Fig. 1

STM images (sample bias voltage 1.0 V; tunneling current 1 nA) of the single crystal Ru(0001). a Prepared according to the ion bombardment protocol (see Methods) followed by etching at 1100 K in O₂ at 5 × 10⁻⁸ mbar during 30 min. b The same as a but with additional oxidation at 630 K in O₂ at 3 × 10⁻⁵ mbar during 50 min. Both images (a, b) have a visible area of 430 nm × 430 nm and share the same gray scale ruler shown right next to them. c The same as a but with additional oxidation at 630 K in O₂ at 3 × 10⁻⁵ mbar during 40 min; visible area 860 nm × 860 nm, a height–distance cross-section along the black dashed line 1 is given to the left of the image