Elastic metamaterials for independent realization of negativity in   density and stiffness
  In this paper, we present the realization of an elastic metamaterial allowing independent tuning of negative density and stiffness for elastic waves propagating along a designated direction. In electromagnetic (or acoustic) metamaterials, it is now possible to tune permittivity (bulk modulus) and permeability (density) independently. Apparently, the tuning methods seem to be directly applicable for elastic case, but no realization has yet been made due to the unique tensorial physics of elasticity that makes wave motions coupled in a peculiar way. To realize independent tunability, we developed a single-phased elastic metamaterial supported by theoretical analysis and numerical/experimental validations. 
Joo Hwan Oh, Young Eui Kwon, Hyung Jin Lee and Yoon Young Kim
cond-mat.other cond-mat.mtrl-sci