Single-layer metal-on-metal islands driven by strong time-dependent forces

TL;DR

This study investigates how strong static and time-dependent forces influence the non-linear transport of single-layer metal-on-metal islands, revealing phenomena like current enhancement and inversion, with detailed modeling and simulations on copper surfaces.

Contribution

It introduces a combined semi-empirical lattice model with master equation and Monte Carlo methods to analyze driven island transport, highlighting new effects such as current inversion and odd-even size dependence.

Findings

Pulsed fields can increase island current compared to static fields.

Current inversion occurs in electrophoretic ratchet conditions.

Strong odd-even effects are observed in driven islands.

Abstract

Non-linear transport properties of single-layer metal-on-metal islands driven with strong static and time-dependent forces are studied. We apply a semi-empirical lattice model and use master equation and kinetic Monte Carlo simulation methods to compute observables such as the velocity and the diffusion coefficient. Two types of time-dependent driving are considered: a pulsed rotated field and an alternating field with a zero net force (electrophoretic ratchet). Small islands up to 12 atoms were studied in detail with the master equation method and larger ones with simulations. Results are presented mainly for a parametrization of Cu on Cu(001) surface, which has been the main system of interest in several previous studies. The main results are that the pulsed field can increase the current in both diagonal and axis direction when compared to static field, and there exists a current inversion in the electrophoretic ratchet. Both of these phenomena are a consequence of the coupling of the internal dynamics of the island with its transport. In addition to the previously discovered "magic size" effect for islands in equilibrium, a strong odd-even effect was found for islands driven far out of equilibrium. Master equation computations revealed non-monotonous behavior for the leading relaxation constant and effective Arrhenius parameters. Using cycle optimization methods, typical island transport mechanisms are identified for small islands.