Figure 2: Crystallographic planes with different orientations.

(a), A 3D landscape of scan area, showing two differently orientated surfaces after cleaving. The left facet is the (001) plane, while the right facet is determined as (661) plane. (b), The ball models of surface atomic structures for (001) plane and (661) plane. The basic surface unit cells are marked by blue dashed lines. (c), Local tunneling spectra were taken on the locations within different planes marked as stars in panel (a), showing the variation of the insulating gap in different crystalline planes. The gap size can be estimated to be approximately 164 meV from the fitting of spectra. The fitting curves are shown as black lines in the same plots as the experimental STS data. All spectral surveys were taken with a sample-bias voltage of 20 mV, a tunneling current of 0.1 nA, and bias modulation amplitude of 3 mV_rms.