Reduction of surface coverage of finite systems due to geometrical steps

TL;DR

This paper investigates how surface steps on vicinal surfaces affect molecular coverage, revealing an additional low-temperature transition caused by surface steps, modeled through an extended Ising framework.

Contribution

It introduces an effective two-spin model to describe coverage reduction and additional phase transition due to surface steps in a distorted Ising system.

Findings

Coverage decreases at low temperatures due to surface steps.

An extra low-temperature transition occurs, characterized by diverging susceptibility.

Specific heat diverges with a power law because of surface steps.

Abstract

The coverage of vicinal, stepped surfaces with molecules is simulated with the help of a two-dimensional Ising model including local distortions and an Ehrlich-Schwoebel barrier term at the steps. An effective two-spin model is capable to describe the main properties of this distorted Ising model. It is employed to analyze the behavior of the system close to the critical points. Within a well-defined regime of bonding strengths and Ehrlich-Schwoebel barriers we find a reduction of coverage (magnetization) at low temperatures due to the presence of the surface step. This results in a second, low-temperature transition besides the standard Ising order-disorder transition. The additional transition is characterized by a divergence of the susceptibility as a finite-size effect. Due to the surface step the mean-field specific heat diverges with a power law.