We demonstrate that Coulomb drag between spatially separated quasi-two-dimensional electron and hole gases is strongly enhanced by Coulombic correlations.The correlations modify the carrier concentration dependence and the temperature dependence of transresistance and remove the persistent order of magnitude disagreement between the experimental data and the theories based on the mean field (random phase) approximation. Disorder scattering is shown to influence the results, particularly strongly at low concentrations.A great deal of attention has been recently devoted to double layer systems in which two quasi-two-dimensional subsystems (electron or hole gases) are placed in parallel planes separated by a potential barrier thick enough to prevent particles from tunneling across it but allowing for the interactions between the particles on both its sides. In such systems many body correlations due to Coulomb in-

A systematic approach on collective diffusion in an interacting lattice gas adsorbed on a non-homogeneous substrate is formulated. It is based on a variational Ritz procedure of determining a diffusive eigenvalue of a transition rate matrix describing microscopic kinetics of particle migration processes in the system. Form of a trial eigenvector and a choice of variational parameters are discussed and justified on physical grounds. Reed-Ehrlich factorization of the collective diffusion coefficient into the thermodynamic and kinetic factors is explicitly shown to emerge naturally from the variational approach, and closed expressions for both factors are derived. Validity of the approach is tested by applying it to the simplest case of diffusion of noninteracting adparticles across steps on a stepped substrate ͑modeled by a Schwoebel barrier͒. The coverage dependence of collective diffusion coefficient, obtained in an algebraic form, agrees very well with the results of Monte Carlo simulations. It is demonstrated that the results obtained provide a substantial improvement over the mean-field theory results for the same system. Generalizations necessary to include interparticle interactions are listed and discussed.

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A variational approach to microscopic kinetics of an interacting lattice gas is presented. It accounts for the equilibrium correlations in the system and allows one to derive an algebraic expression for the particle density (coverage) dependent chemical diffusion coefficient for a wide variety of interaction models. Detailed derivation is presented for a one dimensional case for which the results are compared with the results of Monte Carlo simulations. Generalization and an application to the simplest case of the two dimensional lattice gas is briefly described.

Collective diffusion is investigated within the kinetic lattice gas model for systems of particles in one dimension with repulsive long-range interactions which are known to result in a staircaselike phase diagram with an infinite sequence of incompressible crystalline phases separated one from another by unstable compressible liquidlike phases. Using a recently proposed ͓Gortel and Załuska-Kotur, Phys. Rev. B 70, 125431 ͑2004͔͒ variational method, an analytic expression for the particle density dependence of the diffusion coefficient is derived in which commonly postulated static and kinetic factors are unambiguously identified. It is shown that while the static factor exhibits singular coverage dependence due to a sharp drop of compressibility when the system enters a crystalline phase, the kinetic factor may substantially modify this behavior. Depending on details of the activated state interactions controlling the migration kinetics the diffusion coefficient may also be singular or, at another extreme, it may be a continuously smooth function of density. In view of these observations recent results on efficient low temperature self-reorganization through devil's staircase phases in the dense Pb/ Si͑111͒-ͱ 3 ϫ ͱ 3 are discussed.