Fig. 6

Effect of surface potassium (K) dosing on RbFe₂As₂. a–c Topographic images of RbFe₂As₂ with various K coverage (Kc = 0.012 ML, 0.046 ML, 0.17 ML, respectively). K atoms appear as random bright spots at low Kc. (Setpoints of a: V_b = 1 V, I = 50 pA; b: V_b = 1 V, I = 20 pA, c: V_b = 0.2 V, I = 200 pA; scale bars: 10 nm). d Spatially averaged dI/dV spectra of RbFe₂As₂ with various Kc (V_b = 50 mV, I = 200 pA, ΔV = 1 mV, T = 4.5 K for all the spectra). e Low-energy dI/dV spectra of RbFe₂As₂ at Kc = 0.17 ML (V_b = 15 mV, I = 300 pA, ΔV = 1 mV, T = 4.5 K), taken along the arrow in panel c. A spatially uniform gap with the size of 2Δ = 7.0 meV is observed. f A representative dI/dV map taken at Kc = 0, T = 4.5 K (V_b = 10 mV, I = 200 pA, ΔV = 1 mV; scale bar: 10 nm). g Selected FFT images of dI/dV maps with Kc = 0 (scale bar: 0.3 Å^-1). The FFTs are mirror symmetrized along (π, π) and (π, −π) directions. h A representative dI/dV map taken at Kc = 0.17 ML (V_b = 10 mV, I = 200 pA, ΔV = 1 mV, T = 4.5 K; scale bar: 10 nm). i Selected FFT images of the dI/dV maps with Kc = 0.17 ML (scale bar: 0.3 Å^-1); FFTs are also mirror symmetrized along (π, π) and (π, −π) directions. j FFT profiles along the (π, 0) direction for Kc = 0 (averaged over a 30° angle, as indicated in the E = 2 meV image in panel g). Parabolic fit (dashed curve) gives E_b = 25 meV and q_F = 0.19 Å⁻¹. k FFT profiles along the (π, 0) direction (averaged over a 30° angle) for Kc = 0.17 ML. Parabolic fit (dashed curve) gives E_b = 50 meV and q_F = 0.34 Å⁻¹