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We consider two independent lattice harmonic crystals in dimension d greater than or equal to 3 constrained to live in the upper half-plane and to lie one above the other in a large region. We identify the leading order asymptotics of this model, both from the point of view of probability estimates and of pathwise behavior: this gives a rather complete picture of the phenomenon via a detailed analysis of the underlying entropy-energy competition. From the technical viewpoint, with respect to earlier work on sharp constants for harmonic entropic repulsion, this model is lacking certain monotonicity properties and the main tool that allows to overcome this difficulty is the comparison with suitable rough substrate models. (C) 2003 Elsevier B.V. All rights reserved.

We consider the harmonic crystal, or massless free field, phi = {phix}(xis an element ofZd), d greater than or equal to 3, that is the centered Gaussian field with covariance given by the Green function of the simple random walk on Z(d). Our main aim is to obtain quantitative information on the repulsion phenomenon that arises when we condition phi(x) to be larger than sigmax, sigma = {sigmax}(xis an element ofZd) is an IID field (which is also independent of phi), for every x in a large region D-N = N D boolean AND Z(d), with N a positive integer and D a bounded subset of R d. We are mostly motivated by results for given typical realizations of sigma (quenched set-up), since the conditioned harmonic crystal may be seen as a model for an equilibrium interface, living in a (d I)-dimensional space, constrained not to go below an inhomogeneous substrate that acts as a hard wall. We consider various types of substrate and we observe that the interface is pushed away from the wall much more than in the case of a flat wall as soon as the upward tail of sigma(0) is heavier than Gaussian, while essentially no effect is observed if the tail is sub-Gaussian. In the critical case, that is the one of approximately Gaussian tail, the interplay of the two sources of randomness, phi and sigma, leads to an enhanced repulsion effect of additive type. This generalizes work done in the case of a flat wall and also in our case the crucial estimates are optimal Large Deviation type asymptotics as N NE arrow infinity of the probability that phi lies above or in D-N.