Enhanced interface repulsion from quenched hard–wall randomness

 We consider the harmonic crystal, or massless free field, , , 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 to be larger than , is an IID field (which is also independent of ϕ), for every x in a large region , with N a positive integer and D a bounded subset of ℝ ^d . We are mostly motivated by results for given typical realizations of σ (quenched set–up), since the conditioned harmonic crystal may be seen as a model for an equilibrium interface, living in a (d+1)–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 σ 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, ϕ and σ, 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 of the probability that ϕ lies above σ in D _N . Received: 6 February 2002 / Revised version: 23 May 2002 / Published online: 30 September 2002 Mathematics Subject Classification (2000): 82B24, 60K35, 60G15 Keywords or phrases: Harmonic Crystal – Rough Substrate – Quenched and Annealed Models – Entropic Repulsion – Gaussian fields – Extrema of Random Fields – Large Deviations – Random Walks