  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.
