  The driving forces behind electronic nematicity in the iron pnictides remain
hotly debated. We use atomic-resolution variable-temperature scanning tunneling
spectroscopy to provide the first direct visual evidence that local electronic
nematicity and unidirectional antiferroic (stripe) fluctuations persist to
temperatures almost twice the nominal structural ordering temperature in the
parent pnictide NaFeAs. Low-temperature spectroscopic imaging of
nematically-ordered NaFeAs shows anisotropic electronic features that are not
observed for isostructural, non-nematic LiFeAs. The local electronic features
are shown to arise from scattering interference around crystalline defects in
NaFeAs, and their spatial anisotropy is a direct consequence of the structural
and stripe-magnetic order present at low temperature. We show that the
anisotropic features persist up to high temperatures in the nominally
tetragonal phase of the crystal. The spatial distribution and energy dependence
of the anisotropy at high temperatures is explained by the persistence of large
amplitude, short-range, unidirectional, antiferroic (stripe) fluctuations,
indicating that strong density wave fluctuations exist and couple to near-Fermi
surface electrons even far from the structural and density wave phase
boundaries.
