Extended Data Fig. 6: Tip-induced collapse of a flat band.
From: Cascade of electronic transitions in magic-angle twisted bilayer graphene
Measurements on MATBG can strongly depend on the STM tip, so we calibrate the tip against the Shockley surface state of Cu(111). Damage to or contamination of the tip causes distortions to observed features. Copper and tungsten, incidentally, have work functions similar to that of graphene. a, dI/dV(Vb, Vg) measured at the centre of an AA site in device A, with a twist angle of 1.01° and 0.2% strain (same data as Extended Data Fig. 2a). The lack of symmetry in dI/dV(Vb, Vg) indicates that the tip condition is already non-optimal. b, Same as a after lightly crashing the tip into the sample (initial tunnelling parameters: Vb = −200 mV, I = 0.5 nA, 551.7 Hz sinusoidal modulation of Vrms = 1 mV). The crash further degraded the tip, causing the valence flat band to appear to cross EF without pinning. This can be caused by changes in tip-induced band bending7 and tip-induced doping37.
