Negative static dielectric constant in a nano-colloid

We have investigated the dielectric properties of assemblies of urea-coated Ba$_{0.8}$Rb$_{0.4}$TiO(C$_{2}$O$_{4})_{2}$ nanoparticles. The effective dielectric constant, $\varepsilon $', is negative below a certain bias dependent frequency. $\varepsilon $' follows a plasma-like dispersion with an extremely low plasma frequency and damping rate. This dielectric response is linear with bias fields larger than 2.5 kV/mm. Additionally, after a step increase of the electric field, the transient current changes sign and is opposite to the applied field. These two properties rule out electrochemical effects as well as carrier trapping/de-trapping as the source of the negative dielectric constant. More importantly, this latter feature verifies that the static dielectric function, $\varepsilon $'(k,$\omega $=0), is negative. Furthermore, measurements on cold pressed (solid) pellets of the nanoparticles demonstrate that the effect is not due to rearrangement or large scale motion of the particles. In fact, the data suggests that the negative static $\varepsilon $' is correlated to the interfacial conductivity of the nanoparticles.