  We find, with the use of first-principles calculations, that a
single-atom-thick boron-nitride (BN) sheet exhibits an unusual nonlinear
electromechanical effect: it becomes macroscopically polarized when bent
out-of-plane. The direction of the induced polarization is in the plane of the
film and it depends non-analitically on the corrugation wave vector k. The
magnitude of the polarization can reach very high values in spite of being at
least quadratic in atomic displacements due to BN sheets being able to tolerate
large mechanical strains. The discovered effect can find many applications, in
particular, in a new type of efficient and reliable nanogenerators.
