Fig. 1: Identification of surface terminations of Ba_1−xK_xFe₂As₂ (x ≈ 0.77).

a Crystal structure of Ba_1−xK_xFe₂As₂ (x ≈ 0.77). b Its two possible cleaved surfaces: Ba/K- and As- terminated surfaces (side view). c STM topographic image obtained from the disordered Ba/K terminated surface [(V, I) = (80 mV, 80 pA), image size: (100 nm)²]. d Differential conductance (dI/dV) as a function of bias voltage (V) and position (x) spectra measured along the red arrowed line in c [spectroscopic set-point (V_s, I_s) = 15 mV, 200 pA; amplitude of bias modulation used \({V}_{{{{{\rm{mod}}}}}}=0.25\,{{{{\rm{mV}}}}}\)]. Spectra were obtained at a temperature of 4.2 K. Spectra are vertically offset for clarity. e Histogram of the superconducting gap sizes (Δ_SC), calculated by measuring the separation distance between the coherence peaks. f Large-scale topographic image of As-terminated surface obtained after the displacement of the Ba/K atoms to the edges of the imaged region [(V, I) = (80 mV, 100 pA), image size: (110 nm)²]. g Atomically-resolved topographic image obtained from the central part of f [(V, I) = ( − 10 mV, 300 pA), image size: (40 nm)²]. The image shows that the As-terminated surface exhibits (1 × 1) As- unit cell, and \((\sqrt{2}\times \sqrt{2})\) spatial periodicity originating from surface reconstruction. h Fast Fourier transform of g, where black circles mark the (1 × 1) unit cell Bragg peaks, blue circles mark the peaks associated with the \((\sqrt{2}\times \sqrt{2})\) surface reconstruction, and red squares mark the additional ordering peaks.