Tip-induced mobilization upon cooling of Ni monolayers on Re(0001)

Abstract

Usually, cooling a metal sample down to cryogenic temperatures leads to immobilization of the surface atoms. In this study, we demonstrate a movement of Ni adatoms at $4.6\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ on Ni films grown on a Re(0001) single crystal, while the surface is rigid at room temperature. The mobility is observed from 2 to 20 atomic-layer-thick films. Measurements at intermediate temperatures reveal an increasing mobility with decreasing temperature. The observed velocity of advancing steps is consistent with a model considering a sudden release of material, eventually triggered by the tip, followed by free diffusion. According to the model, an increasing length scale for the mobilization event with decreasing temperature, tentatively explained by epitaxial strain, is responsible for the inverse mobilization behavior.