We determine the Poisson ratio of self-avoiding fixed-connectivity membranes,
modeled as impenetrable plaquettes, to be sigma=-0.37(6), in statistical
agreement with the Poisson ratio of phantom fixed-connectivity membranes
sigma=-0.32(4). Together with the equality of critical exponents, this result
implies a unique universality class for fixed-connectivity membranes. Our
findings thus establish that physical fixed-connectivity membranes provide a
wide class of auxetic (negative Poisson ratio) materials with significant
potential applications in materials science.Comment: 4 pages, 3 figures, LaTeX (revtex) Published version - title changed,
one figure improved and one reference change