Nematic superconducting state in iron pnictide superconductors

TL;DR

This paper reports the discovery of a nematic superconducting state in Ba0.5K0.5Fe2As2, characterized by twofold oscillations in magnetoresistivity, indicating broken rotational symmetry in the superconducting phase.

Contribution

It provides experimental evidence of nematic superconductivity in iron pnictides and explains its origin as a mixture of s-wave and d-wave components influenced by crystal stress.

Findings

Large twofold oscillations near superconducting transition

Nematic superconductivity arises from s-wave and d-wave mixture

Weak anisotropy of stress likely induces nematic state

Abstract

Nematic order often breaks the tetragonal symmetry of iron-based superconductors. It arises from regular structural transition or electronic instability in the normal phase. Here, we report the observation of a nematic superconducting state, by measuring the angular dependence of the in-plane and out-of-plane magnetoresistivity of Ba0.5K0.5Fe2As2 single crystals. We find large 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, the sample was designed as Corbino-shape for in-plane and mesa-shape for out-of-plane measurements. Theoretical analysis shows that the nematic superconductivity arises from the weak mixture of the quasi-degenerate s-wave and d-wave components of the superconducting condensate, most probably induced by a weak anisotropy of stresses inherent to single crystals.