Auxetic properties and anisotropy of elastic material constants of 2D   crystalline media
  Anisotropies of Young's modulus E, the shear modulus G, and Poisson's ratio of all 2D symmetry systems are studied. Simple necessary and sufficient conditions on their elastic compliances are derived to identify if any of these crystals are completely auxetic, non-auxetic and auxetic. Particular attention is paid to 2D crystals of quadratic symmetry. All mechanically stable quadratic crystals are characterized by three parameters belonging to a prism with the stability triangle in the base. Regions in the stability triangle in which quadratic materials are completely auxetic, non-auxetic, and auxetic are established. Examples of all types of auxetic properties of crystals of oblique and rectangular symmetry are presented. 
Cz. Jasiukiewicz, T. Paszkiewicz, S. Wolski