A model of linear chain submonolayer structures: Application to Li/W(112) and Li/Mo(112)

We propose a lattice gas model to account for linear chain structures adsorbed on (112) faces of tungsten and molybdenum. This model includes a dipole-dipole interaction as well as a long-range indirect (oscillatory) interaction of the form similar to cos(2k(F)r + phi)/r, where k(F) is the wavevector of electrons at the corresponding Fermi surface and phi is a phase shift. It is assumed that the structures are stabilized by an attractive indirect interaction along the chains. We have explicitly demonstrated that the periodic ground states strongly depend on a competition between the dipole-dipole and long-range indirect interactions. The effect of temperature in our model of linear chain structures is studied within the molecular-field approximation. The numerical results clearly show that for the dipole-dipole interaction only, all long-periodic linear chain phases are suppressed to low temperatures while phases with periods 2, 3, and 4 dominate the phase diagram. However, when the long-range indirect interaction becomes important, the long-periodic linear chain phases start to fill up the phase diagram and develop a high thermal stability. We have chosen model parameters in order to reconstruct a sequence of long-periodic phases (for coverages less than 0.5) as observed experimentally at T = 77 K for Li/Mo(112) and Li/W(112). It would be interesting to verify our model and assumptions by checking experimentally the corresponding phase diagrams.