Voltage-Induced Tunneling Conduction in Granular Metals at Low Temperatures

We have observed a new conduction mechanism in granular metals at low temperatures and high electric fields $\mathcal{E}$ characterized by a field-dependent conductivity ${\sigma }_0\exp \left(\frac{-{\mathcal{E}}_0}{\mathcal{E}}\right)$. A theory based on a simple model of field-induced electron-hole generation in the bulk of the granular metal predicts the observed field dependence and gives expressions for ${\sigma }_0$ and ${\mathcal{E}}_0$ which are functions of the granular-metal parameters: metal grain size, and thickness and height of the tunneling barriers separating the metal grains. Agreement between theory and experiment is satisfactory.