Dynamic response of Ag monolayers adsorbed on Au(100) upon an oscillatory variation of the chemical potential: A Monte Carlo simulation study

Abstract

Based on the fact that the underpotential electrochemical deposition of Ag atoms on the $Au(100)$ surface exhibits sharp first-order phase transitions at well-defined values of the (coexistence) chemical potential ($μ_{coex}$), we performed extensive simulations aimed at investigating the hysteretic dynamic behavior of the system close to coexistence upon the application of a periodic signal of the form $μ(t) = μ_{coex} + μ_{o}sin(2Πt/τ) $, where $μ_{o}$ and $τ$ are the amplitude and the period of the sweep, respectively. For relatively short periods and small enough amplitudes the system becomes trapped either at low or high Ag coverage states, as evidenced by recording hysteresis loops. This scenario is identified as dynamically ordered states (DOS), such that the relaxation time $(τ_{relax})$ of the corresponding metastable state obeys $τ_{relax} > τ$. On the other hand, by properly increasing $μ_{o}$ or/and $τ$, one finds that the $Ag$ coverage gently follows the external drive (here $τ_{relax} < τ$) and the system is said to enter into dynamically disordered states (DDS), where hysteresis loops show almost symmetric limiting cycles. This symmetry breaking between limiting cycles driven by an external signal is discussed in terms of the concept of (out-of-equilibrium) Dynamic Phase Transitions between DOS and DDS, similar to those encountered when a magnetic system is placed in the presence of a variable external magnetic field. However, a careful finite-size scaling study reveals that, at least at $T = 300K$, the $Ag/Au(100)$ system does not exhibit true second-order phase transitions but rather a crossover behavior between states. A diagram showing the location of the ordered and disordered states in the $μ$ versus $τ$ plane is obtained and discussed.