▎ 摘　　要

We demonstrate that a key elastic parameter of a suspended crystalline membrane-the Poisson ratio (PR) nu-is a nontrivial function of the applied stress sigma and of the system size L, i. e., nu =nu(L)(sigma). We consider a generic two-dimensional membrane embedded into space of dimensionality 2 + d(c). (The physical situation corresponds to d(c) = 1.) A particularly important application of our results is to freestanding graphene. We find that at a very low stress, when the membrane exhibits linear response, the PR nu(L) (0) decreases with increasing system size L and saturates for L -> infinity at a value which depends on the boundary conditions and is essentially different from the value nu = -1/3 previously predicted by the membrane theory within a self-consistent scaling analysis. By increasing sigma, one drives a sufficiently large membrane (with the length L much larger than the Ginzburg length) into a nonlinear regime characterized by a universal value of PR that depends solely on d(c), in close connection with the critical index eta controlling the renormalization of bending rigidity. This universal nonlinear PR acquires its minimum value nu(min) = -1 in the limit d(c) -> infinity, when eta -> 0. With the further increase of sigma, the PR changes sign and finally saturates at a positive nonuniversal value prescribed by the conventional elasticity theory. We also show that one should distinguish between the absolute and differential PR (nu and nu(diff), respectively). While coinciding in the limits of very low and very high stress, they differ in general: nu not equal nu(diff). In particular, in the nonlinear universal regime, nu(diff) takes a universal value which, similarly to the absolute PR, is a function solely of d(c) (or, equivalently, eta) but is different from the universal value of nu. In the limit of infinite dimensionality of the embedding space, d(c) -> infinity (i. e., eta -> 0), the universal value of nu(diff) tends to -1/3, at variance with the limiting value -1 of nu Finally, we briefly discuss generalization of these results to a disordered membrane.