Fig. 2: Breaking of fourfold rotational symmetry.

a, b Fermi surface maps recorded at T = 16 K (hν = 75 eV) for two sample azimuthal angles that are 90^∘ apart as indicated in the schematics. The azimuthal angle rotation was operated in situ and hence the sample surface is identical. k_∥ (k_⊥) represents momentum parallel (perpendicular) to the electron-analyser slit. The spectral intensity was integrated within E_F ± 20 meV, washing-out spectral gaps within the integration window. c, d Fermi surfaces from the tight-binding model (see Supplementary note 1). The sheets not observed in the experiment are indicated in grey. e–h Energy distribution maps along M_a–Γ, X–M_b, M_b–Γ, and X–M_a as shown in c and d. The energy distribution maps were recorded at hν = 37 eV except for g where hν = 31 eV incident light was selected to optimise the photoelectron matrix element. Systematic photon-energy dependence between 30 and 40 eV is presented in Supplementary Figs. S2 and S3. The band structure along with M_a–Γ (X–M_a) and M_b–Γ (X–M_b) is inequivalent.