Superconductivity-induced nematicity

TL;DR

This paper reveals that LiFeAs exhibits nematic order within its superconducting state, with spontaneous rotational symmetry breaking observed in the gap structure and Fermi surface distortions, disappearing above T_c, indicating a novel superconductivity-induced nematicity.

Contribution

It demonstrates for the first time that superconductivity in LiFeAs induces nematic order, linking nematicity directly to the superconducting state without prior structural or magnetic transitions.

Findings

Spontaneous rotational symmetry breaking in the superconducting gap.

Unidirectional distortion of Fermi pockets in LiFeAs.

Nematic deformations vanish above T_c.

Abstract

The role of nematic order for the mechanism of high-temperature superconductivity is highly debated. In most iron-based superconductors (IBS) the tetragonal symmetry is broken already in the normal state, resulting in orthorhombic lattice distortions, static stripe magnetic order, or both. Superconductivity then emerges, at least at weak doping, already from the state with broken $C_4$ rotational symmetry. One of the few stoichiometric IBS, lithium iron arsenide, superconducts below 18 K and does not display either structural or magnetic transition in the normal state. Here we demonstrate, using angle-resolved photoemission, that even superconducting state in LiFeAs is also a nematic one. We observe spontaneous breaking of the rotational symmetry in the gap amplitude on all Fermi surfaces, as well as unidirectional distortion of the Fermi pockets. Remarkably, these deformations disappear above superconducting $T_c$. Our results demonstrate the realization of a novel phenomenon of superconductivity-induced nematicity in IBS, emphasizing the intimate relation between them. We suggest a theoretical explanation based on the emergence of a secondary instability inside the superconducting state, which leads to the nematic order and s-d mixing in the gap function.