Figure 8.

STM measurement of topography and tunneling spectra on Nd_1−xSr_xNiO₂ film

(A) 3D illustration of the topographic image after topotactic reaction method; one can see the surface roughness about 1–2 nm.

(B) A tunneling spectrum with a V-shape, which is measured on the rough surface. The Dynes model fitting yields a gap function Δ = 3.9cos2θ (meV).

(C) A tunneling spectrum with full gap feature, which is measured on the rough surface. The Dynes model fitting is Δ = 2.35(0.85 + 0.15cos4θ) (meV).

(D) 3D illustration of the topographic image after a long-time vacuum annealing. The roughness becomes much smaller. We can see a clear step with the height about 0.17 nm, being consistent with half of the unit cell height.

(E) A typical V-shape spectrum measured on the smooth surface. A gap function Δ = 3.95(0.95cos2θ + 0.05cos6θ) (meV) is used in the fitting.

(F) A mixture of the two-gap features on the spectrum is measured on the smooth surface. Dynes fitting results, Δ₁ = 5.3(0.8cos2θ + 0.2cos6θ) (meV), Δ₂ = 2 meV, p₁ = 85%, and p₂ = 15%. Adapted from Gu et al.¹⁷⁴