  The Poisson's ratio of a material characterizes its response to uniaxial
strain. Materials normally possess a positive Poisson's ratio - they contract
laterally when stretched, and expand laterally when compressed. A negative
Poisson's ratio is theoretically permissible but has not, with few exceptions
of man-made bulk structures, been experimentally observed in any natural
materials. Here, we show that the negative Poisson's ratio exists in the
low-dimensional natural material black phosphorus, and that our experimental
observations are consistent with first principles simulations. Through
application of uniaxial strain along zigzag and armchair directions, we find
that both interlayer and intralayer negative Poisson's ratios can be obtained
in black phosphorus. The phenomenon originates from the puckered structure of
its in-plane lattice, together with coupled hinge-like bonding configurations.
