Fig. 4: The CO phase boundary and STS at the boundary.

a Differential conductance map of the CO phase boundary measured at a sample bias of +200 mV and tunneling current of 500 pA. The dashed line indicates the approximate position of the CO phase boundary. b We used line profiles along the atomic rows in b direction to define the CO phase boundary. The CO phase shows up as a wavy corrugation (height change), which suddenly terminates. The spatial position where the modulation disappears and the contrast becomes constant is taken as boundary. c Average tunneling spectrum measured at the CO phase boundary with two representative error bars given at positive and negative bias.