Step edge selection during ion erosion of Cu(001)

Upon Ar⁺ ion erosion of a Cu(001) surface, we observed with HR-LEED faceted structures on the surface oriented preferentially along ⟨100⟩ at 200 K and along ⟨110⟩ at 330 K. These results are evaluated with a geometric rule, relating under kinetic conditions the orientation of the surviving steps to their advance rates. Application of this generic rule demonstrates that the ⟨100⟩ steps advance slowest. This reveals enhanced interlayer mass transport across these steps. We also show that a transition from kinetically favored step orientation into the thermodynamically favored one occurs at very short time scales even near room temperature.