Fig. 1: Atomic-scale nematicity on FeAs.
From: Atomic-scale imaging of electronic nematicity in ferropnictides
a Electrical resistivity versus temperature curve for a 10-UC BaFe2As2 film epitaxially grown on insulating SrTiO3(001) substrate. The black arrow indicates the nematic transition temperature (TN) around 134 K. b STM topography (20 nm × 20 nm) acquired on FeAs-terminated surface, exhibiting a 2√2 × 2√2 reconstruction induced by 1/8 Ba adsorption. The black square outlines the super unit cell formed by Ba adatoms. Setpoint: V = 1.0 V and I = 50 pA. c STM topography in the same FOV as in (b), but taken at a different setpoint of V = 20 mV and I = 100 pA. The dashed black lines mark DWs separating orthogonally nematic domains I and II. d Structural model of \(2\surd 2\times 2\surd 2\) surface reconstruction due to Ba adsorption on the FeAs plane. Ring-like structure in the zoom-in STM topography (3.6 nm × 3.6 nm, V = 100 mV, I = 100 pA) is formed by four top-layer As atoms, with Ba adatoms adsorbed at their centers. e Site-resolved dI/dV spectra measured within domain I, as indicated in (d). The dashed blue line represents a vertical offset from the solid blue line to facilitate comparison between the spectra taken at the Bx and By sites. f Contrast-enhanced STM topography from the red-boxed region in (c), highlighting dragged Ba adatoms and 4aFe-periodic stripes aligned along the Fe-Fe directions. g Zoom-in of Ba adatoms within domain I, domain II and DW (top panel) as well as the line profiles along x and y directions within domains I and II (bottom panel), respectively. Colored dots mark Ba adatoms (black), Bx (red) and By (magenta) bridge sites. h Spatial map of nematic order parameter NT(r) extracted from STM topography T(r) in (c). i Histogram of NT(r) values from the nematic map in (h), displaying a bimodal distribution.
