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Visualization of Electron Nematicity and Unidirectional Antiferroic Fluctuations at High Temperatures in NaFeAs

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.