  The significance of a negative dielectric constant has long been recognized.
We report here the observation of a field-induced large negative dielectric
constant of aggregates of oxide nano-particles at frequencies below ~ 1 Hz at
room temperature. The accompanying induced charge detected opposes the electric
field applied in the field-induced negative dielectric constant state. A
possible collective effect in the nano-particle aggregates is proposed to
account for the observations. Materials with a negative dielectric constant are
expected to provide an attraction between similar charges and unusual
scattering to electromagnetic waves with possible profound implications for
high temperature superconductivity and communications.
