Studying the unconventional coupling of electronic quantum effects and atomic diffusion in Pb on Si(111)

• Applicants: Professorin Dr. Janet Anders; Professorin Dr. Regina Hoffmann-Vogel; Professor Dr. Ralf Metzler
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 513075417

Diffusion is not only important to understand fundamental physical processes such as electric currents, pattern formation in surface science, or molecular reactions in living biological cells, but it is also decisive in technology applications, for instance, when materials are grown industrially on Si substrates in computer chip manufacturing. Growing metals on semiconductors leads to new challenges and insights, due to the strong differences in electronic properties between the two materials. The goal of the project is to elucidate the link between electronic effects and unconventional diffusion of Pb on Si(111). For standard atomic diffusion, we would expect that the mean squared displacement increases with time as t^alpha, with alpha=1, and that the particle is driven by concentration gradients, moving, on average, to reduce these gradients. For Pb on Si(111) it has been found that Pb travels ballistically (alpha=2) over mesoscopic distances. These observations have been connected to the new notion of ultrafast spreading and collective superdiffusion. In addition, it has been found that neighboring Pb islands on Si(111) move in similar directions, opposite to the gradient that, in principle, should drive them. Such apparent contradictions demonstrate the need for both better experimental data to exclude possible artifacts and better modelling to understand how the origins of this unconventional behaviour could be explained. We will study the unconventional diffusion of Pb on the Si(111) surface by scanning force microscopy in ultra-high vacuum combined with theoretical calculations and simulations. We will first confirm the previously claimed but not directly observed occurrence of Pb atomic superdiffusion for different system parameters. We will analyse the observed atomic motion and provide a quantitative stochastic model for the superdiffusive dynamics. Moreover, we will investigate how this unconventional diffusion arises from either classical or quantum electronic noise effects. Experimentally we can measure, as a function of time, the Pb island size and concentration, related to the number N of atoms and the electronic work function Phi above the islands and above the wetting layer. The chemical potential mu of electrons is here linked to the electronic work function Phi, the observable that is accessible by the Kelvin method. The chemical potential of the electrons, in turn, is linked to the chemical potential of the atoms in this system, as demonstrated previously. This relation will allow us to understand the thermodynamic energy Ndmu, that drives the diffusion of the system and thus give us the full information needed to understand the diffusion process.

• DFG Programme: Research Grants