The surface diffusion of single adatoms has been intensively studied over the last decades, [1][2][3] due to its importance in thin film and crystal growth. [4] Once individual atoms are adsorbed on a surface they can meet, thus forming larger clusters. However, the diffusion of even the simplest cluster, a dimer, on a surface is by far not yet understood. [5][6][7][8][9][10][11][12][13] The diffusion dynamics can be strongly affected by the coupling of the intramolecular motion to the translational motion of the centre of mass (CM) of the cluster. [12][13][14][15] Herein, we present a simple, one-dimensional model for studying the Hamiltonian and diffusive dynamics of a dimer, which is relevant to systems where quasione-dimensional motion takes place.[16] The deterministic dynamics of this model is characterized by a complex behavior, dominated by nonlinear effects, parametric resonances and chaotic features. At variance with the case of a single atom, at T ¼ 6 0 the role of the internal degrees of freedom of the dimer is responsible for deviations from activated behavior of the diffusion coefficient. In Section 1, we briefly outline our model. In Sections 2 and 3, we discuss the nonlinear deterministic and thermal dynamics, respectively, and compare the two situations in Section 4. Concluding remarks are given in Section 5.
ModelWe consider the deterministic and thermal dynamics of a dimer moving on a periodic one-dimensional substrate. The particle-substrate interaction is a sinusoidal function of amplitude 2 U 0 and period a, and the interparticle interaction is given by a harmonic potential with spring constant K and equilibrium length l. We use Langevin dynamics to deal with finite temperature T. The equations of motion for the two atoms of mass m and of coordinates x 1 and x 2 composing the dimer are given by Equation (1):where the effect of finite temperature T is taken into account by the stochastically fluctuating forces f i , satisfying the conditions hf i (t)i = 0 and hf i (t)f j (0)i = 2mhk B Td ij d(t), and by the damping term mhẋ i . In the following, we will use representative[a] C.