Contrast Reversal and Shape Changes of Atomic Adsorbates Measured with Scanning Tunneling Microscopy

Systematic, quantitative comparisons between scanning tunneling microscopy (STM) experiments and first principles simulations of $\mathrm{O}(2\times 2)/\mathrm{Ru}(0001)$ have been performed. The shape of the atomic adsorbates in the images depends strongly on the tunneling resistance and changes reversibly from circular (high resistance) to triangular (low resistance). In addition, after adsorption of oxygen on the STM tip we observe a contrast reversal on the surface, confirmed by extensive numerical simulations.