines with various forms of order, such as the "nematic" order which breaks the rotational symmetry of the lattice. Investigation of these ordered phases sheds crucial light on the superconductivity itself. Here we report a low-temperature scanning tunneling microscopy (STM) study on RbFe2As2, a heavily hole-doped Fe-based superconductor (FeSC). We observe significant symmetry breaking in its electronic structure and magnetic vortex which differentiates the ({\pi}, {\pi}) and ({\pi}, -{\pi}) directions of the unfolded Brillouin zone (BZ). It is thus a novel nematic state, distinct from the nematicity of undoped/lightly-doped FeSCs which breaks the ({\pi}, 0) / (0, {\pi}) equivalence. Moreover, we observe a clear "V"-shaped superconducting gap which can be well fitted with a nodal gap function. The gap is found to be suppressed on surface Rb vacancies and at step edges, and the suppression is particularly strong at the [110]-oriented edges. This is possibly due to a dx2-y2 like pairing component with nodes along the [110] directions. We further demonstrated that such ({\pi}, {\pi}) nematic state can be suppressed via surface electron doping to RbFe2As2, and the superconductivity is subsequently enhanced. Our results thus highlight the intimate connection between nematicity and superconducting pairing in iron-based superconductors.