Dynamic Response of Silver Monolayers Adsorbed on Gold(100) upon an Oscillatory Variation of the Chemical Potential: a Monte Carlo Study.

M. CECILIA GIMÉNEZ; EZEQUIEL V. ALBANO

JOURNAL OF PHYSICAL CHEMISTRY

American Chemical Society

Lugar: EE. UU.; Año: 2007 p. 1809 - 1815

0022-3654

On the basis 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 (mu_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 mu(t) = mu_coex + mu_o sin(2¶t/tau), where mu_o and tau 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 shown by recording hysteresis loops. This scenario is identified as dynamically ordered states (DOS) such that the relaxation time (tau_relax) of the corresponding metastable state obeys tau_relax > tau. On the other hand, by properly increasing mu_o and/or tau, one finds that the Ag coverage gently follows the external drive (here tau_relax < tau) 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 ofthe 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 =300 K 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 mu versus tau plane is obtained and discussed.