Imaging anomalous nematic order and strain in optimally doped BaFe$_2$(As,P)$_2$

TL;DR

This study uses ultrafast optical and microscopy techniques to reveal a nematic phase in BaFe$_2$(As,P)$_2$ that exists above the superconducting transition temperature, influenced by local strain and domain structure.

Contribution

It demonstrates the existence of an anomalous nematic phase above $T_c$ and links it to local strain variations, providing new insights into nematicity in iron-based superconductors.

Findings

Nematic order exists above $T_c$ without a bulk phase transition.

Nematicity varies on a 50-100 μm scale and correlates with local strain.

Nematic order is most prominent in regions of weak, isotropic strain.

Abstract

We present the strain and temperature dependence of an anomalous nematic phase in optimally doped BaFe$_2$(As,P)$_2$. Polarized ultrafast optical measurements reveal broken 4-fold rotational symmetry in a temperature range above $T_c$ in which bulk probes do not detect a phase transition. Using ultrafast microscopy, we find that the magnitude and sign of this nematicity vary on a ${50{-}100}~\mu$m length scale, and the temperature at which it onsets ranges from 40 K near a domain boundary to 60 K deep within a domain. Scanning Laue microdiffraction maps of local strain at room temperature indicate that the nematic order appears most strongly in regions of weak, isotropic strain. These results indicate that nematic order arises in a genuine phase transition rather than by enhancement of local anisotropy by a strong nematic susceptibility. We interpret our results in the context of a proposed surface nematic phase.