Surface diffusion is the key mass transport process and is commonly described as a Fickian random walk. Using STM and surface diffraction, we show that intercalated Pb under graphene on SiC moves by collective diffusion much faster than random-walk diffusion above a critical coverage ${\theta }_{\mathrm{c}}$ higher than the Pb(111) density. Massive deintercalation of the compressed intercalated layer is observed unexpectedly at room temperature (RT), well below the typical deintercalation temperature of $\sim 550{}^{\circ }\mathrm{C}$. The layer forms after several intercalation/deintercalation cycles, such that its coverage is high, as confirmed by the large drop in the intensity of the diffraction spots. Pb islands with uniform rings at their perimeters that grow at low temperatures are distinctive signatures of collective diffusion ideally studied with STM. LEEM videos of the Pb intercalation reported in the literature further support these nonclassical results.

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• Growth
• Intercalation
• Nucleation
• Nucleation on surfaces
• Surface diffusion
• Graphene
• Interfaces
• Thin films
• Electron diffraction
• Epitaxy
• Scanning tunneling microscopy
• Surface scattering