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